Oscilloscope Floating using UPS

[JJ_023]

So, this is more of a thought experiment to learn from. 

If you took a name brand scope, for example RIGOL MSO5074 and wanted to make it a floating oscilloscope.  For whatever the reason one may want to do that. Could the following method be employed?

Taking a battery backup UPS that is fully charged.  Unplugging it from mains and securing the plug in a way that would prevent accidental contact with anything.  Then plugging the scope and running it off the UPS without any other connections to the UPS.

Would this work and what would be some potential concerns in doing this?

Thank you in advance for any discussion on the topic.

[CountChocula]

I think it might work*, but it would be dangerous. If one of your ground leads gets connected to mains hot, your 'scope is now referenced to 110 or 220V. If your body has a path to earth and you come into contact with the chassis, it will be an… electrifying experience.

Better to float the DUT using an isolation transformer, IMO.


* Might because, while on battery power, most UPSs output at best an approximation of the mains sine wave, and so your 'scope might end up picking up all kinds of noise from it, to the point of being unusable. Not sure, though—I've never tried it.

[JJ_023]

I think it might work*, but it would be dangerous. If one of your ground leads gets connected to mains hot, your 'scope is now referenced to 110 or 220V. If your body has a path to earth and you come into contact with the chassis, it will be an… electrifying experience.

I am aware of this. Thank you for mentioning that.

Better to float the DUT using an isolation transformer, IMO.

Even better would be differential probes.  But that is not the thought experiment I am conducting.  Besides the dangers associated with doing something like this I am wondering if it would work.

* Might because, while on battery power, most UPSs output at best an approximation of the mains sine wave, and so your 'scope might end up picking up all kinds of noise from it, to the point of being unusable. Not sure, though—I've never tried it.

This would be my main concern. I am fully aware that it is an approximate sine wave.  However the oscilloscope internally primarily runs on DC and the oscilloscope itself is designed to handle mains which is not necessarily all that clean at times.  So I am not sure if that would affect the oscilloscope or not.

[Andy Chee]

I’ve used an oscilloscope from a petrol inverter generator before, though not for the specific purpose of floating isolation. YMMV.

[JJ_023]

I’ve used an oscilloscope from a petrol inverter generator before, though not for the specific purpose of floating isolation. YMMV.

Could you elaborate on any issues with that set up?

[MLXXXp]

This would be my main concern. I am fully aware that it is an approximate sine wave.

There are UPSs available that output a true sine wave. They are usually more expensive than an equivalent with the more common stepped approximation output but aren't hard to find.

Just one example of many:
https://www.apc.com/ca/en/product/BR1000MS/apc-backups-pro-1000va-600w-tower-120v-10x-nema-515r-outlets-sine-wave-avr-usb-type-a-+-c-ports-lcd-user-replaceable-battery

[Andy Chee]

Could you elaborate on any issues with that set up?

I do use a small power line conditioner as standard practice with any sensitive electronics connected to the generator, which would’ve cleaned up the noisy mains considerably as far as interference and “squareness” is concerned.

https://www.poweronaustralia.com.au/wp-content/uploads/2014/07/Sola-200-210-Brochure.pdf

https://cdn.hackaday.io/files/21242908817728/Oneac_CB_Series.pdf

[pdenisowski]

* Might because, while on battery power, most UPSs output at best an approximation of the mains sine wave, and so your 'scope might end up picking up all kinds of noise from it, to the point of being unusable.

This is the main application (in my experience) of a line trigger (i.e. the scope triggers on the peaks of the AC mains supply waveform).  If you do have noise that's somehow synchronized with the AC power, using a line trigger will case the noise to appear stationary on the scope display.  There's an example of this in my video on RTB2000 triggering

https://youtu.be/rg6CECPGbAQ?t=454

[pdenisowski]

If you took a name brand scope, for example RIGOL MSO5074 and wanted to make it a floating oscilloscope.  For whatever the reason one may want to do that. Could the following method be employed?

Taking a battery backup UPS that is fully charged.  Unplugging it from mains and securing the plug in a way that would prevent accidental contact with anything.  Then plugging the scope and running it off the UPS without any other connections to the UPS.

Would this work and what would be some potential concerns in doing this?

Several scope manufacturers (including R&S) sell battery powered oscilloscopes and one of the main applications is making (high voltage) floating measurements.  What you're describing is more or less the same thing - building a battery powered scope.

That said, I'm working on a video discussing isolated and floating measurements, and my "official" recommendation will be "don't float the scope".  There are so many things that can go wrong, many of which are dangerous to the scope, the user, or both.  Differential probes are the way to go in almost all circumstances.

But if one is determined to "float the scope,", a (good) isolation transformer is the best choice.  You also want to make sure there are no LAN, USB, etc. cables connected to your scope.

[JJ_023]

* Might because, while on battery power, most UPSs output at best an approximation of the mains sine wave, and so your 'scope might end up picking up all kinds of noise from it, to the point of being unusable.

This is the main application (in my experience) of a line trigger (i.e. the scope triggers on the peaks of the AC mains supply waveform).  If you do have noise that's somehow synchronized with the AC power, using a line trigger will case the noise to appear stationary on the scope display.  There's an example of this in my video on RTB2000 triggering

https://youtu.be/rg6CECPGbAQ?t=454

Interesting video. Thank you for sharing.

[JJ_023]

Several scope manufacturers (including R&S) sell battery powered oscilloscopes and one of the main applications is making (high voltage) floating measurements.  What you're describing is more or less the same thing - building a battery powered scope.

That was basically my train of thought and was wondering if there would be any effect on the measurements taken by the scope by doing something like that. To me it seems like it would not affect the scope in any capacity but I am not 100% sure that's why I posted.

That said, I'm working on a video discussing isolated and floating measurements, and my "official" recommendation will be "don't float the scope".  There are so many things that can go wrong, many of which are dangerous to the scope, the user, or both.  Differential probes are the way to go in almost all circumstances.

But if one is determined to "float the scope,", a (good) isolation transformer is the best choice.  You also want to make sure there are no LAN, USB, etc. cables connected to your scope.

I agree with you, if you read some of my other replies I am well aware that differential probes are the way to go. And I am well aware of isolation transformers.

My post was more of an idea exploring post on whether a mains oscilloscope could be floated, essentially turned into a battery-powered scope without degrading the functionality of the scope. 

I fully understand being aware of any unwanted connections. 

Thank you for the warnings.

[Smokey]

Like what everyone else said already.  Yer gonna die!  But... If you don't mind dying....
...  you just need a 3-2 plug adapter to float the scope.  Or breaking the ground lug off the cable does the same thing.  No UPS required.

[magic]

I can confirm that the plug adapter of death works. It has a risk though: if you apply really high voltage to the scope's ground for even more certain death, primary to secondary isolation of the scope's transformer may break down. No problems, however, if you simply "ground" it to another mains potential, particularly if it's the same phase.

I have also heard (not tried yet) that modern DSOs with universal switching PSUs can work from DC, and it may not even take a lot of it (tens of volts). Perhaps a battery bank is all you need. I imagine that features like "AC line" triggering may not work...

[MrAl]

Hello there,


I always see people get super paranoid about these kinds of measurements yet they are done every day without incident.

Years ago I used the method of cutting the ground pin off of the scopes line cord plug for literally 100's of measurements.  Many people I knew did the same thing, over and over and over and over and over again.  None of them died.  None of them even got shocked.  These were all CRT type scopes.

The key point here is you just have to pay attention to what you are doing when making ANY measurement of line voltages, 120vac or 230vac, or even 440vac.  It's not limited to scopes.  When you use a regular voltmeter, you have to pay attention to where your hands and feet are, as to what they are touching at all times.  It's either that, or pay someone to do the measurements for you.

These days battery powered scopes are somewhat cheap you can get one now for less than $30 that will do line frequency measurements with ease.  That would be the recommended method for today, but you still have to pay attention to what your hands and feet are touching when you do the measurements.
1.  Don't touch any metal connected to any wire.
2.  Don't stand on concrete or any electrically conductive floor, don't stand in water.

Do some reading on the web for how to make line voltage measurements and precautions, you should be ok.

[Vovk_Z]

I can confirm that the plug adapter of death works. It has a risk though:

It has a risk probably if you use to work with grounded instruments. But in some areas we don't have proper grounding at all since ages and it's ok too.
And now we have differential relays. We may use isolation transformers too. So I don't see grounding as an only thing between you life and death.

[Ian.M]

Back in the day of all metal CRO cases and front panels, everybody with the slightest trace of common sense knew that floating a scope was high risk, especially if there were any grounds nearby and a risk of the user touching both.   Also this was before ELCBs/RCDs were in common use so no 'magic' circuit breaker was there to save your misguided ass.

The major hidden hazard with floating a modern plastic cased DSO, is you have absolutely no idea of the creepage and clearance distances between the touchable surfaces of the operating controls and the chassis ground plane of the front panel PCB(s), or other chassis referenced circuits.  To a casual eye, it looks 'safe' with insulating plastic knobs and buttons, but a little conductive dirt buildup on the side of a button or in a knob grubscrew hole could easily be a serious shock hazard.   Then there's the issue of how to handle probes safely when they were never intended to provide more than basic insulation on their grounded parts you've now floated.

Many of the major scope manufacturers have specific technical briefs advising that instruments not designed for floating operation should never be floated, so why are we second guessing them here in a beginners subforum?

[tggzzz]

I always see people get super paranoid about these kinds of measurements yet they are done every day without incident.

Years ago I used the method of cutting the ground pin off of the scopes line cord plug for literally 100's of measurements.  Many people I knew did the same thing, over and over and over and over and over again.  None of them died.  None of them even got shocked.  These were all CRT type scopes.

The key point here is you just have to pay attention to what you are doing when ...

Exactly the same can be said about walking into the road without looking.

(And why did you bring up the display technology (CRT)? That is completely and utterly irrelevant)

[Solder_Junkie]

I had a requirement to measure a DC current switch-on surge by using a low Ohm resistor in series with the load. As the Voltage across the resistor was only a few mV, and I didn’t have a differential probe, I used a battery to power the equipment and used my mains powered scope across the series resistor. The equipment under test was isolated from ground and the mains powered scope was earthed via the mains plug earth pin. The DC Voltage was only 12 Volts, so powering from a battery was not a problem.

No safety issues and no noise pickup. It would have been hard to use a differential probe for this measurement as the peak Voltage was only just over 20 mV and most differential probes appear to have a lot of attenuation for measuring high Voltages.

Only a fool disconnects the earth from mains powered test equipment, it’s there to protect the user in case of failure.

SJ

[magic]

Many of the major scope manufacturers have specific technical briefs advising that instruments not designed for floating operation should never be floated, so why are we second guessing them here in a beginners subforum?

Moreover, much of the equipment that one would probe in such way has specific warnings about hazardous voltages and no user servicable parts inside...

[jonpaul]

Bonjour, bravo to our friends at R&S for the good advise.....

.....my "official" recommendation will be "don't float the scope".

Since 1970s, in HV, SMPS, electronic ballast, medical and avionics design, debug and mfg, we use  mains isolation transformers like Signal DU-1, DU-5 to float the DUT/PSU NOT  the scope!!!

Thus no need for diff probes, or the other kluges mentioned.

Jon

[JJ_023]

Like what everyone else said already.  Yer gonna die!  But... If you don't mind dying....
...  you just need a 3-2 plug adapter to float the scope.  Or breaking the ground lug off the cable does the same thing.  No UPS required.

If you want to be accurate you're the 1st 1 to say that.  I also think you misunderstood the point of the post ( probably my fault for not doing a good enough job explaining).  I am not asking on how you should float a scope. Or why. 

What I am asking is can you take a Mains oscilloscope and turn it into a battery-powered oscilloscope using a UPS that is disconnected from mains.  The byproduct would be a floating scope.  What my question was trying to ascertain was whether it would affect the functionality of the scope by powering it of UPS power supply that is disconnected from the mains.

[JJ_023]

I can confirm that the plug adapter of death works. It has a risk though: if you apply really high voltage to the scope's ground for even more certain death, primary to secondary isolation of the scope's transformer may break down. No problems, however, if you simply "ground" it to another mains potential, particularly if it's the same phase.

I have also heard (not tried yet) that modern DSOs with universal switching PSUs can work from DC, and it may not even take a lot of it (tens of volts). Perhaps a battery bank is all you need. I imagine that features like "AC line" triggering may not work...

I think you misunderstood the intention of the post. However you make a valid point about features like AC line Triggering possibly not working.  That was the purpose of the post to explore ideas like that.  If somebody knows a definitive answer to that it would be greatly appreciated.

[JJ_023]

Hello there,


I always see people get super paranoid about these kinds of measurements yet they are done every day without incident.

Years ago I used the method of cutting the ground pin off of the scopes line cord plug for literally 100's of measurements.  Many people I knew did the same thing, over and over and over and over and over again.  None of them died.  None of them even got shocked.  These were all CRT type scopes.

The key point here is you just have to pay attention to what you are doing when making ANY measurement of line voltages, 120vac or 230vac, or even 440vac.  It's not limited to scopes.  When you use a regular voltmeter, you have to pay attention to where your hands and feet are, as to what they are touching at all times.  It's either that, or pay someone to do the measurements for you.

These days battery powered scopes are somewhat cheap you can get one now for less than $30 that will do line frequency measurements with ease.  That would be the recommended method for today, but you still have to pay attention to what your hands and feet are touching when you do the measurements.
1.  Don't touch any metal connected to any wire.
2.  Don't stand on concrete or any electrically conductive floor, don't stand in water.

Do some reading on the web for how to make line voltage measurements and precautions, you should be ok.

I agree with everything that you are saying.  I measure voltages over 240 at least once a week and feel very comfortable doing so.  Since so many people misunderstood what I was trying to ascertain with this post, I will take the blame for not doing a better job of articulating the question.

The idea was can you take a mains powered oscilloscope and turn it into a battery-operated oscilloscope utilizing a UPS that is completely disconnected from the mains.  The byproduct would be a floated oscilloscope.  However that is not what I'm trying to figure out.  What I am trying to ascertain is how would it affect the operation of the oscilloscope and the measurements taken by it if it is utilized in this manner.  The application doesn't even necessarily have to do with high-voltage.  Maybe you need to check something on an automobile or some other circuit that doesn't have mains power available. 

[JJ_023]

Back in the day of all metal CRO cases and front panels, everybody with the slightest trace of common sense knew that floating a scope was high risk, especially if there were any grounds nearby and a risk of the user touching both.   Also this was before ELCBs/RCDs were in common use so no 'magic' circuit breaker was there to save your misguided ass.

The major hidden hazard with floating a modern plastic cased DSO, is you have absolutely no idea of the creepage and clearance distances between the touchable surfaces of the operating controls and the chassis ground plane of the front panel PCB(s), or other chassis referenced circuits.  To a casual eye, it looks 'safe' with insulating plastic knobs and buttons, but a little conductive dirt buildup on the side of a button or in a knob grubscrew hole could easily be a serious shock hazard.   Then there's the issue of how to handle probes safely when they were never intended to provide more than basic insulation on their grounded parts you've now floated.

Many of the major scope manufacturers have specific technical briefs advising that instruments not designed for floating operation should never be floated, so why are we second guessing them here in a beginners subforum?

Not only do I agree with what you said but I can personally confirm having numerous experiences where dust particles containing metal ( industrial environment) have gotten into places you would never think and either made certain parts of the equipment malfunction or completely shut down the whole machine.

You misunderstood the post. 

The idea was can you take a mains powered oscilloscope and turn it into a battery-operated oscilloscope utilizing a UPS that is completely disconnected from the mains.  The byproduct would be a floated oscilloscope. 

However that is not what I'm trying to figure out.  What I am trying to ascertain is how would it affect the operation of the oscilloscope and the measurements taken by it if it is utilized in this manner.  The application doesn't even necessarily have to do with high-voltage.  Maybe you need to check something on an automobile or some other circuit that doesn't have mains power available.

[JJ_023]

Many of the major scope manufacturers have specific technical briefs advising that instruments not designed for floating operation should never be floated, so why are we second guessing them here in a beginners subforum?

Moreover, much of the equipment that one would probe in such way has specific warnings about hazardous voltages and no user servicable parts inside...

I would disagree with that statement. If there were no serviceable parts then things that got broken would never get fixed. 

[JJ_023]

Bonjour, bravo to our friends at R&S for the good advise.....

.....my "official" recommendation will be "don't float the scope".

Since 1970s, in HV, SMPS, electronic ballast, medical and avionics design, debug and mfg, we use  mains isolation transformers like Signal DU-1, DU-5 to float the DUT/PSU NOT  the scope!!!

Thus no need for diff probes, or the other kluges mentioned.

Jon

Thank you for the model suggestions and the advice.  What I really am looking for as far as input is was to explore the following idea.  Which I unfortunately did not articulate well enough.

The idea was can you take a mains powered oscilloscope and turn it into a battery-operated oscilloscope utilizing a UPS that is completely disconnected from the mains.  The byproduct would be a floated oscilloscope. 

However that is not what I'm trying to figure out.  What I am trying to ascertain is how would it affect the operation of the oscilloscope and the measurements taken by it if it is utilized in this manner.  The application doesn't even necessarily have to do with high-voltage.  Maybe you need to check something on an automobile or some other circuit that doesn't have mains power available.

[magic]

I doubt there would be serious problems running a scope from a UPS, particularly any modern one with switching PSU.

Line trigger will trigger off the UPS output rather than true mains, obviously.

You may not even need a UPS, just enough batteries.

If the UPS is disconnected and doesn't pass mains earth then you are floating the scope, and so the thread has turned into another typical "omg somebody is floating a scope" thread.

On a car you generally don't have the high voltage danger issue, although you could connect grounding clips to spark plug cables for a little fun

[CaptDon]

Worst idea ever!!! Due to the line filter built into the power input of the scope the scope's probe ground will be capacitively coupled and balanced at about 60 v.a.c. above earth ground. A sure way to blow up SMPS units under test or almost anything else the O.P. is working on. Isolation transformer connected to U.U.T. is best answer. Isolation transformers are cheap enough when balanced against the cost of human life or blown up items under test. Isolation transformers can also be easily and cheaply made from a pair of back to back control transformers of proper VA rating.
E.G. 120 to 24 driving a 24 to 120. Don't cut the ground lug and don't use those adapters, just bad unprofessional work ethic.

[JJ_023]

I doubt there would be serious problems running a scope from a UPS, particularly any modern one with switching PSU.

Line trigger will trigger off the UPS output rather than true mains, obviously.

You may not even need a UPS, just enough batteries.

If the UPS is disconnected and doesn't pass mains earth then you are floating the scope, and so the thread has turned into another typical "omg somebody is floating a scope" thread.

On a car you generally don't have the high voltage danger issue, although you could connect grounding clips to spark plug cables for a little fun

Thank you. This is similar to what I was looking for.

I understand your fun suggestion but no grounding the spark plug cables for me. 

Checking the ignition coils can be fun if you do it the proper way. 

[JJ_023]

Worst idea ever!!! Due to the line filter built into the power input of the scope the scope's probe ground will be capacitively coupled and balanced at about 60 v.a.c. above earth ground. A sure way to blow up SMPS units under test or almost anything else the O.P. is working on. Isolation transformer connected to U.U.T. is best answer. Isolation transformers are cheap enough when balanced against the cost of human life or blown up items under test. Isolation transformers can also be easily and cheaply made from a pair of back to back control transformers of proper VA rating.
E.G. 120 to 24 driving a 24 to 120. Don't cut the ground lug and don't use those adapters, just bad unprofessional work ethic.

Thank you for the warnings.  I think you misunderstood the intent of the post.  It is not to create a floating oscilloscope.

The idea was can you take a mains powered oscilloscope and turn it into a battery-operated oscilloscope utilizing a UPS that is completely disconnected from the mains.  The byproduct would be a floated oscilloscope.

However that is not what I'm trying to figure out.  What I am trying to ascertain is how would it affect the operation of the oscilloscope and the measurements taken by it if it is utilized in this manner.  The application doesn't even necessarily have to do with high-voltage.  Maybe you need to check something on an automobile or some other circuit that doesn't have mains power available.

[Thunderer]

Under no circumstances, nor new ways of measuring with a scope, the scope should ever be floating. The scope should be solidly grounded, for your own protection, and that of your gear. If you want to float something, it should be the object you want to measure.

Second, the fact that some portable scopes are floating by construction is in line with their construction: if you use the provided probes, you cannot touch the metallic part of the probe. Which is not the case with the bench scopes that we try to make them floating but cutting the ground or powering from an UPS/generator.

STOP inventing dangerous ways to kill people. The argument that you will pay attention on what you do, that you are using the safest methods and/or approaches, again STOP! You do not know who reads your posts and what level of understanding the risks they have.

