EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: analias on July 03, 2018, 02:07:22 pm
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In a shared environment I discovered a 1980s Tek scope that was plugged into the mains via a cable marked "earth disconnected". In other words, somebody had floated the scope - and then left it as a boobytrap for the next poor sod.
I have two questions...
Firstly, what internal damage might have occurred? I can think of:
- transformer insulation. A Tek note indicates that can result in future electric shock, even when it is no longer floated
- mains filter X/Y caps (think RIFA!) and potentially smoothing caps. That could result in fire and smoke damage
- other components near the SMPS
are there any other types of damage?
Secondly, what is (a) necessary and (b) sufficient to find any damage, and preferably determine none has occurred? I can think of:
- visual inspection of the capacitors and "nearby" component. Certainly necessary, probably sufficient given that the risk is merely fire
- PAT testing the mains input and transformer; if there are multiple "classes" of test, which is appropriate? Presumably necessary, but I don't know that it is sufficient given that electric shock is a risk
are there any other types of test?
Unfortunately we know neither the purpose for which it was floated, nor the "floating" voltage. Hence that cannot be used to help assess any damage done to the scope.
Please note I'm not interested in a discussion as to whether or not "floating a scope" is safe/normal/dangerous, since that's been done to death (ho ho) in other threads, e.g. https://www.eevblog.com/forum/testgear/floating-scopes/msg1238945/#msg1238945 (https://www.eevblog.com/forum/testgear/floating-scopes/msg1238945/#msg1238945)
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Plug it into a GFCI protected outlet. If it don't trip, it's fine.
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Plug it into a GFCI protected outlet. If it don't trip, it's fine.
Which failure modes/mechanisms does that apply to, and why?
I rather doubt it would check a dodgy RIFA capacitor!
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Better advice may come if you tell us the model #.
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Why would you expect it to be damaged? Floating it isn't really advisable technique but it shouldn't damage the scope. It's not a bad idea to test any equipment for electrical leakage but other than that just use it.
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Maybe they mean if you don't have the filter the transformer can be damaged by fast transients a bit? You are floating the capacitors in the filter, so you basically have small series inductance infront of the transformer primary. Not sure what they can do.
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Better advice may come if you tell us the model #.
Probably not, since I'm after information about the damage that can be done to a generic scope. While this forum is better than many, there is a tendency to descend into minutiae and miss the big picture.
Having said that, it is probably relevant to mention that it is an SMPS PSU rather than a linear PSU, from the 24xxA series.
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Why would you expect it to be damaged? Floating it isn't really advisable technique but it shouldn't damage the scope.
The manufacturer states that scopes in general can be damaged by floating. Basically the "floating" voltage is added to the voltage applied to the transformer. Look at a generic scope's circuit diagram, and consider what the voltage differences are if the scope's "ground" is at, to pick a large value, 1kV relative to the mains ground.
Excerpt from Tektronix' "Fundamentals of Floating Measurements and Isolated Input Oscilloscopes Application Note"
https://info.tek.com/www-fundamentals-of-floating-measurements-and-isolated-input-oscilloscopes_ty.html
Advantages
Although floating equipment is a method that uses existing equipment to make floating measurements and remove ground loops on lower frequency signals, it is an unsafe and dangerous practice and should never be done.
Trade-Offs
This technique is dangerous, not only from the standpoint of elevated voltage present on the oscilloscope (a shock hazard to the operator), but also due to cumulative stresses on the oscilloscope’s power transformer insulation. This stress may not cause immediate failure but can lead to future dangerous failures (a shock and fire hazard), even after returning the oscilloscope to properly grounded operation.
At higher frequencies, severing the ground may not break the ground loop as the line-powered instrument exhibits a large parasitic capacitance when floated above earth ground. The floating measurement can be corrupted by ringing. Floating oscilloscopes do not have balanced inputs. The reference side (the “ground” clip on the probe) has a significant capacitance to ground. Any source impedance the reference is connected to will be loaded during fast common-mode transitions, attenuating the signal. Worse yet, the high capacitance can damage some circuits. Connecting the oscilloscope common to the upper gate in an inverter may slow the gate-drive signal, preventing the device from turning off and destroying the input bridge. This failure is usually accompanied by a miniature fireworks display right on your bench.
