Oscilloscope Floating using UPS

[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.