This kind of topics are like the ones explaining how to do a bomb, but says: do not do it at home! Well, someone will do it, and Lady Luck may not be in the neighborhood.

[JJ_023]

Under no circumstances, nor new ways of measuring with a scope, the scope should ever be floating. The scope should be solidly grounded, for your own protection, and that of your gear. If you want to float something, it should be the object you want to measure.

Second, the fact that some portable scopes are floating by construction is in line with their construction: if you use the provided probes, you cannot touch the metallic part of the probe. Which is not the case with the bench scopes that we try to make them floating but cutting the ground or powering from an UPS/generator.

STOP inventing dangerous ways to kill people. The argument that you will pay attention on what you do, that you are using the safest methods and/or approaches, again STOP! You do not know who reads your posts and what level of understanding the risks they have.

This kind of topics are like the ones explaining how to do a bomb, but says: do not do it at home! Well, someone will do it, and Lady Luck may not be in the neighborhood.

I appreciate you taking the time to respond as well as your concerns. 

I also think it's ridiculous that you think ideas can't be explored such as this 1 because someone may not understand them and therefore be in harm's way. 

I totally get the idea that people can misinterpret, it's clear that you misinterpreted this particular post, but that is a poor excuse not to discuss concept and ideas to further educate and explore concepts. IMHO

[Thunderer]

Many people I knew did the same thing, over and over and over and over and over again.  None of them died.  None of them even got shocked. 

The fact that you did not know/see something, it does not mean it does not exist.

I am pretty sure there are things you never saw (in real life), and yet you believe they exist when you see a photo or a text mentioning them. So why your thinking is so selective?

Again, some people can live while doing properly some dangerous jobs. Yet, by your text, you think anyone can do those dangerous things, while you have no idea what background they have. Just follow some rules... until we make a mistake or decide a shortcut is worthed.

[Thunderer]

I also think it's ridiculous that you think ideas can't be explored such as this 1 because someone may not understand them and therefore be in harm's way. 

Stay safe, buddy! I do safety for a living, that's why I posted what I posted. Clearly, you did not get my message. Again, stay safe!

PS: You asked about concerns, what answers were you expected if the safety disturbs you?!

[JJ_023]

Many people I knew did the same thing, over and over and over and over and over again.  None of them died.  None of them even got shocked. 

The fact that you did not know/see something, it does not mean it does not exist.

I am pretty sure there are things you never saw (in real life), and yet you believe they exist when you see a photo or a text mentioning them. So why your thinking is so selective?

Again, some people can live while doing properly some dangerous jobs. Yet, by your text, you think anyone can do those dangerous things, while you have no idea what background they have. Just follow some rules... until we make a mistake or decide a shortcut is worthed.

You're making a point based on someone else's post which doesn't even tie into the main topic of this post. 

Based on your line of thinking you are doing the same exact thing.  You are assuming without knowing.  You don't know what's going to be measured.  It could be an LED connected to a resistor connected to a 5 V battery pack.  Please stay on topic.

[JJ_023]

I also think it's ridiculous that you think ideas can't be explored such as this 1 because someone may not understand them and therefore be in harm's way. 

Stay safe, buddy! I do safety for a living, that's why I posted what I posted. Clearly, you did not get my message. Again, stay safe!

PS: You asked about concerns, what answers were you expected if the safety disturbs you?!

  I appreciate you taking the time.  You make legitimate points, however they are completely off topic.  The concerns are not with safety. 

[Thunderer]

You're making a point based on someone else's post which doesn't even tie into the main topic of this post. 

Based on your line of thinking you are doing the same exact thing.  You are assuming without knowing.  You don't know what's going to be measured.  It could be an LED connected to a resistor connected to a 5 V battery pack.  Please stay on topic.

Buddy, why don't you exactly explain why you'd need separation (floating) if it is not to measure HV signals? You see that most posts are about safety. The answers are as good as the question is.

PS: If you need to measure floating signals, do as I did: build yourself a differential probe (it works even in HV cases if you do it properly). I can help you (with knowledge) to build one. You can buy one too, no obligation to build one.

I will assume something now, and it is based on thousands of similar cases found on forums: one beginner, with not even the basics in possession, asks a question on a forum (or more, the same question). The question is not asked to get real insight on the issue he/she is asking about. The question is asked to get his/her assumption the validation that he/she knew it. That way, the person is highly irritated when the answers do not validate his/her expectations. Most of the time, the person will end up saying that nobody understand him/her, or that nobody knows the answer. The same will repeat on other forums. Bottom line, most people on forums are offtopic or simply dumb, unless they validate his/her expectations.

End of the story. Stay safe, you may need it more than you believe it!

[magic]

Buddy, why don't you exactly explain why you'd need separation (floating) if it is not to measure HV signals?

Already did several times, including to you directly.
Now, why can't you read the thread you are responding to?

[bdunham7]

The idea was can you take a mains powered oscilloscope and turn it into a battery-operated oscilloscope utilizing a UPS that is completely disconnected from the mains.  The byproduct would be a floated oscilloscope.  However that is not what I'm trying to figure out.  What I am trying to ascertain is how would it affect the operation of the oscilloscope and the measurements taken by it if it is utilized in this manner.  The application doesn't even necessarily have to do with high-voltage.  Maybe you need to check something on an automobile or some other circuit that doesn't have mains power available.

Of course what you propose will "work", as long as the UPS can be cold-started or you have it connected to line power and then unplug it.  The issues you will run into are probably mostly related to noise, as the inputs are not differential and not designed to have a significant AC voltage on the ground.  The effect of this will vary a lot with frequency and so forth.  At low frequencies and moderate voltages, such as you might find in many basic automotive circuits, this may work just fine.  At RF frequencies or very high voltages, these issues may be more of a problem.  If you're trying to look at high-side gate drive signals on an SMPS, the combined capacitive loading of your scope/UPS combo may be an issue as well.  And, of course, if you try to float the combo up too high you'll have corona discharge issues.  Depending on the exact model of scope, both pure-sine and modified-sine UPS versions will likely work, but the modified-sine may have additional noise issues. 

[JJ_023]

Buddy, why don't you exactly explain why you'd need separation (floating) if it is not to measure HV signals? You see that most posts are about safety. The answers are as good as the question is.

Again I never stated I needed floating to measure high-voltage signals.  You misinterpreted the whole purpose of this post.  I don't doubt you have a lot of valuable knowledge to share but you can't share it unless you take the time to understand the question that is being asked.

PS: If you need to measure floating signals, do as I did: build yourself a differential probe (it works even in HV cases if you do it properly). I can help you (with knowledge) to build one. You can buy one too, no obligation to build one.

Despite us not understanding one another on this particular subject I would gladly welcome your help or anybody's when it comes down to building a differential probe.  So if that time arises you can expect a PM from me.  Thank you for the offer, hopefully it will still be valid.

I will assume something now, and it is based on thousands of similar cases found on forums: one beginner, with not even the basics in possession, asks a question on a forum (or more, the same question). The question is not asked to get real insight on the issue he/she is asking about. The question is asked to get his/her assumption the validation that he/she knew it. That way, the person is highly irritated when the answers do not validate his/her expectations. Most of the time, the person will end up saying that nobody understand him/her, or that nobody knows the answer. The same will repeat on other forums. Bottom line, most people on forums are offtopic or simply dumb, unless they validate his/her expectations.

End of the story. Stay safe, you may need it more than you believe it!

This is really ironic.  Because this is exactly what you are doing in this regard, despite the fact that you claim you have a lot of experience.  Maybe my original question was not articulated properly enough but I have made several attempts to correct what I am seeking to ascertain as far as knowledge. 

I have zero qualms about looking ignorant or stupid when I ask a question provided I learn and educate myself in the process.  I will at times ask for validation of an idea I may have but I will switch my position in a heartbeat if I am educated on why my position is invalid, and thank somebody for pointing out my faulty thinking.

[tggzzz]

Many people I knew did the same thing, over and over and over and over and over again.  None of them died.  None of them even got shocked. 

The fact that you did not know/see something, it does not mean it does not exist.

I am pretty sure there are things you never saw (in real life), and yet you believe they exist when you see a photo or a text mentioning them. So why your thinking is so selective?

Again, some people can live while doing properly some dangerous jobs. Yet, by your text, you think anyone can do those dangerous things, while you have no idea what background they have. Just follow some rules... until we make a mistake or decide a shortcut is worthed.

Exactly. I wonder if he will take note now that two people have independently made the same point. Somehow I doubt it.

I also wonder why he mentioned "These were all CRT type scopes" as if the display technology has any relevance. Mind you, that kind of comment is useful for attentive readers: they might presume that such a misunderstanding is a hint that he misunderstands other more fundamental safety topics.

[JJ_023]

The idea was can you take a mains powered oscilloscope and turn it into a battery-operated oscilloscope utilizing a UPS that is completely disconnected from the mains.  The byproduct would be a floated oscilloscope.  However that is not what I'm trying to figure out.  What I am trying to ascertain is how would it affect the operation of the oscilloscope and the measurements taken by it if it is utilized in this manner.  The application doesn't even necessarily have to do with high-voltage.  Maybe you need to check something on an automobile or some other circuit that doesn't have mains power available.

Of course what you propose will "work", as long as the UPS can be cold-started or you have it connected to line power and then unplug it.  The issues you will run into are probably mostly related to noise, as the inputs are not differential and not designed to have a significant AC voltage on the ground.  The effect of this will vary a lot with frequency and so forth.  At low frequencies and moderate voltages, such as you might find in many basic automotive circuits, this may work just fine.  At RF frequencies or very high voltages, these issues may be more of a problem.  If you're trying to look at high-side gate drive signals on an SMPS, the combined capacitive loading of your scope/UPS combo may be an issue as well.  And, of course, if you try to float the combo up too high you'll have corona discharge issues.  Depending on the exact model of scope, both pure-sine and modified-sine UPS versions will likely work, but the modified-sine may have additional noise issues.

This is along the lines of what I am looking for.  Let's assume the UPS can be cold started (I actually don't know of any that can't).

I was thinking that it may induce noise from the inside circuitry but not at low frequencies. 

However, how would the oscilloscope differentiate between mains and a UPS, the unit primarily runs on DC inside?

The other thought of reasoning is.  When people have floated a scope by disabling the ground (for whatever reason they do it) are they subjugated to the same type of noise?

I understand your frequency comments.  As well as high-frequency when getting into the high-voltage space.

As far as your SMPS comment.  My question is.  Would this apply to a battery-powered oscilloscope as well?  Most battery-powered oscilloscopes are similar to desktop versions in the regard that the inputs all share a common ground.

Thanks for your insight.  It's just when you answer some questions new ones develop for me.

[bdunham7]

I was thinking that it may induce noise from the inside circuitry but not at low frequencies. 

However, how would the oscilloscope differentiate between mains and a UPS, the unit primarily runs on DC inside?

Sure, even a so-called low frequency inverter that uses a large transformer still typically has a HF switching primary so there may be some 50kHz or so noise in there.  The difference is that when connected normally and grounded, the ground components of the scope are at 0V and not changing, thus there is no electric field at those points.  Floating and applying a changing voltage to the scopes ground points changes that.

When people have floated a scope by disabling the ground (for whatever reason they do it) are they subjugated to the same type of noise?

As far as your SMPS comment.  My question is.  Would this apply to a battery-powered oscilloscope as well?  Most battery-powered oscilloscopes are similar to desktop versions in the regard that the inputs all share a common ground.

Floating a scope by just breaking the ground has widely varying results, some of which depend on the nature of the scope's PSU and how it is internally coupled to the ground point.  As far as the performance of a cheap scope that just happens to be powered by batteries, IDK--probably crap.  Good ones that I'm familiar with--Fluke Scopemeters and Tektronix TPS scopes--take this into account and perform fairly well, although the TPS and the newer, better Scopemeters actually do not have common grounds for the inputs. 

[JJ_023]

Sure, even a so-called low frequency inverter that uses a large transformer still typically has a HF switching primary so there may be some 50kHz or so noise in there.  The difference is that when connected normally and grounded, the ground components of the scope are at 0V and not changing, thus there is no electric field at those points.  Floating and applying a changing voltage to the scopes ground points changes that.

This makes perfect sense to me.  Thanks for the concise explanation.

Floating a scope by just breaking the ground has widely varying results, some of which depend on the nature of the scope's PSU and how it is internally coupled to the ground point.  As far as the performance of a cheap scope that just happens to be powered by batteries, IDK--probably crap.  Good ones that I'm familiar with--Fluke Scopemeters and Tektronix TPS scopes--take this into account and perform fairly well, although the TPS and the newer, better Scopemeters actually do not have common grounds for the inputs.

I’ve actually used a fluke handheld.  They do perform well.  I guess a better question would be how are they accomplishing this if they are truly different from a mains powered oscilloscope running essentially on a battery?

I know some of the better scopes do not have common grounds but that is usually not on a common case occurrence from what I understand.  Is this a correct assumption?

Again thanks for explaining what may seem rudimentary to you.

[Vovk_Z]

why are we second guessing them here in a beginners subforum?

So, the only logical answer to this and all other similar questions here in Beginners subforum is 'No'.
And logical answer to the same question given in other than Beginners subforums is 'Yes'.

[tggzzz]

The only logical answer is to learn from the experts that base their recommendations on solid engineering theory and practice.

In this case, suitable starting points are Tektronix'
"Fundamentals of Floating Measurements and Isolated Input Oscilloscopes"
https://www.tek.com/en/documents/application-note/fundamentals-floating-measurements-and-isolated-input-oscilloscopes
and
"Floating Oscilloscope Measurements … And Operator Protection"
http://www.tek.com/dl/51W_10640_1.pdf
both of which discuss various techniques.

For other starting points, see the "Praxis and Safety" refs at https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/

[magic]

When people have floated a scope by disabling the ground (for whatever reason they do it) are they subjugated to the same type of noise?

As far as your SMPS comment.  My question is.  Would this apply to a battery-powered oscilloscope as well?  Most battery-powered oscilloscopes are similar to desktop versions in the regard that the inputs all share a common ground.

Floating a scope by just breaking the ground has widely varying results, some of which depend on the nature of the scope's PSU and how it is internally coupled to the ground point.  As far as the performance of a cheap scope that just happens to be powered by batteries, IDK--probably crap.  Good ones that I'm familiar with--Fluke Scopemeters and Tektronix TPS scopes--take this into account and perform fairly well, although the TPS and the newer, better Scopemeters actually do not have common grounds for the inputs.

It depends a lot on how the scope is grounded to the DUT. You never truly float a scope (leave its ground completely unconnected) - it's always connected to the DUT somehow, either with alligator clips at the probes or through mains earth cabling if you omit those and both devices are earthed. Otherwise the two devices have no common point of reference and the waveform has unknown DC offset and may jump wildly with any picked up noise.

The most problematic case is when people try to use a floating scope as a poor man's differential oscilloscope: connect its ground to some sensitive and/or high impedance node in the DUT in order to take measurements with respect to that node rather than ground. In such case noise current coupled into oscilloscope ground through its transformer causes the node's voltage to fluctuate and DUT operation may be disrupted and/or other non-disrupted signals will appear to fluctuate (with respect to the "weak" reference node) even though it's the reference node which fluctuates. Such "measurement" may show problems which simply don't exist or didn't exist before the oscilloscope was connected to the device.

OTOH, grounding the scope to some ground, quasi-ground or power supply rail tends to work much better because such nodes are able to absorb scope's ground noise. I suspect that the most common real world use case for floating scopes (and the reason why safety nuts are going nuts) is grounding the scope to a "hot ground", i.e. the negative of rectified mains. I have done it once or twice when repairing SMPS and observed no unusual behavior, it simply worked as if the hot ground were ordinary ground.

UPS powered scope in a car may be uncharted territory so far, but based on the above I think it has a decent chance to work if you don't try to be clever and ground the scope and the UPS to the car's chassis. (You could also earth the whole car to some available metal structure).

[Thunderer]

Despite us not understanding one another on this particular subject I would gladly welcome your help or anybody's when it comes down to building a differential probe.  So if that time arises you can expect a PM from me.  Thank you for the offer, hopefully it will still be valid.

Of course, I may even ship you a PCB for the probe I built.

[JJ_023]

The only logical answer is to learn from the experts that base their recommendations on solid engineering theory and practice.

In this case, suitable starting points are Tektronix'
"Fundamentals of Floating Measurements and Isolated Input Oscilloscopes"
https://www.tek.com/en/documents/application-note/fundamentals-floating-measurements-and-isolated-input-oscilloscopes
and
"Floating Oscilloscope Measurements … And Operator Protection"
http://www.tek.com/dl/51W_10640_1.pdf
both of which discuss various techniques.

For other starting points, see the "Praxis and Safety" refs at https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/

Thank you for sharing these links and information it is greatly appreciated and has contributed to my knowledge base.  I have read both links as well as 90% of the links on "Scope Probe Reference Material".  I will finish the rest within the next day or so.

[JJ_023]

It depends a lot on how the scope is grounded to the DUT. You never truly float a scope (leave its ground completely unconnected) - it's always connected to the DUT somehow, either with alligator clips at the probes or through mains earth cabling if you omit those and both devices are earthed. Otherwise the two devices have no common point of reference and the waveform has unknown DC offset and may jump wildly with any picked up noise.

Is this a typo ?  if you omit those and both devices are earthed

I do get the gist of this and it makes sense to me.

The most problematic case is when people try to use a floating scope as a poor man's differential oscilloscope: connect its ground to some sensitive and/or high impedance node in the DUT in order to take measurements with respect to that node rather than ground. In such case noise current coupled into oscilloscope ground through its transformer causes the node's voltage to fluctuate and DUT operation may be disrupted and/or other non-disrupted signals will appear to fluctuate (with respect to the "weak" reference node) even though it's the reference node which fluctuates. Such "measurement" may show problems which simply don't exist or didn't exist before the oscilloscope was connected to the device.

Thank you for expanding on this it makes sense.

OTOH, grounding the scope to some ground, quasi-ground or power supply rail tends to work much better because such nodes are able to absorb scope's ground noise. I suspect that the most common real world use case for floating scopes (and the reason why safety nuts are going nuts) is grounding the scope to a "hot ground", i.e. the negative of rectified mains. I have done it once or twice when repairing SMPS and observed no unusual behavior, it simply worked as if the hot ground were ordinary ground.

I can definitely see the dangers associated with the negative of rectified mains.  What would be the purpose of taking such a measurement with an oscilloscope when you can take one with a DMM?

I have done this countless times with a DMM and never had a need for anything more.

UPS powered scope in a car may be uncharted territory so far, but based on the above I think it has a decent chance to work if you don't try to be clever and ground the scope and the UPS to the car's chassis. (You could also earth the whole car to some available metal structure).

1st of all that was just a possible application given as a possible scenario.  In that particular situation both the UPS and oscilloscope would be outside the car.   Also since the scope is going to attach to a grounding point on the car I am not sure I understand your point. 

Could you possibly elaborate?

Thank you for your lengthy response.

[tggzzz]

The only logical answer is to learn from the experts that base their recommendations on solid engineering theory and practice.

In this case, suitable starting points are Tektronix'
"Fundamentals of Floating Measurements and Isolated Input Oscilloscopes"
https://www.tek.com/en/documents/application-note/fundamentals-floating-measurements-and-isolated-input-oscilloscopes
and
"Floating Oscilloscope Measurements … And Operator Protection"
http://www.tek.com/dl/51W_10640_1.pdf
both of which discuss various techniques.

For other starting points, see the "Praxis and Safety" refs at https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/

Thank you for sharing these links and information it is greatly appreciated and has contributed to my knowledge base.  I have read both links as well as 90% of the links on "Scope Probe Reference Material".  I will finish the rest within the next day or so.

You are welcome; it is always a pleasure to provide pointers to someone that is prepared to think and learn.

Don't forget that there is no such thing as "ground" or "earth"; they are merely simplifications that have some utility in some circumstances, but are misleading in others.

Don't forget that capacitors, resistors, and inductors don't really exist in a practical or theoretical sense.
Not practical, since every resistor you buy also comes with "free" capacitance and inductance.
Not theoretical, since the the R,L,C (and earth/ground) concepts are simplifications of the way electromagnetic fields interact. Very useful simplifications in many circumstances, but they are of diminishing use at microwave frequencies and when trying to model EMI/EMC effects in terms of "parasitic" R,L,C.

The latter is of particular importance when considering "floating" components, since interactions will occur even when there is no direct connection.

For example, I remember touching the front flap of a Eurocard rack, and getting a slight transient electric shock. The engineer and technician correctly pointed out that the flap was insulated (plastic hinges and catch), and so it couldn't give me a shock (incorrect!). They were so confident of that they wouldn't touch it themselves. Basically, since it was electrically isolated, it was floating at ~120V, halfway between live and neutral.