In addition it is conceivable that other components may be stressed, but that is likely to be scope specific.
It's not a bad idea to test any equipment for electrical leakage but other than that just use it.
Do you have any specific tests in mind?
What qualifications do you have to make such a statement?
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Unfortunately we know neither the purpose for which it was floated, nor the "floating" voltage. Hence that cannot be used to help assess any damage done to the scope.
Probably none. The usual justification for floating a 'scope isn't to expose it to much higher voltages than normal, it's just so that they can put the ground clip wherever they like.
are there any other types of test?
Switch it on and leave it for a day. If it appears to work OK then it probably is OK.
Secondly, what is (a) necessary and (b) sufficient to find any damage, and preferably determine none has occurred?
I don't think that you can *garantee* that, not without putting in more work that the 'scope is worth.
This scope is just as likely to be dangerous through age as it is to be dangerous through being floated.
I can think of:
- visual inspection of the capacitors and "nearby" component. Certainly necessary, probably sufficient given that the risk is merely fire
You should be doing that anyway if it's from the 1980s: Visually inspect all the caps and check the voltages at all the test points.
Some people would be replacing all the caps anyway, simply because of age.
If you want to replace the X/Y caps out of paranoia then fair enough. It's cheap/easy to do.
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Maybe they mean if you don't have the filter the transformer can be damaged by fast transients a bit?
The principal reason for the X/Y filter caps is to prevent EMI escaping from the scope's SMPS back into the mains lead, not to protect the SMPS.
You are floating the capacitors in the filter, so you basically have small series inductance infront of the transformer primary. Not sure what they can do.
Can you explain that in more detail?
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Unfortunately we know neither the purpose for which it was floated, nor the "floating" voltage. Hence that cannot be used to help assess any damage done to the scope.
Probably none. The usual justification for floating a 'scope isn't to expose it to much higher voltages than normal, it's just so that they can put the ground clip wherever they like.
Indeed, but who knows why this scope was floated? Perhaps they were testing an off-line SMPS.
are there any other types of test?
Switch it on and leave it for a day. If it appears to work OK then it probably is OK.
It is a shared environment with relatively inexperienced people. I don't care too much about the scope, but some people are (or ought to be) concerned about the next user of the scope.
I'm certainly not in that location to babysit it for that kind of time. I can't guarantee anybody else is either.
Secondly, what is (a) necessary and (b) sufficient to find any damage, and preferably determine none has occurred?
I don't think that you can *garantee* that, not without putting in more work that the 'scope is worth.
This scope is just as likely to be dangerous through age as it is to be dangerous through being floated.
While no guarantees are possible, it it prudent to demonstrably take appropriate precautions. I'm interested in what those precautions should be.
While the mains caps could be problematic and cause smoke/fire, that is unlikely to be lethal and can probably be dealt with even by non-expert users.
I see no reason why a Tektronix scope in good cosmetic condition is likely to be a shock hazard, unless it has been overstressed.
I can think of:
- visual inspection of the capacitors and "nearby" component. Certainly necessary, probably sufficient given that the risk is merely fire
You should be doing that anyway if it's from the 1980s: Visually inspect all the caps and check the voltages at all the test points.
Some people would be replacing all the caps anyway, simply because of age.
If you want to replace the X/Y caps out of paranoia then fair enough. It's cheap/easy to do.
It isn't my scope and I have neither the time nor inclination. There are very few people in that environment with the skill to replace caps; I can't presume their inclination.
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The manufacturer states that scopes in general can be damaged by floating. Basically the "floating" voltage is added to the voltage applied to the transformer. Look at a generic scope's circuit diagram, and consider what the voltage differences are if the scope's "ground" is at, to pick a large value, 1kV relative to the mains ground.
Trade-Offs
....cumulative stresses on the oscilloscope’s power transformer insulation. This stress may not cause immediate failure but can lead to future dangerous failures (a shock and fire hazard), even after returning the oscilloscope to properly grounded operation.
