Oscilloscope Floating using UPS

[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