[JJ_023]

Despite us not understanding one another on this particular subject I would gladly welcome your help or anybody's when it comes down to building a differential probe.  So if that time arises you can expect a PM from me.  Thank you for the offer, hopefully it will still be valid.

Of course, I may even ship you a PCB for the probe I built.

That would be great.  Thank you.  If you have a Gerber file that would probably be easier.  What I would really be interested in is a detailed explanation of how your circuit works and hopefully answers to whatever questions that generates. 

To be honest before I attempted that I would probably do a modification to a current MicSig DP10007 HV Differential Probe that is outlined in the following post.

https://www.eevblog.com/forum/testgear/new-eevblog-hv-probe/150/

[JJ_023]

You are welcome; it is always a pleasure to provide pointers to someone that is prepared to think and learn.

All I can say is I love to learn I do it professionally and as a hobby.   If you are willing to provide pointers then I am willing to listen and ask questions. 

Don't forget that there is no such thing as "ground" or "earth"; they are merely simplifications that have some utility in some circumstances, but are misleading in others.

To me ground is a reference point.  Having said that.  What would be some examples that are very misleading?

Don't forget that capacitors, resistors, and inductors don't really exist in a practical or theoretical sense.
Not practical, since every resistor you buy also comes with "free" capacitance and inductance.

This I am aware of.  Resistors capacitors and inductors all have inductance capacitance and resistance each on their own.
 

Not theoretical, since the the R,L,C (and earth/ground) concepts are simplifications of the way electromagnetic fields interact. Very useful simplifications in many circumstances, but they are of diminishing use at microwave frequencies and when trying to model EMI/EMC effects in terms of "parasitic" R,L,C.

I have very little experience and very modest understanding of microwave frequencies.  This is something I would probably venture into further down the road.

Right now my main concern is getting an understanding of analog, especially towards power supply design concepts.  Right now I was reading a bunch of stuff on current sensing using shunts and DAC’s.  And just a better understanding of op amps overall.  When I have a bit of time I will post on that subject asking for clarification.

The latter is of particular importance when considering "floating" components, since interactions will occur even when there is no direct connection.

For example, I remember touching the front flap of a Eurocard rack, and getting a slight transient electric shock. The engineer and technician correctly pointed out that the flap was insulated (plastic hinges and catch), and so it couldn't give me a shock (incorrect!). They were so confident of that they wouldn't touch it themselves. Basically, since it was electrically isolated, it was floating at ~120V, halfway between live and neutral.

I totally understand where you are coming from.  Having done a considerable amount of Heliarc welding which uses high-frequency starts I have seen and experienced unexpected events. 

One of which baffles me to today.  I got shocked by the high-frequency with the ground clamp lying on a concrete floor nowhere near the weldment.  Wearing thick boots and full PPE with no perspiration. 

Still not sure how that happened and it has happened more than once on different machines.

[Smokey]

...
But if one is determined to "float the scope,", a (good) isolation transformer is the best choice.  You also want to make sure there are no LAN, USB, etc. cables connected to your scope.

Ethernet should be fully galvanically isolated with transformers.  USB is float-breaker though, ya.

[johansen]

fairly certain my old TEK 475 oscope survived being floated at +/- 170vdc at 120KHZ.. no problem, with a cut ground pin.

doesn't mean the waveform was accurate, due to the resistance, inductance, and coupling of the 10 foot long oscope leads... but, it didn't blow up anything.

[magic]

Is this a typo ?  if you omit those and both devices are earthed

I meant: if you omit grounding clips but the DUT and the scope are both earthed, then the scope is still grounded to DUT ground through mains earth cables.

I can definitely see the dangers associated with the negative of rectified mains.  What would be the purpose of taking such a measurement with an oscilloscope when you can take one with a DMM?

Observing switching waveforms, diagnosing repetitive shutdown issues, monitoring ripple on primary controller's low voltage power supply.

Usually DMM checks are enough to find what's wrong with a simple SMPS and often a few part swaps do the job without further damage, but I wanted to be particularly pedantic about verifying everything when working on more complex or valuable PSUs.

1st of all that was just a possible application given as a possible scenario.  In that particular situation both the UPS and oscilloscope would be outside the car.   Also since the scope is going to attach to a grounding point on the car I am not sure I understand your point.

My point is that no one is known to have tried, but if the scope is grounded to the car (rather than used as a poor man's differential probe*) then the result hopefully won't be too bad.

It must be said that both the scope and the car have some capacitance to earth so they aren't truly floating. For example, if there is high frequency noise between the UPS output and its chassis, the chassis is effectively earthed capacitively and noise can be pushed out through the output cable. Then coupled to scope ground, which is the car's ground. All circuits in the car are powered by its battery/alternator and referenced to the car's ground and they might be expected to track the noise too, but they may not if they have their own capacitance to earth and only a high impedance connection with the rest of the car. So it's possible that UPS switching noise could be found all of a sudden on some unshielded analog sensor signal, for instance.

* The thing about diff probes is that they have high and equal input impedance on both IN+ and IN- and they are designed to have no noise sources on IN-. A floated scope is obviously not a diff probe - IN- has high capacitance to ground and possibility of PSU noise.

[tggzzz]

fairly certain my old TEK 475 oscope survived being floated at +/- 170vdc at 120KHZ.. no problem, with a cut ground pin.

doesn't mean the waveform was accurate, due to the resistance, inductance, and coupling of the 10 foot long oscope leads... but, it didn't blow up anything.

I'm fairly certain I've walked into the road without looking... no problem.

With your 475, what were the voltages across the transformer windings and "nearby" PSU components. Were they rated for that voltage? Were they subtly damaged if the rating was exceeded?

[tggzzz]

You are welcome; it is always a pleasure to provide pointers to someone that is prepared to think and learn.

All I can say is I love to learn I do it professionally and as a hobby.   If you are willing to provide pointers then I am willing to listen and ask questions. 

Don't forget that there is no such thing as "ground" or "earth"; they are merely simplifications that have some utility in some circumstances, but are misleading in others.

To me ground is a reference point.  Having said that.  What would be some examples that are very misleading?

Two grounds/earths might not be at the same potential.

Consider a transmission line where the speed of light means that one end could change potential and it will take a noticeable time for that to reach the other end. N.B. a metre is 5ns, i.e. >> than modern logic transition times

Consider two pieces of industrial equipment, both grounded but to different phases or via 100m long cabling. Any resistance plus current => voltage difference.

Consider the advice as to what to do if caught in a field in a lightning storm: keep your feet together. That's because if there is a nearby lightning strike you don't want the electric currents in the earth to "decide" that your legs are a low resistance.

Consider the "ground" on an aircraft which has been flying through dust/clouds and picked up a static charge relative to the runway.

Of course not all those are relevant to you, but they do serve to make it clear that not all ground/earth points are at the same potential.

I have very little experience and very modest understanding of microwave frequencies.  This is something I would probably venture into further down the road.

Sensible decision

If you do, you will come to realise that the electric power is transmitted through the dielectrics (in electromagnetic fields), not along the conductors (by electrons) Yes Virginia, you aren't in Kansas any more (Oops, mixed cultural references )

I totally understand where you are coming from.  Having done a considerable amount of Heliarc welding which uses high-frequency starts I have seen and experienced unexpected events. 

One of which baffles me to today.  I got shocked by the high-frequency with the ground clamp lying on a concrete floor nowhere near the weldment.  Wearing thick boots and full PPE with no perspiration. 

Still not sure how that happened and it has happened more than once on different machines.

Realising what you don't know is the key starting point for improvement The older you get and the more experienced you become, the more you realise that you don't know very much.

Those that close their eyes to what they might not know can be dangerous.

Have fun, safely.

[MrAl]

I always see people get super paranoid about these kinds of measurements yet they are done every day without incident.

Years ago I used the method of cutting the ground pin off of the scopes line cord plug for literally 100's of measurements.  Many people I knew did the same thing, over and over and over and over and over again.  None of them died.  None of them even got shocked.  These were all CRT type scopes.

The key point here is you just have to pay attention to what you are doing when ...

Exactly the same can be said about walking into the road without looking.

(And why did you bring up the display technology (CRT)? That is completely and utterly irrelevant)

Hello,

You have no idea what you are talking about.
How can I be so sure?
I already mentioned this but you, as usual, fail to understand.
I have worked with a LOT of tech's who ALL used scopes with the ground pin cut off, and NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.  Let me repeat that so MAYBE you will read it and understand this time.

NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
Not ever, over years and years of working with them in several different companies.  That's three, four, five companies.  Plus, we're talking about a LOT of technicians, and who were not reprimanded in any way by the upper management.  In fact, they were instructed to do it this way.
How can this be?  It be, period.

I am not trying to force anyone to do this, just stating some facts about this.  If a given reader does not like this idea, that's ok with me.  If you feel it is dangerous, then don't do it.  Don't like it?  Then don't do it.  But don't spread misinformation.

Also, mentioning a CRT scope may be irrelevant to you, but not to me.  You're welcome to talk about any kind of scope you like however, feel free.

Now if you want to go ahead and keep bitching about this, go right ahead, and I'll start to ignore what you have to say also.

[magic]

What I find most amusing is that the same guy talks about unpowered fixed-wing flying coffins.
Now, this is a dangerous activity which actually kills people in the real world

[xavier60]

This might not be useful to know. Some mains powered equipment that have a SMPS, will run on DC.
My HP 54645a DSO will run on 85VDC at 0.9A.
There is a chance that this could damage some equipment.

[Ian.M]

OSHA standard 1910.334 - Use of equipment, section (3) Grounding-type equipment specifically prohibits cutting the ground pin:

Attachment plugs and receptacles may not be connected or altered in a manner which would prevent proper continuity of the equipment grounding conductor at the point where plugs are attached to receptacles. ...

and later in the section, explicitly prohibits the use of adapters that interrupt the ground conductor.

Assuming this was in the USA, MrAl and his 'tech' friends exposed the company to potentially massive liability, and should all have received a formal written warning for the first offence of interrupting a grounding conductor, escalating to retraining and even dismissal for repeated offenses.

Even ignoring the fact that OSHA regulations are written in blood, i.e. are codified in response to serious or fatal industrial accidents, or serious negligent harm to employee health, to attempt to protect all employees in similar industries, so should be obeyed even if (and especially if) you do not understand the rationale behind them, the fines OSHA  would have imposed if even one disgruntled (ex) employee had reported cutting ground pins, + the disruption from the resulting surprise inspection to confirm the report, would almost certainly have exceeded the cost of providing isolated differential probes to all techs that actually needed to make floating scope measurements.

Cutting ground pins, like drunk driving, may once have been socially acceptable even though it was and is prohibited/illegal.  It is no longer acceptable, and any colleague or supervisor that proposes or performs it is an active danger to your personal safety.  If management wont listen and act, gather evidence and report it to OSHA or your equivalent national workplace safety authority.

[shapirus]

NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.

I remember somebody mentioning (in a similar thread a year or so ago) a case of somebody getting a lethal shock from an ungrounded scope. I don't remember exact details, but I think it happened when that person was alone, so apparently he touched something grounded and the potential present on the scope's housing or controls at the same time, since that's the only way you can get a shock in this context, and nobody was around to save him.

Yes, of course using the scope without grounding is dangerous. But the dangers will only stay a possibility and never cause actual harm, unless (or should I say "until"?) the operator, and those who stay close to the device, fail to observe a particular safety precaution. Similarly, the hand grenade with the pin removed is dangerous, but will not cause harm as long as you keep the lever pressed.

When it comes to employment, insurance, and employer's liability (wherever the latter two are a thing), it's easier to mandate that all the scopes in the company must be grounded at all times than to make sure no human errors are made (which is impossible anyway), and cheaper to simply buy differential probes for those areas of work where they are required.

The difference between the "strong no" and "why, possible, as long as you know what you're doing" positions, I guess, is the difference between the people who received education or training on this subject in countries where earthing in the electric networks has been widespread for a very long time, and the rest.

There are scopes which don't have the earth pin in their mains plugs at all. They work. I have one of them (an old soviet-made CRT scope).
However, I'm not positive that it doesn't have a dedicated wire and clip to be connected to mains earth *in addition* to the main plug, that has to be checked. I imagine that earthing would have been present at the work places at the factories for which those scopes were made in the first place, probably by way of earthing every metal part of the building's constructions and work places furniture etc., so maybe the scopes were supposed to be connected to that. I will check that some time later, as the scope isn't with me right now.

[pdenisowski]

Many of the major scope manufacturers have specific technical briefs advising that instruments not designed for floating operation should never be floated, so why are we second guessing them here in a beginners subforum?

I can tell you that all of our (R&S) internal training and documentation about floating and isolated measurements very strongly discourage floating the scope. 

Like, bold, capital letters, in red with lots of exclamation points

Sure, it can be done.  And I may or may not have done it myself on one or more occasions.  But an isolation transformer and/or differential probe is cheap insurance and doesn't make safety (as) dependent on operator skill.

[pdenisowski]

I’ve actually used a fluke handheld.  They do perform well.  I guess a better question would be how are they accomplishing this if they are truly different from a mains powered oscilloscope running essentially on a battery?

I know some of the better scopes do not have common grounds but that is usually not on a common case occurrence from what I understand.  Is this a correct assumption?

With the exception of truly handheld scopes (like the R&S RTH or similar models), the grounds of all scope channels are tied to each other and to instrument ground.  This is true for all of our benchtop scopes.

I'm not aware of a modern, non-handheld / battery powered scope that isn't constructed this way (all grounds connected together).

[pdenisowski]

The only logical answer is to learn from the experts that base their recommendations on solid engineering theory and practice.

In this case, suitable starting points are Tektronix'
"Fundamentals of Floating Measurements and Isolated Input Oscilloscopes"
https://www.tek.com/en/documents/application-note/fundamentals-floating-measurements-and-isolated-input-oscilloscopes

Although they are a competitor , I have to give a shout-out to Tek for also stating that floating the scope is "an unsafe and dangerous practice and should never be done!"

[tggzzz]

I always see people get super paranoid about these kinds of measurements yet they are done every day without incident.

Years ago I used the method of cutting the ground pin off of the scopes line cord plug for literally 100's of measurements.  Many people I knew did the same thing, over and over and over and over and over again.  None of them died.  None of them even got shocked.  These were all CRT type scopes.

The key point here is you just have to pay attention to what you are doing when ...

Exactly the same can be said about walking into the road without looking.

(And why did you bring up the display technology (CRT)? That is completely and utterly irrelevant)

Hello,

You have no idea what you are talking about.
How can I be so sure?
I already mentioned this but you, as usual, fail to understand.
I have worked with a LOT of tech's who ALL used scopes with the ground pin cut off, and NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.  Let me repeat that so MAYBE you will read it and understand this time.

NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
Not ever, over years and years of working with them in several different companies.  That's three, four, five companies.  Plus, we're talking about a LOT of technicians, and who were not reprimanded in any way by the upper management.  In fact, they were instructed to do it this way.
How can this be?  It be, period.

I am not trying to force anyone to do this, just stating some facts about this.  If a given reader does not like this idea, that's ok with me.  If you feel it is dangerous, then don't do it.  Don't like it?  Then don't do it.  But don't spread misinformation.

Some people have been killed. Your shouted assertions don't change that.

Here’s an anecdote about someone that "was WELL AWARE of the danger and took a lot of precautions to prevent shock . . . Bottom Line: He died anyway". https://groups.io/g/TekScopes/message/10754

Also, mentioning a CRT scope may be irrelevant to you, but not to me.  You're welcome to talk about any kind of scope you like however, feel free.

Why, exactly, is the display type relevant to the points being made? Repeated assertions are not a reason.

Now if you want to go ahead and keep bitching about this, go right ahead, and I'll start to ignore what you have to say also.

From your responses (i.e. not answers) you have already started that

[tggzzz]

I've just received a new-old toy[1], a THS720A handheld mains/battery scope with isolated inputs (Sorry, pdenisowski, not R&S )

At first glance the BNC connector looked surprisingly dirty, but the OP might be interested in the reason it isn't.



As you can see, the "outer shield" is plastic, not metal.

[1] no I don't need it. TEA syndrome.

[JJ_023]

Is this a typo ?  if you omit those and both devices are earthed

I meant: if you omit grounding clips but the DUT and the scope are both earthed, then the scope is still grounded to DUT ground through mains earth cables

That's what I thought was the implication, but I wanted to make sure.

I can definitely see the dangers associated with the negative of rectified mains.  What would be the purpose of taking such a measurement with an oscilloscope when you can take one with a DMM?

Observing switching waveforms, diagnosing repetitive shutdown issues, monitoring ripple on primary controller's low voltage power supply.

Usually DMM checks are enough to find what's wrong with a simple SMPS and often a few part swaps do the job without further damage, but I wanted to be particularly pedantic about verifying everything when working on more complex or valuable PSUs.

This makes a lot of sense thank you for the elaboration.  When you take such measurements what are some of the proportions that you take?  I assume you utilize a differential probe.

1st of all that was just a possible application given as a possible scenario.  In that particular situation both the UPS and oscilloscope would be outside the car.   Also since the scope is going to attach to a grounding point on the car I am not sure I understand your point.

My point is that no one is known to have tried, but if the scope is grounded to the car (rather than used as a poor man's differential probe*) then the result hopefully won't be too bad.

It must be said that both the scope and the car have some capacitance to earth so they aren't truly floating. For example, if there is high frequency noise between the UPS output and its chassis, the chassis is effectively earthed capacitively and noise can be pushed out through the output cable. Then coupled to scope ground, which is the car's ground. All circuits in the car are powered by its battery/alternator and referenced to the car's ground and they might be expected to track the noise too, but they may not if they have their own capacitance to earth and only a high impedance connection with the rest of the car. So it's possible that UPS switching noise could be found all of a sudden on some unshielded analog sensor signal, for instance.

Everything you said makes sense to me.  As I explore this particular topic further my understanding is that a lot of the battery powered scopes are very similar to mains powered scopes (I'm talking about reputable manufacturers).  Very few of them have isolated inputs. 

So if that is the case how is utilizing a mains oscilloscope under battery power from a UPS any different in terms of measurement capability under a particular scenario (let's say testing automobile circuits)?  I understand that there will be extra capacitance and other things to contend with.  Both scopes should have the same disadvantages, or am I missing something?

* The thing about diff probes is that they have high and equal input impedance on both IN+ and IN- and they are designed to have no noise sources on IN-. A floated scope is obviously not a diff probe - IN- has high capacitance to ground and possibility of PSU noise.

This I am aware of.  I've actually done a bit of reading on differential probes.  Thank you for pointing that out though.

[JJ_023]

fairly certain my old TEK 475 oscope survived being floated at +/- 170vdc at 120KHZ.. no problem, with a cut ground pin.

doesn't mean the waveform was accurate, due to the resistance, inductance, and coupling of the 10 foot long oscope leads... but, it didn't blow up anything.

I'm fairly certain I've walked into the road without looking... no problem.

With your 475, what were the voltages across the transformer windings and "nearby" PSU components. Were they rated for that voltage? Were they subtly damaged if the rating was exceeded?

From what I read from some of the suggested reading on floating a scope is you actually in danger the oscilloscope specifically with the transformer.  I too wondered what the thresholds would be for damage.  I guess like anything else it would depend on the design of the oscilloscope.

[JJ_023]

Two grounds/earths might not be at the same potential.

Consider a transmission line where the speed of light means that one end could change potential and it will take a noticeable time for that to reach the other end. N.B. a metre is 5ns, i.e. >> than modern logic transition times

Consider two pieces of industrial equipment, both grounded but to different phases or via 100m long cabling. Any resistance plus current => voltage difference.

Consider the advice as to what to do if caught in a field in a lightning storm: keep your feet together. That's because if there is a nearby lightning strike you don't want the electric currents in the earth to "decide" that your legs are a low resistance.

Consider the "ground" on an aircraft which has been flying through dust/clouds and picked up a static charge relative to the runway.

Of course not all those are relevant to you, but they do serve to make it clear that not all ground/earth points are at the same potential.

I understand all of the examples that you gave.  I am totally aware that 2 grounds can have different potential.  And as I stated before to me a ground is a reference point. So what am I missing or are we on the same page?

Realising what you don't know is the key starting point for improvement The older you get and the more experienced you become, the more you realise that you don't know very much.

Those that close their eyes to what they might not know can be dangerous.

Have fun, safely.

I will take that as a gentleman's way of saying you have no clue how I got shocked by th HF. lol

[JJ_023]

Also, mentioning a CRT scope may be irrelevant to you, but not to me.  You're welcome to talk about any kind of scope you like however, feel free.

From a neutral stance could you please elaborate on why the CRT scope would make a difference?  I am curious to see what the logic behind that is.

Thank you.

