Oscilloscope Floating using UPS

[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.