This is all true in theory, but unlikely in practice. If the previous users were playing with high voltages they'd be more likely to kill it outright than to leave it in that state.
If you're as risk-averse as seem to be implying then you shouldn't be using any test gear with an undocumented history, and probably no 1980s test gear at all.
PS: If you want to be 99% sure that the above hasn't happened to your transformer then the only way is to pull it out of the power supply and attach it to an insulation tester.
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The manufacturer states that scopes in general can be damaged by floating. Basically the "floating" voltage is added to the voltage applied to the transformer. Look at a generic scope's circuit diagram, and consider what the voltage differences are if the scope's "ground" is at, to pick a large value, 1kV relative to the mains ground.
Trade-Offs
....cumulative stresses on the oscilloscope’s power transformer insulation. This stress may not cause immediate failure but can lead to future dangerous failures (a shock and fire hazard), even after returning the oscilloscope to properly grounded operation.
This is all true in theory, but unlikely in practice. If the previous users were playing with high voltages they'd be more likely to kill it outright than to leave it in that state.
If you're as risk-averse as seem to be implying then you shouldn't be using any test gear with an undocumented history, and probably no 1980s test gear at all.
:) Actually most people think that I do things which indicate I'm not adverse to calculated risks. However what I do to myself is my business, whereas I would get rather annoyed if someone else did it to me.
PS: If you want to be 99% sure that the above hasn't happened to your transformer then the only way is to pull it out of the power supply and attach it to an insulation tester.
I don't know whether that is possible, nor whether it would be sufficient. It is almost certainly "not economic".
Which in-situ tests, e.g. via the mains lead, might be sufficient?
If the scope is damaged by such a test, it is preferable to a user being damaged by the scope.
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While no guarantees are possible, it it prudent to demonstrably take appropriate precautions. I'm interested in what those precautions should be.
The only sensible tests would be:
* Inspect caps, check voltages at test points.
* Switch it on for a day or so and see if anything bad happens (put it in a place with a concrete floor and nothing around it).
While the mains caps could be problematic and cause smoke/fire, that is unlikely to be lethal and can probably be dealt with even by non-expert users.
I see no reason why a Tektronix scope in good cosmetic condition is likely to be a shock hazard, unless it has been overstressed.
Even if it's been overstressed it's unlikely to a shock hazard. All the external metal parts are grounded.
It isn't my scope and I have neither the time nor inclination.
In that case it's a simply a game of chance.
But... if it powers on OK and survives the first week then it's a game of chance with a high probability of winning, IMHO.
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... pull it out of the power supply and attach it to an insulation tester.
I don't know whether that is possible, nor whether it would be sufficient. It is almost certainly "not economic".
This is the real point being made (I think).
Which in-situ tests, e.g. via the mains lead, might be sufficient?
a) Open it up, make sure that no critters made any nests inside and that no caps are obviously bulging/leaking.
b) Power it on for a day, sniff it before/during/after. If the smell gets worse then be suspicious of it.
c) Check voltages at test points towards the end of test (b).
If you have the manual then it might have a list of general diagnostic tests.
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While no guarantees are possible, it it prudent to demonstrably take appropriate precautions. I'm interested in what those precautions should be.
The only sensible tests would be:
* Inspect caps, check voltages at test points.
* Switch it on for a day or so and see if anything bad happens (put it in a place with a concrete floor and nothing around it).
While the mains caps could be problematic and cause smoke/fire, that is unlikely to be lethal and can probably be dealt with even by non-expert users.
Hey, I said that too. We are in agreement ;)
I see no reason why a Tektronix scope in good cosmetic condition is likely to be a shock hazard, unless it has been overstressed.
Even if it's been overstressed it's unlikely to a shock hazard. All the external metal parts are grounded.
Except when they aren't :( Maybe some other idiot "that knows better" will float it again.
It isn't my scope and I have neither the time nor inclination.
In that case it's a simply a game of chance.