[JJ_023]

This might not be useful to know. Some mains powered equipment that have a SMPS, will run on DC.
My HP 54645a DSO will run on 85VDC at 0.9A.
There is a chance that this could damage some equipment.

To somebody more experienced this might make sense.  Could you please elaborate in more complete thoughts so that I could understand this better. 

Thank you.

[JJ_023]

  But an isolation transformer and/or differential probe is cheap insurance and doesn't make safety (as) dependent on operator skill.

In your opinion which would be better?  An isolation transformer on the DUT or Utilizing a differential probe (please don't say both). 

I have read most of the literature that was recommended from Tektronix on the subject matter.  I was wondering what your recommendation is since each has its advantages over the other.

Also in which case would one be preferred over the other?

[JJ_023]

I've just received a new-old toy[1], a THS720A handheld mains/battery scope with isolated inputs (Sorry, pdenisowski, not R&S )

At first glance the BNC connector looked surprisingly dirty, but the OP might be interested in the reason it isn't.



As you can see, the "outer shield" is plastic, not metal.

[1] no I don't need it. TEA syndrome.

I can tell it's plastic.  What do the probes look like if you do not mind?

[johansen]

fairly certain my old TEK 475 oscope survived being floated at +/- 170vdc at 120KHZ.. no problem, with a cut ground pin.

doesn't mean the waveform was accurate, due to the resistance, inductance, and coupling of the 10 foot long oscope leads... but, it didn't blow up anything.

I'm fairly certain I've walked into the road without looking... no problem.

With your 475, what were the voltages across the transformer windings and "nearby" PSU components. Were they rated for that voltage? Were they subtly damaged if the rating was exceeded?

Tek used a fairly high quality transformer.. with i think, a shield inside between primary and secondary.

i would expect them to work fine at even much higher voltages.

[tggzzz]

I've just received a new-old toy[1], a THS720A handheld mains/battery scope with isolated inputs (Sorry, pdenisowski, not R&S )

At first glance the BNC connector looked surprisingly dirty, but the OP might be interested in the reason it isn't.



As you can see, the "outer shield" is plastic, not metal.

[1] no I don't need it. TEA syndrome.

I can tell it's plastic.  What do the probes look like if you do not mind?

Metal bit at the blunt end is covered with plastic, so that small fingers would be needed to touch metal. I received P6117, but he normal type would be P5102

https://w140.com/tekwiki/wiki/File:Tek_p5102_2.jpg

[JJ_023]

fairly certain my old TEK 475 oscope survived being floated at +/- 170vdc at 120KHZ.. no problem, with a cut ground pin.

doesn't mean the waveform was accurate, due to the resistance, inductance, and coupling of the 10 foot long oscope leads... but, it didn't blow up anything.

I'm fairly certain I've walked into the road without looking... no problem.

With your 475, what were the voltages across the transformer windings and "nearby" PSU components. Were they rated for that voltage? Were they subtly damaged if the rating was exceeded?

Tek used a fairly high quality transformer.. with i think, a shield inside between primary and secondary.

i would expect them to work fine at even much higher voltages.

Up to what voltage would you be comfortable with and not worry about damaging the oscilloscope?

[johansen]

fairly certain my old TEK 475 oscope survived being floated at +/- 170vdc at 120KHZ.. no problem, with a cut ground pin.

doesn't mean the waveform was accurate, due to the resistance, inductance, and coupling of the 10 foot long oscope leads... but, it didn't blow up anything.

I'm fairly certain I've walked into the road without looking... no problem.

With your 475, what were the voltages across the transformer windings and "nearby" PSU components. Were they rated for that voltage? Were they subtly damaged if the rating was exceeded?

Tek used a fairly high quality transformer.. with i think, a shield inside between primary and secondary.

i would expect them to work fine at even much higher voltages.

Up to what voltage would you be comfortable with and not worry about damaging the oscilloscope?

What voltage do you expect the transformer to arc over internally?

Im talking about ancient crt. The whole case is aluminum.. dont touch it.

[tggzzz]

fairly certain my old TEK 475 oscope survived being floated at +/- 170vdc at 120KHZ.. no problem, with a cut ground pin.

doesn't mean the waveform was accurate, due to the resistance, inductance, and coupling of the 10 foot long oscope leads... but, it didn't blow up anything.

I'm fairly certain I've walked into the road without looking... no problem.

With your 475, what were the voltages across the transformer windings and "nearby" PSU components. Were they rated for that voltage? Were they subtly damaged if the rating was exceeded?

Tek used a fairly high quality transformer.. with i think, a shield inside between primary and secondary.

i would expect them to work fine at even much higher voltages.

Up to what voltage would you be comfortable with and not worry about damaging the oscilloscope?

Whatever is in the oscilloscope specification. Beyond that you have to do a thorough reverse engineering job, and hope you haven't missed something. Or you can gamble.

[MrAl]

I always see people get super paranoid about these kinds of measurements yet they are done every day without incident.

Years ago I used the method of cutting the ground pin off of the scopes line cord plug for literally 100's of measurements.  Many people I knew did the same thing, over and over and over and over and over again.  None of them died.  None of them even got shocked.  These were all CRT type scopes.

The key point here is you just have to pay attention to what you are doing when ...

Exactly the same can be said about walking into the road without looking.

(And why did you bring up the display technology (CRT)? That is completely and utterly irrelevant)

Hello,

You have no idea what you are talking about.
How can I be so sure?
I already mentioned this but you, as usual, fail to understand.
I have worked with a LOT of tech's who ALL used scopes with the ground pin cut off, and NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.  Let me repeat that so MAYBE you will read it and understand this time.

NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
Not ever, over years and years of working with them in several different companies.  That's three, four, five companies.  Plus, we're talking about a LOT of technicians, and who were not reprimanded in any way by the upper management.  In fact, they were instructed to do it this way.
How can this be?  It be, period.

I am not trying to force anyone to do this, just stating some facts about this.  If a given reader does not like this idea, that's ok with me.  If you feel it is dangerous, then don't do it.  Don't like it?  Then don't do it.  But don't spread misinformation.

Some people have been killed. Your shouted assertions don't change that.

Here’s an anecdote about someone that "was WELL AWARE of the danger and took a lot of precautions to prevent shock . . . Bottom Line: He died anyway". https://groups.io/g/TekScopes/message/10754

Also, mentioning a CRT scope may be irrelevant to you, but not to me.  You're welcome to talk about any kind of scope you like however, feel free.

Why, exactly, is the display type relevant to the points being made? Repeated assertions are not a reason.

Now if you want to go ahead and keep bitching about this, go right ahead, and I'll start to ignore what you have to say also.

From your responses (i.e. not answers) you have already started that

I do not see *too* much difference between using a scope and using a volt meter or ammeter.  You have to know a little about how to do it with either kind of instrument.  Heck, don't use a toaster.  Don't use a space heater.  Hide under the covers until you grow old and die of natural causes (ha ha). You can get killed just crossing the street.
Life is dangerous, no way around it.

This reminds me of that movie, don't know if anyone here has seen it.  It's named:
"A Million Ways To Die In The West".
Really funny with a dash of old west history.  Highly recommended.
The point is, there are a million ways to die in just about any era.

The bottom line might be if you don't feel comfortable doing it, then don't.  That goes for a volt meter too though, or any other kind of test equipment that can be live sometimes.

In my old rock and roll band days, we were performing outside one day for a big big party and I got stung by the microphone.  I was standing on the grass and the mic had the plug in the outlet in such a way as to make the chassis of the amplifier hot.
That has killed people too from what I have read.  So don't become a singer, you might get killed (ha ha).

[johansen]

Dont do this at home.

I didn't even feel a tingle. Warm dry steel toed boots, clean dry floor.

[MrAl]

OSHA standard 1910.334 - Use of equipment, section (3) Grounding-type equipment specifically prohibits cutting the ground pin:

Attachment plugs and receptacles may not be connected or altered in a manner which would prevent proper continuity of the equipment grounding conductor at the point where plugs are attached to receptacles. ...

and later in the section, explicitly prohibits the use of adapters that interrupt the ground conductor.

Assuming this was in the USA, MrAl and his 'tech' friends exposed the company to potentially massive liability, and should all have received a formal written warning for the first offence of interrupting a grounding conductor, escalating to retraining and even dismissal for repeated offenses.

Even ignoring the fact that OSHA regulations are written in blood, i.e. are codified in response to serious or fatal industrial accidents, or serious negligent harm to employee health, to attempt to protect all employees in similar industries, so should be obeyed even if (and especially if) you do not understand the rationale behind them, the fines OSHA  would have imposed if even one disgruntled (ex) employee had reported cutting ground pins, + the disruption from the resulting surprise inspection to confirm the report, would almost certainly have exceeded the cost of providing isolated differential probes to all techs that actually needed to make floating scope measurements.

Cutting ground pins, like drunk driving, may once have been socially acceptable even though it was and is prohibited/illegal.  It is no longer acceptable, and any colleague or supervisor that proposes or performs it is an active danger to your personal safety.  If management wont listen and act, gather evidence and report it to OSHA or your equivalent national workplace safety authority.

Shoulda, coulda, woulda, mighta, but they did not.  Why not?  Why didn't they do anything when they came around one day?
What they did do was make everyone wear safety glasses, that's about it.

I don't think there is anything in this world that would stop some people from cutting the ground pin, or using a piece of test equipment with a three to two prong adapter.  For me though, I don't have to anymore, and that's why I always recommend a battery powered scope now.  They are not too expensive for the frequency we are talking about.  In fact, you can get one for under $40 or even less.  I got one brand new for $25 some years ago, it was the kind that uses that ARM chip, and only has one input.  Comes with one probe but no batteries you have to get your own batteries.

Some bench meters have a metal case too.
It is true though that there has been more attention to the three prong plug over the last decades than before that.

[MrAl]

Dont do this at home.

I didn't even feel a tingle. Warm dry steel toed boots, clean dry floor.

Wow now that's guts.

[xavier60]

This might not be useful to know. Some mains powered equipment that have a SMPS, will run on DC.
My HP 54645a DSO will run on 85VDC at 0.9A.
There is a chance that this could damage some equipment.

To somebody more experienced this might make sense.  Could you please elaborate in more complete thoughts so that I could understand this better. 

Thank you.

If I wanted to adapt my DSO to battery operation, not necessarily for the purpose of making measurements referenced to hazardous voltages, I would power it from an SLA battery boosted to 85VDC using a boost(flyback) regulator.

[JJ_023]

If I wanted to adapt my DSO to battery operation, not necessarily for the purpose of making measurements referenced to hazardous voltages, I would power it from an SLA battery boosted to 85VDC using a boost(flyback) regulator.

[/quote]

Why would your approach be preferred over utilizing a UPS (where there is nothing to build and it is a finished product in its own case)?

Could you please explain how you would run a mains oscilloscope on 85VDC (that voltage doesn't seem like it would be enough)? 

Thank you.

[johansen]

If I wanted to adapt my DSO to battery operation, not necessarily for the purpose of making measurements referenced to hazardous voltages, I would power it from an SLA battery boosted to 85VDC using a boost(flyback) regulator.

Why would your approach be preferred over utilizing a UPS (where there is nothing to build and it is a finished product in its own case)?

Could you please explain how you would run a mains oscilloscope on 85VDC (that voltage doesn't seem like it would be enough)? 

Thank you.
[/quote]

85 is probably enough. A brick of aa batteries could power a scope for a long time.

[tggzzz]

I always see people get super paranoid about these kinds of measurements yet they are done every day without incident.

Years ago I used the method of cutting the ground pin off of the scopes line cord plug for literally 100's of measurements.  Many people I knew did the same thing, over and over and over and over and over again.  None of them died.  None of them even got shocked.  These were all CRT type scopes.

The key point here is you just have to pay attention to what you are doing when ...

Exactly the same can be said about walking into the road without looking.

(And why did you bring up the display technology (CRT)? That is completely and utterly irrelevant)

Hello,

You have no idea what you are talking about.
How can I be so sure?
I already mentioned this but you, as usual, fail to understand.
I have worked with a LOT of tech's who ALL used scopes with the ground pin cut off, and NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.  Let me repeat that so MAYBE you will read it and understand this time.

NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
Not ever, over years and years of working with them in several different companies.  That's three, four, five companies.  Plus, we're talking about a LOT of technicians, and who were not reprimanded in any way by the upper management.  In fact, they were instructed to do it this way.
How can this be?  It be, period.

I am not trying to force anyone to do this, just stating some facts about this.  If a given reader does not like this idea, that's ok with me.  If you feel it is dangerous, then don't do it.  Don't like it?  Then don't do it.  But don't spread misinformation.

Some people have been killed. Your shouted assertions don't change that.

Here’s an anecdote about someone that "was WELL AWARE of the danger and took a lot of precautions to prevent shock . . . Bottom Line: He died anyway". https://groups.io/g/TekScopes/message/10754

Also, mentioning a CRT scope may be irrelevant to you, but not to me.  You're welcome to talk about any kind of scope you like however, feel free.

Why, exactly, is the display type relevant to the points being made? Repeated assertions are not a reason.

Now if you want to go ahead and keep bitching about this, go right ahead, and I'll start to ignore what you have to say also.

From your responses (i.e. not answers) you have already started that

I do not see *too* much difference between using a scope and using a volt meter or ammeter.  You have to know a little about how to do it with either kind of instrument.
<irrelevant rant omitted>

Yes, we realise that you don't understand the differences and similarities, and that you haven't been injured or killed (yet).

We don't understand why you think a scope's display technology is relevant. Please enlighten us.

[xavier60]

If I wanted to adapt my DSO to battery operation, not necessarily for the purpose of making measurements referenced to hazardous voltages, I would power it from an SLA battery boosted to 85VDC using a boost(flyback) regulator.


Why would your approach be preferred over utilizing a UPS (where there is nothing to build and it is a finished product in its own case)?

Could you please explain how you would run a mains oscilloscope on 85VDC (that voltage doesn't seem like it would be enough)? 

Thank you.

Most of my work needs 74VDC, so I have built a variable boost converter that I power from my bench PSU, from an earlier thread.
I used this variable boost converter to see what low voltage DC my DSO was able to be powered by.
Since I'm already comfortable with designing SMPS circuitry, I would rather build a dedicated boost converter to power the DSO.

I have seen mains powered equipment wake up at as low as 50VDC because of the wide operating voltage range.

[JJ_023]

If I wanted to adapt my DSO to battery operation, not necessarily for the purpose of making measurements referenced to hazardous voltages, I would power it from an SLA battery boosted to 85VDC using a boost(flyback) regulator.


Why would your approach be preferred over utilizing a UPS (where there is nothing to build and it is a finished product in its own case)?

Could you please explain how you would run a mains oscilloscope on 85VDC (that voltage doesn't seem like it would be enough)? 

Thank you.

Most of my work needs 74VDC, so I have built a variable boost converter that I power from my bench PSU, from an earlier thread.
I used this variable boost converter to see what low voltage DC my DSO was able to be powered by.
Since I'm already comfortable with designing SMPS circuitry, I would rather build a dedicated boost converter to power the DSO.

I have seen mains powered equipment wake up at as low as 50VDC because of the wide operating voltage range.

I understand that explanation.  Thank you.

[Thunderer]

Given that you have a 120Vac system to power the scope, and as the scope has a SMPS PSU, meaning that the 120Vac is rectified you will find around 170Vdc at the SMPS transformer primary.

So, if you apply 170Vdc then you are good to go. If you apply a lower DC voltage (see below), the SMPS will not start, as the UVLO (undervoltage lockout) will do its job.

My Rigol says 100-240Vac input. For 100Vac the DC link in the SMPS is 141Vdc, for 120Vac is 170Vdc. Any voltage under 141Vdc will not be good for the scope PSU, as it will refuse to start.

To power from an UPS (non-sine waveform) it is completely acceptable for the SMPS PSU in the scope. That modified sine will be rectified anyway.

[xavier60]

As well as the UVLO, some offline SMPS's need to detect the presence of AC to run. One I met recently would run on 60VDC, but with only one polarity.
I wouldn't like to see what happens if a PFC was fed with DC.
The main reason I run some things on DC is just for a first test run from a current limited supply. Mostly washing machine motor controllers.

[Thunderer]

The PFC has a rectifier bridge in front of it. Then, the PFC (acting as a booster) may decide to let through if enough DC voltage, or boost if the DC input is lower than expected.



From here: https://www.ti.com/seclit/ml/slup390/slup390.pdf

Power Factor Correction (PFC)
Circuit Basics
Reproduced from
2020 Texas Instruments Power Supply Design Seminar
SEM2400
Topic 2
TI Literature Number: SLUP390

[johansen]

a smps that starts on one polarity only has a half wave rectifier in front of it. i don't even understand how that would be cheaper than a fwb and a smaller capacitor.

[johansen]

Given that you have a 120Vac system to power the scope, and as the scope has a SMPS PSU, meaning that the 120Vac is rectified you will find around 170Vdc at the SMPS transformer primary.

So, if you apply 170Vdc then you are good to go. If you apply a lower DC voltage (see below), the SMPS will not start, as the UVLO (undervoltage lockout) will do its job.

My Rigol says 100-240Vac input. For 100Vac the DC link in the SMPS is 141Vdc, for 120Vac is 170Vdc. Any voltage under 141Vdc will not be good for the scope PSU, as it will refuse to start.

To power from an UPS (non-sine waveform) it is completely acceptable for the SMPS PSU in the scope. That modified sine will be rectified anyway.

if it is pfc corrected, the undervoltage lockout on the chip is likely to be 10-16 vdc, and that voltage is typically fed from two 1/4w resistors in series summing to around 500K ohms. so without enough volts, it won't work..

once the chip starts up, the feedback coil from the inductor supplies the current the control chip needs. so you may find you can start it on 50-100vdc, but once you get it started it will run with far less voltage, perhaps all the way down to 20vdc.

most other smps work the same way, a tertiary coil or the snubber on the flyback transformer powers the brains of the smps. for a flyback, its going to need a much higher voltage than a pfc circuit, which can operate all the way down to just 5volts dc input (to get 99% power factor, the boost converter has to be operating for 99% the area under the half sine. it will of course be severely power limited.

but lots of welding equipment is done this way now, you can get half the output on 120vac as you can 240vac. you'd get a 4th of the output at 60vac if you could make it not fault.

[JJ_023]

Given that you have a 120Vac system to power the scope, and as the scope has a SMPS PSU, meaning that the 120Vac is rectified you will find around 170Vdc at the SMPS transformer primary.

So, if you apply 170Vdc then you are good to go. If you apply a lower DC voltage (see below), the SMPS will not start, as the UVLO (undervoltage lockout) will do its job.

My Rigol says 100-240Vac input. For 100Vac the DC link in the SMPS is 141Vdc, for 120Vac is 170Vdc. Any voltage under 141Vdc will not be good for the scope PSU, as it will refuse to start.

To power from an UPS (non-sine waveform) it is completely acceptable for the SMPS PSU in the scope. That modified sine will be rectified anyway.

Most scopes have that same 100-240 VAC Operational parameters.  This is why I question how you could make 1 of these units run on a much lower voltage.  That's the part I was struggling with and it did not make sense to me. 

[JJ_023]

As well as the UVLO, some offline SMPS's need to detect the presence of AC to run. One I met recently would run on 60VDC, but with only one polarity.
I wouldn't like to see what happens if a PFC was fed with DC.
The main reason I run some things on DC is just for a first test run from a current limited supply. Mostly washing machine motor controllers.

So what am I missing because to me it seems contradictory to what you posted earlier.  How would you be able to make it run on that low of a voltage. 

[JJ_023]

a smps that starts on one polarity only has a half wave rectifier in front of it. i don't even understand how that would be cheaper than a fwb and a smaller capacitor.

Is there a way you could elaborate in a more detailed manner so I could grasp the concept.

Thank you.

[JJ_023]

Given that you have a 120Vac system to power the scope, and as the scope has a SMPS PSU, meaning that the 120Vac is rectified you will find around 170Vdc at the SMPS transformer primary.

So, if you apply 170Vdc then you are good to go. If you apply a lower DC voltage (see below), the SMPS will not start, as the UVLO (undervoltage lockout) will do its job.

My Rigol says 100-240Vac input. For 100Vac the DC link in the SMPS is 141Vdc, for 120Vac is 170Vdc. Any voltage under 141Vdc will not be good for the scope PSU, as it will refuse to start.