But... if it powers on OK and survives the first week then it's a game of chance with a high probability of winning, IMHO.
Yeah but...
What happens if there is a "problem" and the police or insurance company investigates? A documentary chain of "reasonable precautions" would be helpful ammunition.
Such "reasonable precautions", preferably with justifications, are what I'm after.
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But... if it powers on OK and survives the first week then it's a game of chance with a high probability of winning, IMHO.
Yeah but...
What happens if there is a "problem" and the police or insurance company investigates? A documentary chain of "reasonable precautions" would be helpful ammunition.
If that's a genuine worry then don't use it.
Even if it's been overstressed it's unlikely to a shock hazard. All the external metal parts are grounded.
Except when they aren't :( Maybe some other idiot "that knows better" will float it again.
Put a big sticker on it saying "Do not float!", that's your responsibility dealt with.
As for physical safety: You can't protect yourself from these people, they can float a new device just as easily as an old one.
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What qualifications do you have to make such a statement?
Plug it in, turn it on, float the earth, and then with the tongue if it tastes salty it's alright.
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But... if it powers on OK and survives the first week then it's a game of chance with a high probability of winning, IMHO.
Yeah but...
What happens if there is a "problem" and the police or insurance company investigates? A documentary chain of "reasonable precautions" would be helpful ammunition.
If that's a genuine worry then don't use it.
Even if it's been overstressed it's unlikely to a shock hazard. All the external metal parts are grounded.
Except when they aren't :( Maybe some other idiot "that knows better" will float it again.
Put a big sticker on it saying "Do not float!", that's your responsibility dealt with.
As for physical safety: You can't protect yourself from these people, they can float a new device just as easily as an old one.
There was (and is) such a sign :(
The issue is what to do now that it has been floated :(
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Better advice may come if you tell us the model #.
Probably not, since I'm after information about the damage that can be done to a generic scope. While this forum is better than many, there is a tendency to descend into minutiae and miss the big picture.
Having said that, it is probably relevant to mention that it is an SMPS PSU rather than a linear PSU, from the 24xxA series.
From that era floating scopes was a common occurrence by those that knew how and manufacturers allowed for this now frowned upon usage in their designs.
Other than insulation tests on the SMPS transformer (removed of course) it's certainly a good question you ask.
Personally, if the scope operates correctly I'd reconnect the mains Gnd and just use it ......after a burn in for piece of minds sake.
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Better advice may come if you tell us the model #.
Probably not, since I'm after information about the damage that can be done to a generic scope. While this forum is better than many, there is a tendency to descend into minutiae and miss the big picture.
Having said that, it is probably relevant to mention that it is an SMPS PSU rather than a linear PSU, from the 24xxA series.
From that era floating scopes was a common occurrence by those that knew how and manufacturers allowed for this now frowned upon usage in their designs.
Other than insulation tests on the SMPS transformer (removed of course) it's certainly a good question you ask.
Personally, if the scope operates correctly I'd reconnect the mains Gnd and just use it ......after a burn in for piece of minds sake.
Get it PAT tested, that will reveal any abnormal leakages. In a commercial environment it should be done anyway, especially if its mains lead has been messed with. That's the proper thing to do.
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Better advice may come if you tell us the model #.
Probably not, since I'm after information about the damage that can be done to a generic scope. While this forum is better than many, there is a tendency to descend into minutiae and miss the big picture.
Having said that, it is probably relevant to mention that it is an SMPS PSU rather than a linear PSU, from the 24xxA series.
From that era floating scopes was a common occurrence by those that knew how and manufacturers allowed for this now frowned upon usage in their designs.
Perhaps for valve scopes, but I'm unconvinced that was true for 1980s SMPS/transistorised scopes.
Even in the 70s I knew that floating a scope was To Be Avoided.
Other than insulation tests on the SMPS transformer (removed of course) it's certainly a good question you ask.
Personally, if the scope operates correctly I'd reconnect the mains Gnd and just use it ......after a burn in for piece of minds sake.
Unfortunately it isn't just "personal".
It is a shared environment, and "ingenious fools" are present => it is difficult to make it foolproof.
Regrettably I think that Electro Detective's reply 86 in https://www.eevblog.com/forum/testgear/floating-scopes/ (https://www.eevblog.com/forum/testgear/floating-scopes/) is the appropriate attitude, including ...
"I'm jumping ship (for once) and following the majority.. to recommend LEAVE WELL ALONE if not 100% sure,
or best practice is to dip into the piggy bank for a differential probe as front line measurement cannon fodder :-+
There are TOO MANY VARIABLES that are almost IMPOSSIBLE to document and break down here for the casual user wanting a 'fast fix'
which usually means a good chance for an earth/ground related BANG! "
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Better advice may come if you tell us the model #.
Probably not, since I'm after information about the damage that can be done to a generic scope. While this forum is better than many, there is a tendency to descend into minutiae and miss the big picture.
Having said that, it is probably relevant to mention that it is an SMPS PSU rather than a linear PSU, from the 24xxA series.
From that era floating scopes was a common occurrence by those that knew how and manufacturers allowed for this now frowned upon usage in their designs.
Other than insulation tests on the SMPS transformer (removed of course) it's certainly a good question you ask.
Personally, if the scope operates correctly I'd reconnect the mains Gnd and just use it ......after a burn in for piece of minds sake.
Get it PAT tested, that will reveal any abnormal leakages. In a commercial environment it should be done anyway, especially if its mains lead has been messed with. That's the proper thing to do.
That's certainly my gut feeling. I think it is probably necessary to PAT test, but are there other beneficial tests?
The Tektronix statement I noted earlier, plus high voltages rapidly spreading in the event of a fault, raises concerns.
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I get the concern here but with the many thousands of used analog scopes out there, many bought and sold repeatedly (via eBay and other venues), cycled through several users - surely many have been floated or subject to worse abuse. Many are now sitting on shared benches, hacker spaces, schools, etc and used regularly by novices. Is this really such danger once the obvious defects revealed by inspection and a trial burn in has been done?
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This whole discussion seems a bit obsessive to me. Why are you so particularly concerned with the specific potential effect of the scope having been floated? In any scope with a 30 year history, who knows what kind of abuse, bad line voltage, or just degradation of critical components it may have suffered? If you encounter any scope that old in a multi-user environment, where you feel repsonsible for safety of all users (a classroom setting maybe?), wouldn't you have to decomission it?
@analias, I was wondering -- in your user name, what does the "ias" stand for? :P
Sorry, couldn't resist...
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I get the concern here but with the many thousands of used analog scopes out there, many bought and sold repeatedly (via eBay and other venues), cycled through several users - surely many have been floated or subject to worse abuse. Many are now sitting on shared benches, hacker spaces, schools, etc and used regularly by novices. Is this really such danger once the obvious defects revealed by inspection and a trial burn in has been done?
That's a reasonable question.
The issues are related to
- "am I my brother's keeper" if I (or others) know there is something that might harm them. To put more of an emotional twist on it, replace "brother" with "child" :)
- "plausible deniability" in the event the worst happens
I don't have a neat answer to those, unfortunately.
Neither do I know what I would do if someone's knowing negligence killed one of my children. Sh1t happens and has to be accepted, but knowingly putting others at risk is different.
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This whole discussion seems a bit obsessive to me. Why are you so particularly concerned with the specific potential effect of the scope having been floated? In any scope with a 30 year history, who knows what kind of abuse, bad line voltage, or just degradation of critical components it may have suffered?
See my previous response.
If you encounter any scope that old in a multi-user environment, where you feel repsonsible for safety of all users (a classroom setting maybe?), wouldn't you have to decomission it?
Reasonable questions.
I'm not responsible, other than in the unsatisfactory "safety is everybody's responsibility" mantra - which isn't taken too seriously in that environment. Ownership is unclear.
It may be that decommissioning is the thing to do, but I would like to avoid that if possible. The floating probably hasn't done it any damage, but nobody has indicated the "floating" voltage so we simply don't know.
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I don't have a neat answer to those, unfortunately.