To power from an UPS (non-sine waveform) it is completely acceptable for the SMPS PSU in the scope. That modified sine will be rectified anyway.

if it is pfc corrected, the undervoltage lockout on the chip is likely to be 10-16 vdc, and that voltage is typically fed from two 1/4w resistors in series summing to around 500K ohms. so without enough volts, it won't work..

once the chip starts up, the feedback coil from the inductor supplies the current the control chip needs. so you may find you can start it on 50-100vdc, but once you get it started it will run with far less voltage, perhaps all the way down to 20vdc.

most other smps work the same way, a tertiary coil or the snubber on the flyback transformer powers the brains of the smps. for a flyback, its going to need a much higher voltage than a pfc circuit, which can operate all the way down to just 5volts dc input (to get 99% power factor, the boost converter has to be operating for 99% the area under the half sine. it will of course be severely power limited.

but lots of welding equipment is done this way now, you can get half the output on 120vac as you can 240vac. you'd get a 4th of the output at 60vac if you could make it not fault.

Just to support what you are saying, on welding equipment the output on 120 VAC is over 50% however the duty cycle is reduced considerably more than 50% on what you would find on 240 VAC.

[johansen]

the boost inductor and the boost switch have a hard current limit. this is likely the majority of the reason a lot of PFC corrected equipment probably won't run at all below 50 volts. the other issue is the tickler to supply current to start up the circuitry.

i was relatively surprised to see my 12v portable hard drive power supply work fine on 60vac, its just a regular 100-240v flyback power supply.

[xavier60]

As well as the UVLO, some offline SMPS's need to detect the presence of AC to run. One I met recently would run on 60VDC, but with only one polarity.
I wouldn't like to see what happens if a PFC was fed with DC.
The main reason I run some things on DC is just for a first test run from a current limited supply. Mostly washing machine motor controllers.

So what am I missing because to me it seems contradictory to what you posted earlier.  How would you be able to make it run on that low of a voltage.

No contradiction. For example, my HP DSO's SMPS contains a TL431 and resistor divider monitoring the HVDC, dedicated to UVLO function. Its threshold is set to about 80VDC.
Although its rated supply range is 100 to 240VAC, it has a generous margin. I have just confirmed that it runs fine at 75Vac.

[freda]

As well as the UVLO, some offline SMPS's need to detect the presence of AC to run. One I met recently would run on 60VDC, but with only one polarity.
I wouldn't like to see what happens if a PFC was fed with DC.
The main reason I run some things on DC is just for a first test run from a current limited supply. Mostly washing machine motor controllers.

So what am I missing because to me it seems contradictory to what you posted earlier.  How would you be able to make it run on that low of a voltage.

if its offline SMPS you trying to test/repair, you don't usually even need mains.
50vdc supply to input rectifier, and another separate usually 12v supply to the controller circuit,
is enough to see whether it is functionally working and probe the signals as much as you want,
no danger involved

as to OP, long time ago did power scope from the mains isolator, since it couldn't power the DUT,
but i recall only needed to verify a few measurements, which i could relocate between switching DUT off, the scope performed normally, no extraneous noise signals or whatnot.

Now to those people boasting about cutting the third pin off the power cord!!
I mean don't you know that ultimately the neutral line IS connected to the earth, somewhere.
Y'all just lucky that the power point you hooked into was correctly wired!

[MrAl]

I always see people get super paranoid about these kinds of measurements yet they are done every day without incident.

Years ago I used the method of cutting the ground pin off of the scopes line cord plug for literally 100's of measurements.  Many people I knew did the same thing, over and over and over and over and over again.  None of them died.  None of them even got shocked.  These were all CRT type scopes.

The key point here is you just have to pay attention to what you are doing when ...

Exactly the same can be said about walking into the road without looking.

(And why did you bring up the display technology (CRT)? That is completely and utterly irrelevant)

Hello,

You have no idea what you are talking about.
How can I be so sure?
I already mentioned this but you, as usual, fail to understand.
I have worked with a LOT of tech's who ALL used scopes with the ground pin cut off, and NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.  Let me repeat that so MAYBE you will read it and understand this time.

NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
Not ever, over years and years of working with them in several different companies.  That's three, four, five companies.  Plus, we're talking about a LOT of technicians, and who were not reprimanded in any way by the upper management.  In fact, they were instructed to do it this way.
How can this be?  It be, period.

I am not trying to force anyone to do this, just stating some facts about this.  If a given reader does not like this idea, that's ok with me.  If you feel it is dangerous, then don't do it.  Don't like it?  Then don't do it.  But don't spread misinformation.

Some people have been killed. Your shouted assertions don't change that.

Here’s an anecdote about someone that "was WELL AWARE of the danger and took a lot of precautions to prevent shock . . . Bottom Line: He died anyway". https://groups.io/g/TekScopes/message/10754

Also, mentioning a CRT scope may be irrelevant to you, but not to me.  You're welcome to talk about any kind of scope you like however, feel free.

Why, exactly, is the display type relevant to the points being made? Repeated assertions are not a reason.

Now if you want to go ahead and keep bitching about this, go right ahead, and I'll start to ignore what you have to say also.

From your responses (i.e. not answers) you have already started that

I do not see *too* much difference between using a scope and using a volt meter or ammeter.  You have to know a little about how to do it with either kind of instrument.
<irrelevant rant omitted>

Yes, we realise that you don't understand the differences and similarities, and that you haven't been injured or killed (yet).

We don't understand why you think a scope's display technology is relevant. Please enlighten us.

Well then perhaps you can explain the differences and similarities between making measurements using a scope and a metal case bench volt meter, both of which could be used to measure line voltages of 120vac or 230vac for example.  Even making a voltage measurement using any volt meter could be dangerous.  Stupid people do stupid things, and you could get killed or injured just walking down the street.  Caution is the rule of the day in any case.

A lot of the CRT scopes have metal cases and much higher voltages inside too used for the CRT tube.

It would be better not to tell someone not to cut off the ground prong on a three-conductor line cord plug.  It would be better to explain how to do it safely.  There are some scopes that already had their ground pins cut off and they aren't about to throw them in the garbage they are going to use them like that and you or anyone else is not going to stop them, so explaining how to do it safely seems like a better option.

I don't see any good reason for continuing with this particular conversation I've made all my points clear.

[tggzzz]

Now to those people boasting about cutting the third pin off the power cord!!
I mean don't you know that ultimately the neutral line IS connected to the earth, somewhere.

Yeah. Reminds me of all those "look at me doing X <splat>" yootoob vids, but insufficiently imaginative and entertaining for the Darwin Award.

[tggzzz]

I always see people get super paranoid about these kinds of measurements yet they are done every day without incident.

Years ago I used the method of cutting the ground pin off of the scopes line cord plug for literally 100's of measurements.  Many people I knew did the same thing, over and over and over and over and over again.  None of them died.  None of them even got shocked.  These were all CRT type scopes.

The key point here is you just have to pay attention to what you are doing when ...

Exactly the same can be said about walking into the road without looking.

(And why did you bring up the display technology (CRT)? That is completely and utterly irrelevant)

Hello,

You have no idea what you are talking about.
How can I be so sure?
I already mentioned this but you, as usual, fail to understand.
I have worked with a LOT of tech's who ALL used scopes with the ground pin cut off, and NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.  Let me repeat that so MAYBE you will read it and understand this time.

NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
Not ever, over years and years of working with them in several different companies.  That's three, four, five companies.  Plus, we're talking about a LOT of technicians, and who were not reprimanded in any way by the upper management.  In fact, they were instructed to do it this way.
How can this be?  It be, period.

I am not trying to force anyone to do this, just stating some facts about this.  If a given reader does not like this idea, that's ok with me.  If you feel it is dangerous, then don't do it.  Don't like it?  Then don't do it.  But don't spread misinformation.

Some people have been killed. Your shouted assertions don't change that.

Here’s an anecdote about someone that "was WELL AWARE of the danger and took a lot of precautions to prevent shock . . . Bottom Line: He died anyway". https://groups.io/g/TekScopes/message/10754

Also, mentioning a CRT scope may be irrelevant to you, but not to me.  You're welcome to talk about any kind of scope you like however, feel free.

Why, exactly, is the display type relevant to the points being made? Repeated assertions are not a reason.

Now if you want to go ahead and keep bitching about this, go right ahead, and I'll start to ignore what you have to say also.

From your responses (i.e. not answers) you have already started that

I do not see *too* much difference between using a scope and using a volt meter or ammeter.  You have to know a little about how to do it with either kind of instrument.
<irrelevant rant omitted>

Yes, we realise that you don't understand the differences and similarities, and that you haven't been injured or killed (yet).

We don't understand why you think a scope's display technology is relevant. Please enlighten us.

Well then perhaps you can explain the differences and similarities between making measurements using a scope and a metal case bench volt meter, both of which could be used to measure line voltages of 120vac or 230vac for example.  Even making a voltage measurement using any volt meter could be dangerous.  Stupid people do stupid things, and you could get killed or injured just walking down the street.  Caution is the rule of the day in any case.

A lot of the CRT scopes have metal cases and much higher voltages inside too used for the CRT tube.

It would be better not to tell someone not to cut off the ground prong on a three-conductor line cord plug.  It would be better to explain how to do it safely.  There are some scopes that already had their ground pins cut off and they aren't about to throw them in the garbage they are going to use them like that and you or anyone else is not going to stop them, so explaining how to do it safely seems like a better option.

I don't see any good reason for continuing with this particular conversation I've made all my points clear.

Yes, the metal case is important, whereas the display technology is irrelevant.

Yes, it is better to explain how to do something safely - and that excludes floating the scope.

For amusement, I have a battery powered CRT scope with metal front panel which is designed to be used in the driving rain and stored 12inches under water.

[JJ_023]

As well as the UVLO, some offline SMPS's need to detect the presence of AC to run. One I met recently would run on 60VDC, but with only one polarity.
I wouldn't like to see what happens if a PFC was fed with DC.
The main reason I run some things on DC is just for a first test run from a current limited supply. Mostly washing machine motor controllers.

So what am I missing because to me it seems contradictory to what you posted earlier.  How would you be able to make it run on that low of a voltage.

No contradiction. For example, my HP DSO's SMPS contains a TL431 and resistor divider monitoring the HVDC, dedicated to UVLO function. Its threshold is set to about 80VDC.
Although its rated supply range is 100 to 240VAC, it has a generous margin. I have just confirmed that it runs fine at 75Vac.

Thanks for the elaboration.  I could've sworn that you mentioned DC in the previous post, that was throwing me off, or maybe I misunderstood something.  Now that you mention AC it makes more sense. 

[JJ_023]

As well as the UVLO, some offline SMPS's need to detect the presence of AC to run. One I met recently would run on 60VDC, but with only one polarity.
I wouldn't like to see what happens if a PFC was fed with DC.
The main reason I run some things on DC is just for a first test run from a current limited supply. Mostly washing machine motor controllers.

So what am I missing because to me it seems contradictory to what you posted earlier.  How would you be able to make it run on that low of a voltage.

if its offline SMPS you trying to test/repair, you don't usually even need mains.
50vdc supply to input rectifier, and another separate usually 12v supply to the controller circuit,
is enough to see whether it is functionally working and probe the signals as much as you want,
no danger involved

as to OP, long time ago did power scope from the mains isolator, since it couldn't power the DUT,
but i recall only needed to verify a few measurements, which i could relocate between switching DUT off, the scope performed normally, no extraneous noise signals or whatnot.

Now to those people boasting about cutting the third pin off the power cord!!
I mean don't you know that ultimately the neutral line IS connected to the earth, somewhere.
Y'all just lucky that the power point you hooked into was correctly wired!

It's funny that you mentioned correctly hooked up outlet.  If I am at a new location and if it's something that is important I always check to see if the wiring is correct and never assume that hot and neutral are properly wired.

As far as my post goes.  I was exploring the idea of whether you could run a mains oscilloscope from a UPS and how it would affect the functionality of the oscilloscope.

[xavier60]

As well as the UVLO, some offline SMPS's need to detect the presence of AC to run. One I met recently would run on 60VDC, but with only one polarity.
I wouldn't like to see what happens if a PFC was fed with DC.
The main reason I run some things on DC is just for a first test run from a current limited supply. Mostly washing machine motor controllers.

So what am I missing because to me it seems contradictory to what you posted earlier.  How would you be able to make it run on that low of a voltage.

No contradiction. For example, my HP DSO's SMPS contains a TL431 and resistor divider monitoring the HVDC, dedicated to UVLO function. Its threshold is set to about 80VDC.
Although its rated supply range is 100 to 240VAC, it has a generous margin. I have just confirmed that it runs fine at 75Vac.

Thanks for the elaboration.  I could've sworn that you mentioned DC in the previous post, that was throwing me off, or maybe I misunderstood something.  Now that you mention AC it makes more sense.

I have mentioned DC many times. Many offline SMPS's will happily run from low voltage DC, as others have already noted.

[JJ_023]

Yes, the metal case is important, whereas the display technology is irrelevant.

What is your reasoning for that?  With some of the older styles scopes there are higher voltages associated with the display.  Under normal circumstances I would see your logic because plastic versus metal.  But we are talking about freak haphazard events in which case the higher voltages would make a difference.  Am I not interpreting something correctly?

[JJ_023]

As well as the UVLO, some offline SMPS's need to detect the presence of AC to run. One I met recently would run on 60VDC, but with only one polarity.
I wouldn't like to see what happens if a PFC was fed with DC.
The main reason I run some things on DC is just for a first test run from a current limited supply. Mostly washing machine motor controllers.

So what am I missing because to me it seems contradictory to what you posted earlier.  How would you be able to make it run on that low of a voltage.

No contradiction. For example, my HP DSO's SMPS contains a TL431 and resistor divider monitoring the HVDC, dedicated to UVLO function. Its threshold is set to about 80VDC.
Although its rated supply range is 100 to 240VAC, it has a generous margin. I have just confirmed that it runs fine at 75Vac.

Thanks for the elaboration.  I could've sworn that you mentioned DC in the previous post, that was throwing me off, or maybe I misunderstood something.  Now that you mention AC it makes more sense.

I have mentioned DC many times. Many offline SMPS's will happily run from low voltage DC, as others have already noted.

Okay thanks for the clarification. 

[BlownUpCapacitor]

This is possible, but noise would become a bit of a problem.

Here: https://youtu.be/v-ajyq_DeDA?si=EmOqWxul3k-EGVsv&t=495

Mehdi uses an inverter, essentially a UPS without the function of auto kicking in after mains goes out. During his testing, his measurements are flooded with noise from the inverter, making capturing the signal from the street coils nearly impossible.

But he uses an extension cord that acts like a giant antenna so the noise pick-up will definitely be greater, and maybe using the scope without the giant antenna will work fine. However, considering how noisy inverters generally are, I think this is an important thing to keep in mind.

[JJ_023]

This is possible, but noise would become a bit of a problem.

Here: https://youtu.be/v-ajyq_DeDA?si=EmOqWxul3k-EGVsv&t=495

Mehdi uses an inverter, essentially a UPS without the function of auto kicking in after mains goes out. During his testing, his measurements are flooded with noise from the inverter, making capturing the signal from the street coils nearly impossible.

But he uses an extension cord that acts like a giant antenna so the noise pick-up will definitely be greater, and maybe using the scope without the giant antenna will work fine. However, considering how noisy inverters generally are, I think this is an important thing to keep in mind.

My initial concern when making this post was how the oscilloscope would be affected by the UPS both in its operation as well as the possible skewing of the measurements.

That video although slightly entertaining really doesn't demonstrate much in terms of what I am seeking.  Unless I am misinterpreting something. 

[tggzzz]

Yes, the metal case is important, whereas the display technology is irrelevant.

What is your reasoning for that?  With some of the older styles scopes there are higher voltages associated with the display.  Under normal circumstances I would see your logic because plastic versus metal.  But we are talking about freak haphazard events in which case the higher voltages would make a difference.  Am I not interpreting something correctly?

The voltages on the CRT or CCFL backlight etc are internal and the user cannot access them (unless water is added and then you have bigger problems ).

The user is exposed to the voltages on the case and connectors, so those must be kept within safe limits. If a scope isn't connected to protective mains earth, then the voltage on the case and connectors is undefined. It could float to (roughly!) halfway between live and neutral and give a slight shock. If the user connects a probe shield to anything other than earth (which is the normal "reason" given for floating a scope), the case will be at that voltage and could give a lethal shock.

When using a scope correctly, the voltages on and between transformer windings, and on noise supression capacitors will be within design limits what ever the display technology. If the scope is floated and the probe shield isn't at earth potential, that extra potential will also appear across capacitors and between windings, and could therefore be outside design limits causing damage.

With a floated scope, there will be a significant capacitance between the case (and hence probe's shield) and earth. That extra capacitance could damage the circuit being probed. That might also be the case if a UPS is used, depending on how the UPS is connected.

Hence the problems are all associated with the consequences not being connected to earth plus case/shield voltage, and that is true whatever the display technology.

[JJ_023]

Yes, the metal case is important, whereas the display technology is irrelevant.

What is your reasoning for that?  With some of the older styles scopes there are higher voltages associated with the display.  Under normal circumstances I would see your logic because plastic versus metal.  But we are talking about freak haphazard events in which case the higher voltages would make a difference.  Am I not interpreting something correctly?

The voltages on the CRT or CCFL backlight etc are internal and the user cannot access them (unless water is added and then you have bigger problems ).

The user is exposed to the voltages on the case and connectors, so those must be kept within safe limits. If a scope isn't connected to protective mains earth, then the voltage on the case and connectors is undefined. It could float to (roughly!) halfway between live and neutral and give a slight shock. If the user connects a probe shield to anything other than earth (which is the normal "reason" given for floating a scope), the case will be at that voltage and could give a lethal shock.

When using a scope correctly, the voltages on and between transformer windings, and on noise supression capacitors will be within design limits what ever the display technology. If the scope is floated and the probe shield isn't at earth potential, that extra potential will also appear across capacitors and between windings, and could therefore be outside design limits causing damage.

With a floated scope, there will be a significant capacitance between the case (and hence probe's shield) and earth. That extra capacitance could damage the circuit being probed. That might also be the case if a UPS is used, depending on how the UPS is connected.

Hence the problems are all associated with the consequences not being connected to earth plus case/shield voltage, and that is true whatever the display technology.

Thank you for such an elaborate explanation. All of it makes sense.  Could you please elaborate even further on the following point.

That might also be the case if a UPS is used, depending on how the UPS is connected.

The scenario that I envision is that the oscilloscope is plugged into the UPS which is disconnected from the mains and is operating on battery.  The UPS has a cold start option.

[tggzzz]

Yes, the metal case is important, whereas the display technology is irrelevant.

What is your reasoning for that?  With some of the older styles scopes there are higher voltages associated with the display.  Under normal circumstances I would see your logic because plastic versus metal.  But we are talking about freak haphazard events in which case the higher voltages would make a difference.  Am I not interpreting something correctly?

The voltages on the CRT or CCFL backlight etc are internal and the user cannot access them (unless water is added and then you have bigger problems ).

The user is exposed to the voltages on the case and connectors, so those must be kept within safe limits. If a scope isn't connected to protective mains earth, then the voltage on the case and connectors is undefined. It could float to (roughly!) halfway between live and neutral and give a slight shock. If the user connects a probe shield to anything other than earth (which is the normal "reason" given for floating a scope), the case will be at that voltage and could give a lethal shock.

When using a scope correctly, the voltages on and between transformer windings, and on noise supression capacitors will be within design limits what ever the display technology. If the scope is floated and the probe shield isn't at earth potential, that extra potential will also appear across capacitors and between windings, and could therefore be outside design limits causing damage.

With a floated scope, there will be a significant capacitance between the case (and hence probe's shield) and earth. That extra capacitance could damage the circuit being probed. That might also be the case if a UPS is used, depending on how the UPS is connected.

Hence the problems are all associated with the consequences not being connected to earth plus case/shield voltage, and that is true whatever the display technology.

Thank you for such an elaborate explanation. All of it makes sense.  Could you please elaborate even further on the following point.

That might also be the case if a UPS is used, depending on how the UPS is connected.

The scenario that I envision is that the oscilloscope is plugged into the UPS which is disconnected from the mains and is operating on battery.  The UPS has a cold start option.

The scope is not floating w.r.t. the UPS, but the UPS is floating w.r.t. protective mains earth - and hence the scope is too.

If the UUT is powered by the UPS then they will all have a common reference, and there probably won't be an extra electrocution problen but do not rely on my opinion since I am not an electrician and I have only briefly considered the issue. It looks like there might be a worse stray capacitance problem because the capacitance between the UPS and earth will be added to that between the scope and earth.

If the UUT is not powered by the UPS, then it looks like the scope is floated.

[bdunham7]

My initial concern when making this post was how the oscilloscope would be affected by the UPS both in its operation as well as the possible skewing of the measurements.

That video although slightly entertaining really doesn't demonstrate much in terms of what I am seeking.  Unless I am misinterpreting something.