Neither does anybody here.
I think the "floating" thing is a red herring. Almost all the risk of a floating device is to the users, not the device.
Using this device is really no different to using any old, undocumented piece of test gear. If the chassis is grounded then there's almost zero risk of electric shock from it.
Children might poke wires through the holes in the case, yes, and an old CRO has higher voltages inside it than a modern DSO but that's nothing to do with it having been floated (or otherwise) in the past. I wouldn't place any real emphasis on that. This should be viewed for what it is - an old CRO. The biggest risk is from dried up capacitors and cobwebs inside it.
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...nobody has indicated the "floating" voltage so we simply don't know.
It may not have been floated for "high voltage".
A lot of people floated them simply to avoid accidentally shorting things with the ground clip while working.
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Erm someone floated it and made it clear it was floating by labelling the socket. As such it could be dangerous to persons but there is no reason it is damaged.
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I think the "floating" thing is a red herring. Almost all the risk of floating a device is to the operators, not the device.
Using this device is really no different to using any old, undocumented piece of test gear. If the chassis is grounded then there's almost zero risk of electric shock from it.
Children might poke wires through the holes in the case, yes, and an old CRO has higher voltages inside it than a modern DSO but that's nothing to do with it having been floated (or otherwise) in the past. I wouldn't place any real emphasis on that. This should be viewed for what it is - an old CRO. The biggest risk is from dried up capacitors and cobwebs inside it.
I agree. Visual inspection,check chassis grounding, then power it up and watch it for a while. If all is ok, then spend efforts on proper safety instructions for novice users and some relevant signage perhaps. That’s were the safety payoff is, not in worrying about theoretical but extremely unlikely events related to past use history.
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I don't understand why people obsess over ridiculous specific hypothetical things like this. Floating the scope won't damage it, it's risky to do because the whole scope is live and you could get a shock while the scope is floating, not because it could damage the scope itself. This reminds me a bit about the thread where the guy was so paranoid about the possibility of a lithium ion battery catching fire that he was talking about incorporating a pyrotechnic ejection charge. It borders on mental illness.
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I don't understand why people obsess over ridiculous specific hypothetical things like this. Floating the scope won't damage it, it's risky to do because the whole scope is live and you could get a shock while the scope is floating, not because it could damage the scope itself.
I accept that you don't understand it. However, your assertion is false; see my reply #7 to your earlier assertion.
Do you have any response to the questions I posed at the end reply #7?
This reminds me a bit about the thread where the guy was so paranoid about the possibility of a lithium ion battery catching fire that he was talking about incorporating a pyrotechnic ejection charge. It borders on mental illness.
Such analogies generate more heat than light, and aren't worth discussing.
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Erm someone floated it and made it clear it was floating by labelling the socket. As such it could be dangerous to persons but there is no reason it is damaged.
The immediate risk is to anyone using a floating scope; there's no disagreement there (in this thread at least!).
However, I indicated in response #7 how the scope can suffer latent damage.
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I think the "floating" thing is a red herring. Almost all the risk of floating a device is to the operators, not the device.
Using this device is really no different to using any old, undocumented piece of test gear. If the chassis is grounded then there's almost zero risk of electric shock from it.
Children might poke wires through the holes in the case, yes, and an old CRO has higher voltages inside it than a modern DSO but that's nothing to do with it having been floated (or otherwise) in the past. I wouldn't place any real emphasis on that. This should be viewed for what it is - an old CRO. The biggest risk is from dried up capacitors and cobwebs inside it.
I agree. Visual inspection,check chassis grounding, then power it up and watch it for a while. If all is ok, then spend efforts on proper safety instructions for novice users and some relevant signage perhaps. That’s were the safety payoff is, not in worrying about theoretical but extremely unlikely events related to past use history.
Unfortunately there was signage, which was ignored.
Unfortunately currently instruction is patchy, and instructing novices is frowned upon - but this may be a stimulus to get that changed.
Neither instructions nor signs will help with people that think either it isn't dangerous or they know how to do it safely :(
Since resolution of those real and valid people-related issues cannot usefully be discussed here, I chose (and choose) to concentrate on a specific technical issue.