The video actually shows you exactly what will happen in that particular case, although the specific results will vary quite a bit with different UPS models, arrangement of cables, specific scope, etc etc.  Floating your scope with the UPS will actually work pretty much as you expect, but the additional noise will cause triggering issues and other side effects that may make it more problematic than using an isolation transformer.  These problems will be more pronounced with low signal levels, just like any other noise issue.

[JJ_023]

Yes, the metal case is important, whereas the display technology is irrelevant.

What is your reasoning for that?  With some of the older styles scopes there are higher voltages associated with the display.  Under normal circumstances I would see your logic because plastic versus metal.  But we are talking about freak haphazard events in which case the higher voltages would make a difference.  Am I not interpreting something correctly?

The voltages on the CRT or CCFL backlight etc are internal and the user cannot access them (unless water is added and then you have bigger problems ).

The user is exposed to the voltages on the case and connectors, so those must be kept within safe limits. If a scope isn't connected to protective mains earth, then the voltage on the case and connectors is undefined. It could float to (roughly!) halfway between live and neutral and give a slight shock. If the user connects a probe shield to anything other than earth (which is the normal "reason" given for floating a scope), the case will be at that voltage and could give a lethal shock.

When using a scope correctly, the voltages on and between transformer windings, and on noise supression capacitors will be within design limits what ever the display technology. If the scope is floated and the probe shield isn't at earth potential, that extra potential will also appear across capacitors and between windings, and could therefore be outside design limits causing damage.

With a floated scope, there will be a significant capacitance between the case (and hence probe's shield) and earth. That extra capacitance could damage the circuit being probed. That might also be the case if a UPS is used, depending on how the UPS is connected.

Hence the problems are all associated with the consequences not being connected to earth plus case/shield voltage, and that is true whatever the display technology.

Thank you for such an elaborate explanation. All of it makes sense.  Could you please elaborate even further on the following point.

That might also be the case if a UPS is used, depending on how the UPS is connected.

The scenario that I envision is that the oscilloscope is plugged into the UPS which is disconnected from the mains and is operating on battery.  The UPS has a cold start option.

The scope is not floating w.r.t. the UPS, but the UPS is floating w.r.t. protective mains earth - and hence the scope is too.

If the UUT is powered by the UPS then they will all have a common reference, and there probably won't be an extra electrocution problen but do not rely on my opinion since I am not an electrician and I have only briefly considered the issue. It looks like there might be a worse stray capacitance problem because the capacitance between the UPS and earth will be added to that between the scope and earth.

If the UUT is not powered by the UPS, then it looks like the scope is floated.

Thank you for the elaboration.  I would definitely not be plugging whatever was being tested into the same UPS.

As a matter of fact I would probably use the UPS to float the DUT and use the oscilloscope off of mains, provided the UPS was capable of supplying enough power to the DUT if I had a need for anything to be floating in the equation and didn't have access to an isolation transformer or differential probes.

My whole idea with the UPS powering the oscilloscope was not really to achieve a floating oscilloscope but more exploring the idea of turning a Mains oscilloscope into a battery oscilloscope should the need ever arise.

[JJ_023]

My initial concern when making this post was how the oscilloscope would be affected by the UPS both in its operation as well as the possible skewing of the measurements.

That video although slightly entertaining really doesn't demonstrate much in terms of what I am seeking.  Unless I am misinterpreting something.

The video actually shows you exactly what will happen in that particular case, although the specific results will vary quite a bit with different UPS models, arrangement of cables, specific scope, etc etc.  Floating your scope with the UPS will actually work pretty much as you expect, but the additional noise will cause triggering issues and other side effects that may make it more problematic than using an isolation transformer.  These problems will be more pronounced with low signal levels, just like any other noise issue.

Thank you for that. What would your suggestions be as attempts to limit some of this noise should it occur?

[DavidAlfa]

I've done this plenty of times, skipping the isolation entirely.
Isolated the scope earth, checked the correct plug position (It's reversable in EU) and used neutral as scope ground, so the potential between scope chassis and earth is basically non-existant (maybe 5-10V), this way you won't be shocked.
Of course safety  first, check everything twice, and measure ac voltage between scope chassis and earth to be completely sure.
Also fixing the probe into 10x with some tape to prevent accidental switching / potentially blowing the channel.
Call me whatever but it seems pretty simple to me.

[tggzzz]

I've done this plenty of times, skipping the isolation entirely.
Isolated the scope earth, checked the correct plug position (It's reversable in EU) and used neutral as scope ground, so the potential between scope chassis and earth is basically non-existant (maybe 5-10V), this way you won't be shocked.
Of course safety  first, check everything twice, and measure ac voltage between scope chassis and earth to be completely sure.
Also fixing the probe into 10x with some tape to prevent accidental switching / potentially blowing the channel.
Call me whatever but it seems pretty simple to me.

I've walked into the road without looking plenty of times.

I listened for traffic twice[1]

Call me whatever but it seems pretty simple to me.

[1] I know I'm pretty deaf, so that isn't the equivalent of Dunning-Krueger syndrome

[shapirus]

I've walked into the road without looking plenty of times.

I listened for traffic twice[1]

Call me whatever but it seems pretty simple to me.

This is actually a pretty good analogy, as long as you also look to the sides and not only listen, and take into account traffic coming from a blind spot, which one should anticipate before walking into the road. You must look and listen if you use crosswalks just as well, because they don't make crossing the road inherently safe, and that is regardless of whether they are controlled by traffic lights or not. The same stands for scopes: don't just assume that everything's always all right, check your earth connection (for example, measure voltage between scope chassis and earth in a different wall socket) from time to time or maybe every time if you're paranoid enough.

Understand the dangers and why specific safety precautions exist. Understanding the danger allows to take steps necessary to avoid it, if you need to violate a safety rule, unless a mistake is made.

[tggzzz]

I've walked into the road without looking plenty of times.

I listened for traffic twice[1]

Call me whatever but it seems pretty simple to me.

This is actually a pretty good analogy, as long as you also look to the sides and not only listen, and take into account traffic coming from a blind spot, which one should anticipate before walking into the road. You must look and listen if you use crosswalks just as well, because they don't make crossing the road inherently safe, and that is regardless of whether they are controlled by traffic lights or not. The same stands for scopes: don't just assume that everything's always all right, check your earth connection (for example, measure voltage between scope chassis and earth in a different wall socket) from time to time or maybe every time if you're paranoid enough.

Understand the dangers and why specific safety precautions exist. Understanding the danger allows to take steps necessary to avoid it, if you need to violate a safety rule, unless a mistake is made.

Your final emphasis is important, of course

I'm perfectly willing to take risks with my health and equipment - but only if the benefit (to me) outweighs the risks.

I won't, however, encourage other inexperienced people to take similar risks unless they are being closely supervised by someone who knows the risks and knows how to avoid them. Obvious example: flying a glider.

Other examples: I regularly touch a 250-400V output in order to demonstrate equipment's operation; 5Mohm impedance, so I don't even notice anything. I recently got a tingle from (I think) a 1kV line; high impedance and poor contact, of course. I've just bought a 1kV 250mA supply; I'll be rather more careful with that!

[Ian.M]

I've done this plenty of times, skipping the isolation entirely.
Isolated the scope earth, checked the correct plug position (It's reversable in EU) and used neutral as scope ground, so the potential between scope chassis and earth is basically non-existant (maybe 5-10V), this way you won't be shocked.
Of course safety  first, check everything twice, and measure ac voltage between scope chassis and earth to be completely sure.
Also fixing the probe into 10x with some tape to prevent accidental switching / potentially blowing the channel.
Call me whatever but it seems pretty simple to me.

Anyone considering copying DavidAlfa, should note that Neutral is regarded as a Live conductor.  Line is the obviously hazardous live conductor, with full supply voltage on it, but you are only one failed Neutral connection away from the supply voltage becoming present on Neutral.   Its rare, but people have been electrocuted by failed Neutrals putting dangerous voltages on things they thought were safe to touch.   The fault doesn't even have to be on your property - it could be anywhere between the wall socket and the utility company's distribution transformer, which may be streets away.

The risk is obviously higher for someone in a low rent apartment building in a (relatively) poor country in Europe than for someone in rural North America in a single family property with recently inspected wiring and its own 'pole pig'.  Eliminate all exposed grounds on or near your bench, and stand on an insulating mat and you can reduce the remaining risk considerably

TLDR: Floating your scope is still dangerous even if David is happy with the risk of his Neutral going 'hot' while he's using the scope!

To our O.P:  Ground the <expletive> UPS so the scope is grounded, and don't put the probe ground clip on nodes that have significant voltage with respect to ground.   If you cant ground it properly (e.g. aircraft, moving vehicles), ground it to the airframe or chassis, but you need to understand "equipotential bonding".

[DavidAlfa]

Maybe the part about measuring multiple times and taking great care means nothing to you?

Certain people here have serious OCD, always taking what you want, skipping context and everything else.
I crossed the road in a long straight area with plenty of visibility and looking twice each side.

Crossing a road?? Wahh you will kill yourself!
Whatever!

If I say I measured 5V between neutral and earth, there was 5V between neutral and earth.
Sure, suddenly someone will switch the live polarity!
Or maybe a Starship falls on me!
People die every year due lightnings, stay at home and never go out!

[Ian.M]

Your 5V measured between ground and neutral shows its safe at the instant of measurement.  It tells you there isn't a pre-existing fault.  However it doesn't tell you anything about the odds of Neutral going 'hot' while you are using your floated scope. Only a full electrical inspection back to the distribution transformer could give you enough data to assess that.  Take other measures to mitigate the risk, and if you have dependents make sure your life insurance is paid up, *before* floating your scope!

Approx. 250 people per annum die from being struck by lightning in North America^*.  That's odds of 1 in 1.5 million.  The risk of serious, probably life changing injury form a lightning strike is an order of magnitude higher.   For comparison, the odds of winning big in a US national lottery are around 1 in 300 million.

People are strange - in general they believe they have a good enough chance of win the lottery for them to spend thousands of dollars a year on lottery tickets, but refuse to believe they may get struck by lightning.

As engineers, and well educated techs and hobbyists, we should be able to assess risk in our field and how to mitigate it better than that. e.g. If you even occasionally are tempted to float your scope, and you play the lottery, stop and save up for an isolated differential probe! 

* The majority of lightning related deaths are in Mexico with very few in Canada - for those interested, you could break down the statistics for North America by state and province, and correct for population density and prevalence of lightning to see whether that accounts for the distribution, or whether there are other factors to consider.

[shapirus]

Your 5V measured between ground and neutral shows its safe at the instant of measurement.  It tells you there isn't a pre-existing fault.  However it doesn't tell you anything about the odds of Neutral going 'hot' while you are using your floated scope. Only a full electrical inspection back to the distribution transformer could give you enough data to assess that.

The same is true for the mains earth connection. Except that there is a lower risk of it receiving a live potential, but it's still non-zero.

For the typical case of neutral going hot (which gives 380V instead of the normal 220V between neutral and live as a free bonus -- that's in EU, substitute for whatever it'll be in North America), the overvoltage relay installed in the circuit breaker panel is supposed to handle it. For the case of the user touching a hot chassis of a device and a grounded conductor at the same time, a differential breaker (switch? not sure what the right english term is) installed in the breaker panel will terminate the power supply to prevent a prolonged electric shock.

[DavidAlfa]

Well, please explain how the neutral would become suddenly live?
A guy at the generation center accidentally steps on a switch labeled "phase polarity reversal" - ooops ?
The only way would be the neutral/earth wire line coming loose, for sure everyone would be at risk.
It would be very rare..  again talking about one in a million.
Talking about statistics, tell me the probability of this happening ,over being hit by a lightning strike .
In any case we have RCD for that. Adding a small resistor of 100ohms (Or whatever, calculate it) between scope chassis and earth, so it doesn't cause inmediate short triggering the RCD, but will trigger it if the voltage rises over 30V for any reason.

Let's use an isolation transformer, or an offline UPS.
If you accidentally touch the output phases you would close the circuit and become the load.
Congratulations, you died, no RCD for you.

Everything is a risk in the wrong hands.

[JJ_023]

I've done this plenty of times, skipping the isolation entirely.
Isolated the scope earth, checked the correct plug position (It's reversable in EU) and used neutral as scope ground, so the potential between scope chassis and earth is basically non-existant (maybe 5-10V), this way you won't be shocked.
Of course safety  first, check everything twice, and measure ac voltage between scope chassis and earth to be completely sure.
Also fixing the probe into 10x with some tape to prevent accidental switching / potentially blowing the channel.
Call me whatever but it seems pretty simple to me.

Thanks for your input.  I understood most of what you said. 

Isolated the scope earth, and used neutral as scope ground,

Could you please clarify the following in a little bit more detail.  I want to make sure I am understanding your concept.

[JJ_023]

I've walked into the road without looking plenty of times.

I listened for traffic twice[1]

Call me whatever but it seems pretty simple to me.

This is actually a pretty good analogy, as long as you also look to the sides and not only listen, and take into account traffic coming from a blind spot, which one should anticipate before walking into the road. You must look and listen if you use crosswalks just as well, because they don't make crossing the road inherently safe, and that is regardless of whether they are controlled by traffic lights or not. The same stands for scopes: don't just assume that everything's always all right, check your earth connection (for example, measure voltage between scope chassis and earth in a different wall socket) from time to time or maybe every time if you're paranoid enough.

Understand the dangers and why specific safety precautions exist. Understanding the danger allows to take steps necessary to avoid it, if you need to violate a safety rule, unless a mistake is made.

Your final emphasis is important, of course

I'm perfectly willing to take risks with my health and equipment - but only if the benefit (to me) outweighs the risks.

I won't, however, encourage other inexperienced people to take similar risks unless they are being closely supervised by someone who knows the risks and knows how to avoid them. Obvious example: flying a glider.

Other examples: I regularly touch a 250-400V output in order to demonstrate equipment's operation; 5Mohm impedance, so I don't even notice anything. I recently got a tingle from (I think) a 1kV line; high impedance and poor contact, of course. I've just bought a 1kV 250mA supply; I'll be rather more careful with that!

How do you get 5 M Ohm impedance?  Every time I've measured I get like 1.5M

[DavidAlfa]

To isolate the scope you have to cut the ground conductor from the power wire.

About the resistor:
You know the RCD (Leak to earth detector/ breaker) will usually trip with 30mA (At least in EU, check this for your country and physical electrical installation).
Even if you have only 5V between neutral and earth, making a short will easily exceed 30mA, triggering the RCD and breaking the circuit.
Using a resistor will limit the current at low voltages.
Read the voltage between neutral and earth and calculate the resistor for  about 1/3rd of the RCD rating: in my case 10 mA.
If you got 5V: 5/0.010= 500 Ohm.
The RCD won't trigger in this condition, but if you make any mistake or the neutral voltage magically rises, it will.
Plus it will protect the scope from damage, as the current will be very limited in the worst case: 230V/500= 460mA.
This won't cause a destructive short.

Always check the RCD works and that the voltage causing 30mA flow through the resistor isn't too high ( I would keep the limit under 50V).


Anyways the rule is simple: Avoid touching anything metallic in the scope nor connecting it to anything else but the testing board.

[tggzzz]

People are strange - in general they believe they have a good enough chance of win the lottery for them to spend thousands of dollars a year on lottery tickets, but refuse to believe they may get struck by lightning.

I like to point out that in the UK weekly national lottery there is a lower chance of them winning the jackpot than there is of them dying while the balls are rolling.

They are buying a dream. Provided they realise that, I have no objection.

[tggzzz]

I've walked into the road without looking plenty of times.

I listened for traffic twice[1]

Call me whatever but it seems pretty simple to me.

This is actually a pretty good analogy, as long as you also look to the sides and not only listen, and take into account traffic coming from a blind spot, which one should anticipate before walking into the road. You must look and listen if you use crosswalks just as well, because they don't make crossing the road inherently safe, and that is regardless of whether they are controlled by traffic lights or not. The same stands for scopes: don't just assume that everything's always all right, check your earth connection (for example, measure voltage between scope chassis and earth in a different wall socket) from time to time or maybe every time if you're paranoid enough.

Understand the dangers and why specific safety precautions exist. Understanding the danger allows to take steps necessary to avoid it, if you need to violate a safety rule, unless a mistake is made.

Your final emphasis is important, of course

I'm perfectly willing to take risks with my health and equipment - but only if the benefit (to me) outweighs the risks.

I won't, however, encourage other inexperienced people to take similar risks unless they are being closely supervised by someone who knows the risks and knows how to avoid them. Obvious example: flying a glider.

Other examples: I regularly touch a 250-400V output in order to demonstrate equipment's operation; 5Mohm impedance, so I don't even notice anything. I recently got a tingle from (I think) a 1kV line; high impedance and poor contact, of course. I've just bought a 1kV 250mA supply; I'll be rather more careful with that!

How do you get 5 M Ohm impedance?  Every time I've measured I get like 1.5M

That output supplies voltage to a geiger muller discharge tube, via an internal 5Mohm resistor. The discharge pulls the voltage down, and that increments the dekatron counter. Designed for use in classrooms, so safety is important

[shapirus]

You know the RCD (Leak to earth detector/ breaker) will usually trip with 30mA (At least in EU, check this for your country and physical electrical installation).

Interesting. Are there any regulations? Here in Ukraine they aren't mandatory (for individuals at least), and you can use whatever value you want. Mine, for example, are rated as follows: the "entrypoint" one, that cuts off entire apartment, is 30mA, for fire and overall protection, and the three that protect the bathroom and the kitchen appliances dealing with water are 10mA, for human protection.

[DavidAlfa]

Yeah they're regulated in most countries.
Your values are pretty common, humid places like bathrooms and kitchen have more sensitive RCDs for extra protection.
You can test it easily, just plug a 5K resistor between earth and live.
This will flow 46mA, should instantly trip the breaker.
Although the instant power will be 10W, the breaker should be fast enough to prevent the resistor from burning, but be careful anyways.

Or measure neutral-earth and calculate the resistor for 40-50mA .

[JJ_023]

To isolate the scope you have to cut the ground conductor from the power wire.

About the resistor:
You know the RCD (Leak to earth detector/ breaker) will usually trip with 30mA (At least in EU, check this for your country and physical electrical installation).
Even if you have only 5V between neutral and earth, making a short will easily exceed 30mA, triggering the RCD and breaking the circuit.
Using a resistor will limit the current at low voltages.
Read the voltage between neutral and earth and calculate the resistor for  about 1/3rd of the RCD rating: in my case 10 mA.
If you got 5V: 5/0.010= 500 Ohm.
The RCD won't trigger in this condition, but if you make any mistake or the neutral voltage magically rises, it will.
Plus it will protect the scope from damage, as the current will be very limited in the worst case: 230V/500= 460mA.
This won't cause a destructive short.

Always check the RCD works and that the voltage causing 30mA flow through the resistor isn't too high ( I would keep the limit under 50V).


Anyways the rule is simple: Avoid touching anything metallic in the scope nor connecting it to anything else but the testing board.

Thank you for the elaborate explanation.  I follow what you are saying.  My follow-up question.  How would you typically place this resistor in the circuit in your particular application? (Like solder it on, clip it on, or some other methodology)

[JJ_023]

You know the RCD (Leak to earth detector/ breaker) will usually trip with 30mA (At least in EU, check this for your country and physical electrical installation).

Interesting. Are there any regulations? Here in Ukraine they aren't mandatory (for individuals at least), and you can use whatever value you want. Mine, for example, are rated as follows: the "entrypoint" one, that cuts off entire apartment, is 30mA, for fire and overall protection, and the three that protect the bathroom and the kitchen appliances dealing with water are 10mA, for human protection.

I looked up something briefly for the US.  This is what one website said. 

30 mA RCD is required to trip at a current between 18 mA to 28 mA

[DavidAlfa]

As I said, in practice neutral failure is like a meteorite falling at your house, chances are extremely low.
For one-day, ocasional job, nothing will happen, just connect the  mains polarity so the board "negative" has low potential against protective earth.
And be extra careful with every you do, you're working with live voltage, still 100V+,isolated or not (320V DC in EU when rectified and filtered, 'Merican snowflakes, be real man haha ).
Literally everything connected to the board will be dangerous.
Sooo don't insert/remove a USB drive on your scope while mains is connected... ! .
But using this as a regular setup/everyday is a bad idea, needs proper isolation and safety.

[JJ_023]

As I said, in practice neutral failure is like a meteorite falling at your house, chances are extremely low.
For one-day, ocasional job, nothing will happen, just connect the  mains polarity so the board "negative" has low potential against protective earth.
And be extra careful with every you do, you're working with live voltage, still 100V+,isolated or not (320V DC in EU when rectified and filtered, 'Merican snowflakes, be real man haha ).
Literally everything connected to the board will be dangerous.
Sooo don't insert/remove a USB drive on your scope while mains is connected... ! .
But using this as a regular setup/everyday is a bad idea, needs proper isolation and safety.

Thank you for the detailed explanation.   The reason I asked these questions is not because I am looking to copy what you are doing I'm trying to understand in greater detail what you are saying and learn something in the process.  Because I am asking these questions Indicates to me that I don't know enough about this particular scenario to copy what you are doing. But I am interested in learning and the reasoning behind it.

I actually have differential probes for something like this. 

So the way that I understand what you are saying.  You are looking to create as little potential difference in voltage as possible with your setup.  I am still having a difficult time understanding what you mean by.

just connect the  mains polarity so the board "negative" has low potential against protective earth.

Could you possibly explain this a little bit more so that I can grasp the concept.  What do you exactly mean by mains polarity so the board negative has low potential against protective earth.  I know what all of the terms mean but I am somehow not piecing this together mentally.

I know to you this seems repetitive and I apologize but I am trying to make it click.  I understand the macro concept that you are conveying about creating the least amount of potential against protective earth.  It's the rest that gets me a little confused.

[tggzzz]

Well, please explain how the neutral would become suddenly live?
A guy at the generation center accidentally steps on a switch labeled "phase polarity reversal" - ooops ?
The only way would be the neutral/earth wire line coming loose, for sure everyone would be at risk.
It would be very rare..  again talking about one in a million.

When you make your statements, what kind of earthing systems are you including, and what kind of earthing systems are you excluding. Start by discussing the five main types of earthing system listed in BS 7671: TN-S, TN-C-S, TT, TN-C, and IT. If you can't do that then you really aren't competent to make statements, and are displaying symptoms associated with the Dunning-Krueger syndrome.

Yes, connections do come loose - and intermittently so they are not always detectable. When the wind blew I've had a chattering connection on the mains supply pole outside my house; it destroyed my central heating boiler's controller.

Given that happening, do you really still contend your practices are (a) safe and (b) suitably safe for a beginner?

[MrAl]

I always see people get super paranoid about these kinds of measurements yet they are done every day without incident.

Years ago I used the method of cutting the ground pin off of the scopes line cord plug for literally 100's of measurements.  Many people I knew did the same thing, over and over and over and over and over again.  None of them died.  None of them even got shocked.  These were all CRT type scopes.

The key point here is you just have to pay attention to what you are doing when ...

Exactly the same can be said about walking into the road without looking.

(And why did you bring up the display technology (CRT)? That is completely and utterly irrelevant)

Hello,

You have no idea what you are talking about.
How can I be so sure?
I already mentioned this but you, as usual, fail to understand.
I have worked with a LOT of tech's who ALL used scopes with the ground pin cut off, and NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.  Let me repeat that so MAYBE you will read it and understand this time.

NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
Not ever, over years and years of working with them in several different companies.  That's three, four, five companies.  Plus, we're talking about a LOT of technicians, and who were not reprimanded in any way by the upper management.  In fact, they were instructed to do it this way.
How can this be?  It be, period.

I am not trying to force anyone to do this, just stating some facts about this.  If a given reader does not like this idea, that's ok with me.  If you feel it is dangerous, then don't do it.  Don't like it?  Then don't do it.  But don't spread misinformation.

Some people have been killed. Your shouted assertions don't change that.

Here’s an anecdote about someone that "was WELL AWARE of the danger and took a lot of precautions to prevent shock . . . Bottom Line: He died anyway". https://groups.io/g/TekScopes/message/10754

Also, mentioning a CRT scope may be irrelevant to you, but not to me.  You're welcome to talk about any kind of scope you like however, feel free.

Why, exactly, is the display type relevant to the points being made? Repeated assertions are not a reason.

Now if you want to go ahead and keep bitching about this, go right ahead, and I'll start to ignore what you have to say also.

From your responses (i.e. not answers) you have already started that

I do not see *too* much difference between using a scope and using a volt meter or ammeter.  You have to know a little about how to do it with either kind of instrument.
<irrelevant rant omitted>

Yes, we realise that you don't understand the differences and similarities, and that you haven't been injured or killed (yet).

We don't understand why you think a scope's display technology is relevant. Please enlighten us.

Well then perhaps you can explain the differences and similarities between making measurements using a scope and a metal case bench volt meter, both of which could be used to measure line voltages of 120vac or 230vac for example.  Even making a voltage measurement using any volt meter could be dangerous.  Stupid people do stupid things, and you could get killed or injured just walking down the street.  Caution is the rule of the day in any case.

A lot of the CRT scopes have metal cases and much higher voltages inside too used for the CRT tube.

It would be better not to tell someone not to cut off the ground prong on a three-conductor line cord plug.  It would be better to explain how to do it safely.  There are some scopes that already had their ground pins cut off and they aren't about to throw them in the garbage they are going to use them like that and you or anyone else is not going to stop them, so explaining how to do it safely seems like a better option.

I don't see any good reason for continuing with this particular conversation I've made all my points clear.

Yes, the metal case is important, whereas the display technology is irrelevant.

Not if the metal case correlates to the display technology.  Most of the CRT's I've seen in the past had metal cases.  The new DSO does not, as I have seen so far.  Maybe this has changed now, but it was that way in the past.

Yes, it is better to explain how to do something safely - and that excludes floating the scope.

Only if you ignore everyone else's opinion can you 'exclude' floating the scope.  Read the other replies.
No matter how much you wish and pray, you will never stop everyone from floating the scope.  Thus, tell them how to do it safely.
I like the idea of the ground fault interrupter; I had forgot about that.

For amusement, I have a battery powered CRT scope with metal front panel which is designed to be used in the driving rain and stored 12inches under water.

That's great.  I take mine in the shower with me 

[shapirus]

Gentlemen, this is a very entertaining and useful topic, but please watch your overquoting.

[Veteran68]

In the US, RCDs (we call them GFCI here) are only required by electrical code for wet areas. Bathrooms, kitchens, garages, and exterior outlets typically. They are implemented at the outlet and not at the service panel like I think is common in Europe and other parts of the world.

In most homes you wouldn't find them indoors except for bathrooms and kitchens, and in older homes maybe not at all. Unless someone explicitly installed them around their workbench, most test equipment wouldn't be plugged into one here.

[DavidAlfa]

When you make your statements (...)

A beginner should be never be working with such voltages, isolated or not.
BTW, it's Dunning-Kruger.
If you don't know how to setup something properly, just don't do it.
Even when I used neutral as scope earth to reduce the risk, I would never touch anything.
No messing with the connections with the thing  powered on.
I want to see those 20KHz 400V from the PFC controller... connect everything, check twice, then turn on, only the scope plastic knobs are to be touched, you should have great perception and control of your surroundings, identifying the dangerous sources.
That means board fixed in place, tidy and secured wires.
On modifying anything, powering off is the first thing.
You can't talk everyone like children, literally calling them idiots using the Dunning-Kruger term, shows lots of condescension, like you're the source of wisdom and everyone else are people which miraculously managed to reach their adult age alive, unable to talk without shitting themselves or getting dizzy, while handling ac wires like drumsticks or licking them to check if they're alive.

No anger, just mildy annoyed.

[MrAl]

I've done this plenty of times, skipping the isolation entirely.
Isolated the scope earth, checked the correct plug position (It's reversable in EU) and used neutral as scope ground, so the potential between scope chassis and earth is basically non-existant (maybe 5-10V), this way you won't be shocked.
Of course safety  first, check everything twice, and measure ac voltage between scope chassis and earth to be completely sure.
Also fixing the probe into 10x with some tape to prevent accidental switching / potentially blowing the channel.
Call me whatever but it seems pretty simple to me.

Hi,

I've known so many people that have done this in the past that when I hear people talking about how dangerous this is I just have to wonder that they must be block heads, unable to grasp ideas that they are just brain stuck on and can't get over no matter what you tell them or proof you show them.

Then we have laws that come into effect that are meant to protect the idiots out there.  One kid gets his finger blown off by a firecracker and the whole state gets their fireworks banned.  That doesn't help anyone anyway they still smuggle in fireworks and that gets even more dangerous.
Likewise, an idiot or two dies because they don't know what they are doing and everyone thinks scopes are dangerous.  The same thing happens with voltmeters.  It's all part of the game of life.

I had a scope one time with a defective high voltage transformer inside that put 1000v on the probe tip. Should I stop using scopes or start to preach about the dangers of using CRT scopes.

I happen to be one of the most safety conscious people in the world but sometimes we have to get things done one way or another.

[tggzzz]

I always see people get super paranoid about these kinds of measurements yet they are done every day without incident.

Years ago I used the method of cutting the ground pin off of the scopes line cord plug for literally 100's of measurements.  Many people I knew did the same thing, over and over and over and over and over again.  None of them died.  None of them even got shocked.  These were all CRT type scopes.

The key point here is you just have to pay attention to what you are doing when ...

Exactly the same can be said about walking into the road without looking.

(And why did you bring up the display technology (CRT)? That is completely and utterly irrelevant)

Hello,

You have no idea what you are talking about.
How can I be so sure?
I already mentioned this but you, as usual, fail to understand.
I have worked with a LOT of tech's who ALL used scopes with the ground pin cut off, and NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.  Let me repeat that so MAYBE you will read it and understand this time.

NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
Not ever, over years and years of working with them in several different companies.  That's three, four, five companies.  Plus, we're talking about a LOT of technicians, and who were not reprimanded in any way by the upper management.  In fact, they were instructed to do it this way.
How can this be?  It be, period.

I am not trying to force anyone to do this, just stating some facts about this.  If a given reader does not like this idea, that's ok with me.  If you feel it is dangerous, then don't do it.  Don't like it?  Then don't do it.  But don't spread misinformation.

Some people have been killed. Your shouted assertions don't change that.

Here’s an anecdote about someone that "was WELL AWARE of the danger and took a lot of precautions to prevent shock . . . Bottom Line: He died anyway". https://groups.io/g/TekScopes/message/10754

Also, mentioning a CRT scope may be irrelevant to you, but not to me.  You're welcome to talk about any kind of scope you like however, feel free.

Why, exactly, is the display type relevant to the points being made? Repeated assertions are not a reason.

Now if you want to go ahead and keep bitching about this, go right ahead, and I'll start to ignore what you have to say also.

From your responses (i.e. not answers) you have already started that

I do not see *too* much difference between using a scope and using a volt meter or ammeter.  You have to know a little about how to do it with either kind of instrument.
<irrelevant rant omitted>

Yes, we realise that you don't understand the differences and similarities, and that you haven't been injured or killed (yet).

We don't understand why you think a scope's display technology is relevant. Please enlighten us.

Well then perhaps you can explain the differences and similarities between making measurements using a scope and a metal case bench volt meter, both of which could be used to measure line voltages of 120vac or 230vac for example.  Even making a voltage measurement using any volt meter could be dangerous.  Stupid people do stupid things, and you could get killed or injured just walking down the street.  Caution is the rule of the day in any case.

A lot of the CRT scopes have metal cases and much higher voltages inside too used for the CRT tube.

It would be better not to tell someone not to cut off the ground prong on a three-conductor line cord plug.  It would be better to explain how to do it safely.  There are some scopes that already had their ground pins cut off and they aren't about to throw them in the garbage they are going to use them like that and you or anyone else is not going to stop them, so explaining how to do it safely seems like a better option.

I don't see any good reason for continuing with this particular conversation I've made all my points clear.

Yes, the metal case is important, whereas the display technology is irrelevant.

Not if the metal case correlates to the display technology.  Most of the CRT's I've seen in the past had metal cases.  The new DSO does not, as I have seen so far.  Maybe this has changed now, but it was that way in the past.

Yes, it is better to explain how to do something safely - and that excludes floating the scope.

Only if you ignore everyone else's opinion can you 'exclude' floating the scope.  Read the other replies.
No matter how much you wish and pray, you will never stop everyone from floating the scope.  Thus, tell them how to do it safely.
I like the idea of the ground fault interrupter; I had forgot about that.

For amusement, I have a battery powered CRT scope with metal front panel which is designed to be used in the driving rain and stored 12inches under water.

That's great.  I take mine in the shower with me 

Please fix your incorrect "quoting". You are falsely attributing things I didn't write to me.

Apart from that, you should be aware that:

• the metal case doesn't correlate to technology
• to coin a phrase, "correlation is not causation"
• most scopes have metal cases for reasons not connected to whether they have CRTs or LCDs. Hint: safety, noise performance, and EMI/EMC
• some scopes don't have metal cases for reasons not connected to whether they have CRTs or LCDs
• only a few relatively rare scopes can be safely disconnected from protective mains earth. Such scopes display that capability prominently. Other scopes exclude it in their operating conditions
• opinions are not sacred. Dangerous opinions should be challenged, and incorrect opinions can be challenged
• respect has to be earned, and should be withheld until it has been earned

[DavidAlfa]

I had a scope one time with a defective high voltage transformer inside that put 1000v on the probe tip.

Now that was a spicy probe!
Maybe there was a switch at the back?
"Scope <-> HV isolation tester” .

Well, enough is enough, I'm off this thread.
There's a Spanish quote saying "No explanation will be enough to deaf ears".
We have airbags, safety belts, yet people keep dying in cars.
People die coming from the grocery store.
People die sleeping. People die working out. People die fu**ing.
And I'm totally sure someone died using an isolation transformer.

Accidents are accidents, we can't get hysterical about it.
Dumb mistakes / lacking skills is a different thing, "He plugged everything in a hurry and held the scope with sweating hands”.
That's natural selection, Darwin awards.
There's a 0.00001% chance of something going wrong in everything, like the neutral breaking.

[MrAl]

When you make your statements (...)

A beginner should be never be working with such voltages, isolated or not.
BTW, it's Dunning-Kruger.
If you don't know how to setup something properly, just don't do it.
Even when I used neutral as scope earth to reduce the risk, I would never touch anything.
No messing with the connections with the thing  powered on.
I want to see those 20KHz 400V from the PFC controller... connect everything, check twice, then turn on, only the scope plastic knobs are to be touched, you should have great perception and control of your surroundings, identifying the dangerous sources.
That means board fixed in place, tidy and secured wires.
On modifying anything, powering off is the first thing.
You can't talk everyone like children, literally calling them idiots using the Dunning-Kruger term, shows lots of condescension, like you're the source of wisdom and everyone else are people which miraculously managed to reach their adult age alive, unable to talk without shitting themselves or getting dizzy, while handling ac wires like drumsticks or licking them to check if they're alive.

No anger, just mildy annoyed.

Ha ha, my take also.

Funny I was just replying to one of your posts and when I was done I saw this one.
No matter what you say some people will keep their block head ideas intact no matter how one-sided it is.
I don't think they are dumb, just stubborn.  Insisting and insisting and insisting, as if there was no other opinion in the entire world.
And unfortunately, this is even after your argument subsumes theirs entirely.
I don't mean any disrespect to anyone on any forum, but geeze there's a limit

I see this same discussion come up time and time again on forums like this one.  I also see the never ending discussion on offline power supplies, which use no transformer and therefore are not galvanically isolated.  I always hear the worst yet major companies talk about how to design and use them all the time in app notes and on the web.
It's the same thing really.  State the precautions and be done with it but don't stop everyone from trying to build one. There are immense advantages.

[tggzzz]

When you make your statements (...)

A beginner should be never be working with such voltages, isolated or not.
BTW, it's Dunning-Kruger.
If you don't know how to setup something properly, just don't do it.
Even when I used neutral as scope earth to reduce the risk, I would never touch anything.
No messing with the connections with the thing  powered on.
I want to see those 20KHz 400V from the PFC controller... connect everything, check twice, then turn on, only the scope plastic knobs are to be touched, you should have great perception and control of your surroundings, identifying the dangerous sources.
That means board fixed in place, tidy and secured wires.
On modifying anything, powering off is the first thing.
You can't talk everyone like children, literally calling them idiots using the Dunning-Kruger term, shows lots of condescension, like you're the source of wisdom and everyone else are people which miraculously managed to reach their adult age alive, unable to talk without shitting themselves or getting dizzy, while handling ac wires like drumsticks or licking them to check if they're alive.

No anger, just mildy annoyed.

Now address the rest of the points in my post. Here they are again, since you seem to have missed them before...

When you make your statements, what kind of earthing systems are you including, and what kind of earthing systems are you excluding. Start by discussing the five main types of earthing system listed in BS 7671: TN-S, TN-C-S, TT, TN-C, and IT. If you can't do that then you really aren't competent to make statements, and are displaying symptoms associated with the Dunning-Kruger syndrome.

Yes, connections do come loose - and intermittently so they are not always detectable. When the wind blew I've had a chattering connection on the mains supply pole outside my house; it destroyed my central heating boiler's controller.

Given that happening, do you really still contend your practices are (a) safe and (b) suitably safe for a beginner?

It is a rule-of-thumb that beginners concentrate on how X works, while experienced professional engineers concentrate on how X fails.

In this context you need to think about what happens when conditions change after measurement, and what happens when normal imperfect people are involved (perfect ideal people don't exist in this world).

[MrAl]

Apart from that, you should be aware that:
the metal case doesn't correlate to technology
to coin a phrase, "correlation is not causation"
most scopes have metal cases for reasons not connected to whether they have CRTs or LCDs. Hint: safety, noise performance, and EMI/EMC
some scopes don't have metal cases for reasons not connected to whether they have CRTs or LCDs
only a few relatively rare scopes can be safely disconnected from protective mains earth. Such scopes display that capability prominently. Other scopes exclude it in their operating conditions
opinions are not sacred. Dangerous opinions should be challenged, and incorrect opinions can be challenged
respect has to be earned, and should be withheld until it has been earned

Hello,

In my cases they did correlate that's why I said it.  Is it really that big of a deal anyway though if they did or did not?

In my view you are just being too overprotective about this whole issue, but that's up to you.

TO SUM UP...

You've said it's too dangerous to use a scope with the ground pin cut off or disconnected in some way.  You've sited a death or two to back up your claim.
I've said if it was that dangerous then people all over the world would not be doing it, and sited many technicians that have done it and would still do it today.

So you've made your point and I've made my point.  What else is there?

[DavidAlfa]

Not addressing how thing fails, but chances, which are ridiculously low at any given time.
I understand your point but it's like living your entire life inside an air cushion because coconuts kill a few every year.

[bdunham7]

Not addressing how thing fails, but chances, which are ridiculously low at any given time.
I understand your point but it's like living your entire life inside an air cushion because coconuts kill a few every year.

It's not terribly uncommon to have significant voltage surges on neutral lines and I've seen it happen more than once.  In fact I owned a commercial building that was plagued by this issue and it wasn't a loose connection--although that will do it too.  The most common way neutral voltage can suddenly rise up and smite you is when a large load on a separate circuit that shares the neutral either turns on or off.  And yes, sharing neutral is a thing--perhaps not in houses in your area, but in 3-phase installations (almost anywhere) as well as both commercial buildings and residential houses in the US, you'll see it.  The problem may not be so much that you'll get a shock, but if the neutral voltage goes to 50V it will also supply a great deal of current. 

If you are using your floated scope to look at mains circuits, you are much more likely than the average person to run into a neutral problem since there's at least some possibility that whatever issue you are chasing will turn out to be a mystery neutral disconnect or whatever. 

[tggzzz]

Not addressing how thing fails, but chances, which are ridiculously low at any given time.
I understand your point but it's like living your entire life inside an air cushion because coconuts kill a few every year.

Which bit of the mains fault happening to me do you not understand?

Please provide a justification for your contention "ridiculously low" with references. Take bdunham's points into account.

Do your statements and opinions apply to all types of earthing systems TN-S, TN-C-S, TT, TN-C, and IT.

And it is nothing like living inside an air cushion. I have already noted that I take beneficial risks in my life e.g. making hundreds of forced landings in aircraft, and deliberately "departing from controlled flight" 300m/8s from impacting the ground vertically. Like this, only lower down:



But I don't consider equipment damage and/or electrocution a beneficial risk; do you?

[MrAl]

Not addressing how thing fails, but chances, which are ridiculously low at any given time.
I understand your point but it's like living your entire life inside an air cushion because coconuts kill a few every year.

Which bit of the mains fault happening to me do you not understand?

Please provide a justification for your contention "ridiculously low" with references. Take bdunham's points into account.

Do your statements and opinions apply to all types of earthing systems TN-S, TN-C-S, TT, TN-C, and IT.

And it is nothing like living inside an air cushion. I have already noted that I take beneficial risks in my life e.g. making hundreds of forced landings in aircraft, and deliberately "departing from controlled flight" 300m/8s from impacting the ground vertically. Like this, only lower down:



But I don't consider equipment damage and/or electrocution a beneficial risk; do you?

Hello again,

Hey that's cool that you fly.  Small aircraft, that takes some guts too.  As they say, "Kudos" for that.

It does seem just a bit ironic though that you are not afraid to fly small aircraft but you are afraid of a cut off ground connection on a scope.

I do wish you the best of luck in the flying arena, and I hope you learned to fly by instrumentation also.  That was, as I am sure you know, JFK Jr's short coming.  It was a surprise to him also that he would have to fly in poor visibility conditions.  If you read about it, it's insane that the wife took an hour long shopping trip that delayed the flight that much longer, which was not planned by him.  Thus, when they got near to their destination, conditions were poor for flying by eyesight.  Such a tragedy.
The key point being that it was a surprise that he ran into conditions he never expected to be in and could not have prepared for it, so better to learn to fly by instrumentation while life is still in you.  If you did already that's great.

[tggzzz]

Hey that's cool that you fly.  Small aircraft, that takes some guts too.  As they say, "Kudos" for that.

No, it doesn't take guts.

It takes tuition, understanding, practice, and the knowledge of how to recognise risks and deal with them safely. That's why my daughter became a solo pilot years before she could legally start learning to drive a car. Those 14yo young adults that have gone solo are mature beyond their years.

Those small aircraft can pull +3.5G/-1.5G (some up to +9G), and you will experience that during training.
Those small aircraft can fly higher than commercial airliners.
Those small aircraft's normal take-off acceleration is three times higher than commercial airliners.
Those small aircraft routinely get to >2000ft in 50s. I think fighter aircraft can do that faster.

It does seem just a bit ironic though that you are not afraid to fly small aircraft but you are afraid of a cut off ground connection on a scope.

Not at all. You are closely taught by experts, and have to repeatedly demonstrate that you can recognise when things are about to happen, and that you can cope when they have happened.

Everyday occurrances: the sky not cooperating and spitting you out, or another glider "pinching your bit of grass" where you were planning on landing (go-arounds are obviously not possible).

Accidentally falling out of the sky, you've already seen - and you have to demonstrate you can enter/exit such spinning before you are allowed to go solo. There are yootoob vids of a 10yo girl doing that.

Ditto engine failure during take-off, which is not uncommon.

I do wish you the best of luck in the flying arena, and I hope you learned to fly by instrumentation also. 

Certainly not. None of this radio rubbish either. ATC? What's that?

The key point being that it was a surprise that he ran into conditions he never expected to be in and could not have prepared for it, so better to learn to fly by instrumentation while life is still in you.  If you did already that's great.

In a glider there is no such thing as a flight going "as expected". Every flight is unpredictable and different - and fun. Daughter learned many things that were only indirectly related to gliding, and they have since been very valuable to her. That's why the risks are worth taking.

Can you give any similar benefits for risking electrocution or equipment destruction by floating a scope?

[MrAl]

Hey that's cool that you fly.  Small aircraft, that takes some guts too.  As they say, "Kudos" for that.

No, it doesn't take guts.

It takes tuition, understanding, practice, and the knowledge of how to recognise risks and deal with them safely. That's why my daughter became a solo pilot years before she could legally start learning to drive a car. Those 14yo young adults that have gone solo are mature beyond their years.

Those small aircraft can pull +3.5G/-1.5G (some up to +9G), and you will experience that during training.
Those small aircraft can fly higher than commercial airliners.
Those small aircraft's normal take-off acceleration is three times higher than commercial airliners.
Those small aircraft routinely get to >2000ft in 50s. I think fighter aircraft can do that faster.

It does seem just a bit ironic though that you are not afraid to fly small aircraft but you are afraid of a cut off ground connection on a scope.

Not at all. You are closely taught by experts, and have to repeatedly demonstrate that you can recognise when things are about to happen, and that you can cope when they have happened.

Everyday occurrances: the sky not cooperating and spitting you out, or another glider "pinching your bit of grass" where you were planning on landing (go-arounds are obviously not possible).

Accidentally falling out of the sky, you've already seen - and you have to demonstrate you can enter/exit such spinning before you are allowed to go solo. There are yootoob vids of a 10yo girl doing that.

Ditto engine failure during take-off, which is not uncommon.

I do wish you the best of luck in the flying arena, and I hope you learned to fly by instrumentation also. 

Certainly not. None of this radio rubbish either. ATC? What's that?

The key point being that it was a surprise that he ran into conditions he never expected to be in and could not have prepared for it, so better to learn to fly by instrumentation while life is still in you.  If you did already that's great.

In a glider there is no such thing as a flight going "as expected". Every flight is unpredictable and different - and fun. Daughter learned many things that were only indirectly related to gliding, and they have since been very valuable to her. That's why the risks are worth taking.

Can you give any similar benefits for risking electrocution or equipment destruction by floating a scope?

You really do like to disagree with everything, that's amazing.
And as I said before, your favorite word is "No".
You think you are reprimanding every statement, but you're just looking like you have limited scope in your thinking and understanding of other people.  You don't realize that because of the same.
It's like you would love to tell people not only what you think, but would love to force them to think what you want them to think.  That's not going to happen.
Please stop acting like that it's annoying, and being pedantic does not make you look smart it just makes you look pedantic.
It's also rude.
Funny, you don't even know how to take a compliment.

[pdenisowski]

It is a rule-of-thumb that beginners concentrate on how X works, while experienced professional engineers concentrate on how X fails.

That's a great observation  - thanks!

[Terry Bites]

So many replies that I cant read through them all.
Running a scope from a UPS disonnected from mains is just the same as using a battery powered inverter.
You've made yourself a battery powered scope.
The danger comes from the thing you are measuring. You've got a 50:50 chance of screwing up and making your scope ground live WRT mains earth.
Death will follow.

[JJ_023]

So many replies that I cant read through them all.
Running a scope from a UPS disonnected from mains is just the same as using a battery powered inverter.
You've made yourself a battery powered scope.
The danger comes from the thing you are measuring. You've got a 50:50 chance of screwing up and making your scope ground live WRT mains earth.
Death will follow.

Thank you for your contribution. It's a shame that someone with your knowledge and experience did not read the post in its entirety to get the gist of what I am looking for.  I would have loved to have benefited from what you know.

[tggzzz]

Hey that's cool that you fly.  Small aircraft, that takes some guts too.  As they say, "Kudos" for that.

No, it doesn't take guts.

It takes tuition, understanding, practice, and the knowledge of how to recognise risks and deal with them safely. That's why my daughter became a solo pilot years before she could legally start learning to drive a car. Those 14yo young adults that have gone solo are mature beyond their years.

Those small aircraft can pull +3.5G/-1.5G (some up to +9G), and you will experience that during training.
Those small aircraft can fly higher than commercial airliners.
Those small aircraft's normal take-off acceleration is three times higher than commercial airliners.
Those small aircraft routinely get to >2000ft in 50s. I think fighter aircraft can do that faster.

It does seem just a bit ironic though that you are not afraid to fly small aircraft but you are afraid of a cut off ground connection on a scope.

Not at all. You are closely taught by experts, and have to repeatedly demonstrate that you can recognise when things are about to happen, and that you can cope when they have happened.

Everyday occurrances: the sky not cooperating and spitting you out, or another glider "pinching your bit of grass" where you were planning on landing (go-arounds are obviously not possible).

Accidentally falling out of the sky, you've already seen - and you have to demonstrate you can enter/exit such spinning before you are allowed to go solo. There are yootoob vids of a 10yo girl doing that.

Ditto engine failure during take-off, which is not uncommon.

I do wish you the best of luck in the flying arena, and I hope you learned to fly by instrumentation also. 

Certainly not. None of this radio rubbish either. ATC? What's that?

The key point being that it was a surprise that he ran into conditions he never expected to be in and could not have prepared for it, so better to learn to fly by instrumentation while life is still in you.  If you did already that's great.

In a glider there is no such thing as a flight going "as expected". Every flight is unpredictable and different - and fun. Daughter learned many things that were only indirectly related to gliding, and they have since been very valuable to her. That's why the risks are worth taking.

Can you give any similar benefits for risking electrocution or equipment destruction by floating a scope?

You really do like to disagree with everything, that's amazing.
And as I said before, your favorite word is "No".
You think you are reprimanding every statement, but you're just looking like you have limited scope in your thinking and understanding of other people.  You don't realize that because of the same.
It's like you would love to tell people not only what you think, but would love to force them to think what you want them to think.  That's not going to happen.
Please stop acting like that it's annoying, and being pedantic does not make you look smart it just makes you look pedantic.
It's also rude.
Funny, you don't even know how to take a compliment.

That's wrong in every respect.

I'm not here to fish for complements, and I'm content for other people to assess the relative merits of various people's competence.

Again, can you give any similar benefits for risking electrocution or equipment destruction by floating a scope?

EDIT: if I'm not here to fish for complements, why am I here? Basically it pleases me to learn new things, and to help other people understand to do new things - to "have fun, safely".

I wonder why MrAl is here?

[tggzzz]

It is a rule-of-thumb that beginners concentrate on how X works, while experienced professional engineers concentrate on how X fails.

That's a great observation  - thanks!

You're welcome

It doesn't just apply to engineering, it also applies to the legal profession, financial "professional", driving vehicles, politics, .........

[bdunham7]

Again, can you give any similar benefits for risking electrocution or equipment destruction by floating a scope?

Completing some critical measurements on an important research project that you otherwise would not have the resources to finish?  Fixing someones life support equipment during a storm on a remote island?  I don't know why you harp on this particular angle--is whatever 'benefit' you get from glider flying somehow superior to other people's goals?  We get it, there are risks and scope floating isn't really a thing anymore, but it is possible to overstate the case against it.  The concept is not entirely without utility, although there are admittedly readily available better alternatives nowadays.  Using private aviation, which has a pretty darn high body count in general, as your counterpoint to the incessantly cited example of that one poor engineer that failed at scope floating seems a dubious sort of argument to me.

IMO the main reason not to float your standard oscilloscope is that usually it doesn't work all that well.  Next up I'd cite a signficant risk of damaging your equipment if you do this with mains or high-energy circuits--even automotive systems could cause some pretty sparkly situations.  Personal safety is probably the third thing I'd cite depending on the situation.  Floating an old metal-cased CRT scope--even one of those little portable ones-- to look at a 480V/3PH motor control circuit would certainly be insane, for example. 

[tggzzz]

Again, can you give any similar benefits for risking electrocution or equipment destruction by floating a scope?

Completing some critical measurements on an important research project that you otherwise would not have the resources to finish?  Fixing someones life support equipment during a storm on a remote island?  I don't know why you harp on this particular angle--is whatever 'benefit' you get from glider flying somehow superior to other people's goals?  We get it, there are risks and scope floating isn't really a thing anymore, but it is possible to overstate the case against it.  The concept is not entirely without utility, although there are admittedly readily available better alternatives nowadays.  Using private aviation, which has a pretty darn high body count in general, as your counterpoint to the incessantly cited example of that one poor engineer that failed at scope floating seems a dubious sort of argument to me.

IMO the main reason not to float your standard oscilloscope is ususally that it doesn't work all that well.  Next up I'd cite a signficant risk of damaging your equipment if you do this with mains or high-energy circuits--even automotive systems could cause some pretty sparkly situations.  Personal safety is probably the third thing I'd cite depending on the situation.  Floating an old metal-cased CRT scope--even one of those little portable ones-- to look at a 480V/3PH motor control circuit would certainly be insane, for example.

Anybody can invent "Hollywood movie plot" examples, with about as much validity!

Defining realistic scenarios where there is no better way achieving the same ends is far more difficult.

We are in violent agreement that there are several solid reasons why floating a scope is bad practice.

[GnomeZA]

I feel like this topic has gone completely off the rails.
But here goes:

New Rigol scopes like the DHO804/814 and DHO914/924 now use USB-PD as power input.
How do they differ from a hand-held oscilloscope when powered from a battery bank?

Rather than stories or anecdotes, can we focus on the technical reasons this is a bad idea?
I'd also like to assume that the person is using a probe with the correct attenuation (ie. x100)

[Veteran68]

New Rigol scopes like the DHO804/814 and DHO914/924 now use USB-PD as power input.
How do they differ from a hand-held oscilloscope when powered from a battery bank?

The difference is they include an earth ground point with a separate cable intended for use in a bench scenario. Obviously if someone doesn't connect it, then it's now floating and subject to the same considerations as any floating scope. Whether that's an acceptable risk is debatable and heavily use-case dependent, as is obvious from this off-the-rails discussion.

[tggzzz]

I feel like this topic has gone completely off the rails.
But here goes:

New Rigol scopes like the DHO804/814 and DHO914/924 now use USB-PD as power input.
How do they differ from a hand-held oscilloscope when powered from a battery bank?

Rather than stories or anecdotes, can we focus on the technical reasons this is a bad idea?
I'd also like to assume that the person is using a probe with the correct attenuation (ie. x100)

It will depend on what class of probe is being used; the attenuation is a secondary specification. Some *100 probes will be fine, others may not be depending on how they are connected.

It will depend on what the USB and the earth point on the back of those scopes is connected to.

Key question: is or isn't there a continuous low-resistance connection between the scope and a protective mains earth.

You can avoid all that uncertainty, protect yourself, protect the scope, protect the equipment being tested, avoid coupled noise - if you use a high voltage differential probe. Using such a probe will mean that the remaining uncertainties are those that you might find when using a scope for any application.

[GnomeZA]

The difference is they include an earth ground point with a separate cable intended for use in a bench scenario. Obviously if someone doesn't connect it, then it's now floating and subject to the same considerations as any floating scope. Whether that's an acceptable risk is debatable and heavily use-case dependent, as is obvious from this off-the-rails discussion.

When powered from a battery bank, like I said, the impedance to the mains supply is practically infinite (and no capacitive coupling)
So I would say it is quite different from floating a scope, because in that case you are separated by whatever level of separation is available in the power supply.

Isn't the earth ground connection they provide to reduce common mode noise?

If yes, why is that relevant to a battery connected scope?

I definitely understand why they include that connection, because they sell this as a scope that is powered from USB-PD, which can be powered from an AC->DC which would have a non-significant amount of capacitive coupling at the very least.

But again, powered from a battery bank, how is this different from a hand-held scope?

It will depend on what class of probe is being used; the attenuation is a secondary specification. Some *100 probes will be fine, others may not be depending on how they are connected.

It will depend on what the USB and the earth point on the back of those scopes is connected to.

Like I said a USB-PD power bank.

Key question: is or isn't there a continuous low-resistance connection between the scope and a protective mains earth.

You can avoid all that uncertainty, protect yourself, protect the scope, protect the equipment being tested, avoid coupled noise - if you use a high voltage differential probe. Using such a probe will mean that the remaining uncertainties are those that you might find when using a scope for any application.

I've looked at differential probes, at least the "low cost" kind (ie. micsig & hantek) and they don't appear to have galvanic isolation, so I don't see how they are safer than a battery operated scope.
Other than the input impedance/attenuation provided.

If we don't consider resistors networks to be safe for AC->DC adapters, I don't fully comprehend why differential probes are considered somehow as totally safe and fine.

In my mind both have benefits.  The differential scope has a really high input impedance but the battery operated device has complete isolation.

[tggzzz]

Don't worry about what something is called or the characteristics of today's PSU; do understand about the fundamental physics that doesn't change next year.

I have an example of a Griffin and George counter that I used at school as a teenager. It has an exposed terminal with a voltage set by a front panel control to between 250V and 500V. When you touch the terminal with your finger the dekatron displays count upwards. Why don't you notice the voltage? Because it is series with a 5Mohm resistor.

Some HV differential probes use the same principle to avoid the need for galvanic isolation.

Unsurprisingly, a hypothetical AC DC converter would use much lower resistors!

[GnomeZA]

I've come to the following conclusions.

The reason a UPS powered scope and the Rigol I mentioned (which can be powered from battery using USB-PD power bank) would still be dangerous to use for AC is as follows:
1. These scopes connect the probe alligator clips (ground connector) together, they are all common and connected to earth

This means if you have a scope with multiple channels and you happen to connect the ground connector (alligator clip) to a part of the circuit at different potentials you WILL have a current passing basically with 0 impedance.  It will damage your scope.  If you ensure to connect your ground connectors all to the same point, then you can at least eliminate this risk (to be fair this risk also exists for isolated measurements on DC circuits).

2. The outside metal of the BNC connector on your scope channels are the ground and they would all be at the potential of the alligator clip.  So even if your scope isn't referenced at all to the same supply, once you connect that ground connector, it is now referenced at that voltage.  If that happens to be not be earth, you could shock yourself touching the BNC connectors on your scope.

3. You still need to mind the CAT rating of your probe and the attenuation setting or you could exceed the safe voltage rating of the scope inputs.

---

On the topic of differential probes.

If you sell a mainstream device, it must be either double insulated or galvanically isolated (capacitive, inductive, light, etc.)
You could never sell a mainstream device that relies on resistors for safety.
It would simply not meet the EU and US standards.

Differential probes have a huge impedance, they are not somehow "safe" because they stuck a bunch of resistors in series (I'm sure probes exist with galvanic isolation).
"Safe" in this case means it will not blow up your scope.  Probably.  But if the resistors fails, they will certainly fail unsafe, it is just super unlikely to happen in a lab setting while you probe a device under test.

In my mind if you want safety you probably want to use a high impedance differential probe and isolation transformer (for the device under test) together.

---

On the topic of a floating scope, it is different from a UPS or battery powered scope in the following way, as far as I can tell:
A scope that is mains powered has common mode noise which is pretty much impossible to eliminate (due to unwanted coupling).
This is especially true for switch mode supplies.

Disconnecting the earth conductor means that common mode noise has nowhere to go and it builds up a potential.
If you've ever accidentally used a earth required device without the earth pin and then gotten a shock touching the metal you'll know the voltage can become significant (although the current is comparatively fairly insignificant).

This can both damage your scope and it makes your measurements unreliable as that noise will show up in measurements.
I suspect there are some other scenarios that come into play with a scope that has an earth disconnected I'm not thinking about as I'm a noob.

--

If there are actual experts in this area that can contribute to the science here without resorting to anecdotes about their past or "funny" little quips and actually provide clear and concise explanations that would be much appreciated.

[tggzzz]

I've come to the following conclusions.

The reason a UPS powered scope and the Rigol I mentioned (which can be powered from battery using USB-PD power bank) would still be dangerous to use for AC is as follows:
1. These scopes connect the probe alligator clips (ground connector) together, they are all common and connected to earth

This means if you have a scope with multiple channels and you happen to connect the ground connector (alligator clip) to a part of the circuit at different potentials you WILL have a current passing basically with 0 impedance.  It will damage your scope.  If you ensure to connect your ground connectors all to the same point, then you can at least eliminate this risk (to be fair this risk also exists for isolated measurements on DC circuits).

2. The outside metal of the BNC connector on your scope channels are the ground and they would all be at the potential of the alligator clip.  So even if your scope isn't referenced at all to the same supply, once you connect that ground connector, it is now referenced at that voltage.  If that happens to be not be earth, you could shock yourself touching the BNC connectors on your scope.

3. You still need to mind the CAT rating of your probe and the attenuation setting or you could exceed the safe voltage rating of the scope inputs.

Pretty sound, but you are bit loose with the terminology of "ground", "earth", "common".

It would be better if you explicitly used "probe shield", "scope case", and are more explicit that probe shields and scope cases can be common with or without being earthed/grounded.

Another point to note is that every probe has a curve showing how the voltage must be derated as frequency increases. Cheaper/smaller probes have more onerous derating requirements.

---

On the topic of differential probes.

If you sell a mainstream device, it must be either double insulated or galvanically isolated (capacitive, inductive, light, etc.)
You could never sell a mainstream device that relies on resistors for safety.
It would simply not meet the EU and US standards.

I don't know about the standards, but there is always a resistance involved - even if very high by design

Differential probes have a huge impedance, they are not somehow "safe" because they stuck a bunch of resistors in series (I'm sure probes exist with galvanic isolation).
"Safe" in this case means it will not blow up your scope.  Probably.  But if the resistors fails, they will certainly fail unsafe, it is just super unlikely to happen in a lab setting while you probe a device under test.

Yes.

Failure mechanisms don't have to require resistor failure. The surface of a PCB (etc) containing the resistors can become contaminated with moisture or dirt or oil or... That's why physical separation is required, sometimes involving cuts in PCBs to increase the length between one terminal and another. Very cheap equipment (especially handheld DMMs) can be insufficient in that respect.

In my mind if you want safety you probably want to use a high impedance differential probe and isolation transformer (for the device under test) together.

The isolation transformer might not help. It depends on the failure mechanism being considered.

If there are actual experts in this area that can contribute to the science here without resorting to anecdotes about their past or "funny" little quips and actually provide clear and concise explanations that would be much appreciated.

Have a look at the references in https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/ especially the Tektronix publications and "Safety and Praxis" section. They are written by people and companies that depend on providing solid information to other people. Compare that with random people commenting on a forum - and I include myself in that category