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Better advice may come if you tell us the model #.
Probably not, since I'm after information about the damage that can be done to a generic scope. While this forum is better than many, there is a tendency to descend into minutiae and miss the big picture.
Having said that, it is probably relevant to mention that it is an SMPS PSU rather than a linear PSU, from the 24xxA series.
From that era floating scopes was a common occurrence by those that knew how and manufacturers allowed for this now frowned upon usage in their designs.
Perhaps for valve scopes, but I'm unconvinced that was true for 1980s SMPS/transistorised scopes.
Even in the 70s I knew that floating a scope was To Be Avoided.
Other than insulation tests on the SMPS transformer (removed of course) it's certainly a good question you ask.
Personally, if the scope operates correctly I'd reconnect the mains Gnd and just use it ......after a burn in for piece of minds sake.
Unfortunately it isn't just "personal".
It is a shared environment, and "ingenious fools" are present => it is difficult to make it foolproof.
Regrettably I think that Electro Detective's reply 86 in https://www.eevblog.com/forum/testgear/floating-scopes/ (https://www.eevblog.com/forum/testgear/floating-scopes/) is the appropriate attitude, including ...
"I'm jumping ship (for once) and following the majority.. to recommend LEAVE WELL ALONE if not 100% sure,
or best practice is to dip into the piggy bank for a differential probe as front line measurement cannon fodder :-+
There are TOO MANY VARIABLES that are almost IMPOSSIBLE to document and break down here for the casual user wanting a 'fast fix'
which usually means a good chance for an earth/ground related BANG! "
Quite so, ED's reply is the 'modern' philosophy in today's litigation mad society where practices like floating scopes could never be taught for fear of culpability. It was by necessity back then as the vast bulk of scopes didn't have differential plugins nor were differential probes as widely available as they are now.
What I would offer as comment is a Tek 24** wasn't a cheap scope back then and any that would set it up as a dedicated floating scope would've known the risks to both themselves and instrument.
The likely extreme voltages it ever saw in floating mode would most likely be just rectified mains of which it could handle with comparative ease.
I get that if this scope is to be shared, culpability again rises it's ugly head where really any use of a scope and in fact any test gear exposes the operator to risk.
If this is too much to bear along with usage of a scope with uncertain history.....then buy a new scope !
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In the end, I think future failure modes are unknown and can't possibly be known. How do I know my brand new scope won't self-destruct? It's a Rigol, after all... I have no idea what was done to my Tek 485 in the years before I bought it. Floated? Not floated? Over-ranged? I don't know. Nor do I care, I just plugged it in and have used it for the last 14 years. Used it yesterday and it works fine!
If the risk is perceived to be high, junk the scope. Personally, I would replace the cord to ensure that the chassis and BNC connectors are grounded and put it back in service. I can't possibly predict some inter-winding anomaly and I doubt that I would worry so long as all exposed non-current carrying metallic surfaces were grounded. Even if the scope went into hand grenade mode, I doubt the damage would be all that extensive. A few pops, a few sizzles and that would be about it.
But in a group environment I wouldn't be so cavalier - I would let somebody else buy a different scope. I wouldn't want to be involved with the purchase either. Not my circus, not my monkeys!
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I have been reading this topic and silently wondering how far this thoughtful investigation about possible damages because the ground was disconnected would go. I am trying to understand the magnitude of this problem and somehow all what I see or can explain as a former el. engineer is that it is mostly a liability related issue (versus common sense) than anything else.
Just think for a second - in recent past many countries (like Soviet Union for example) did not have 3 pole plugs at all and it did not prevent them from creating, debugging, manufacturing, using different electronic devices to allow people to go to space or to the bottom of the ocean. I don't want to switch the topic and start a discussion whether or not my example covers all bases, but I think you got my point.
And when I think about liability issues in North America (let me be politically correct, since not all countries in North America are the same) I think that cases like Liebeck v. McDonald's Restaurants, when a 79-year-old woman who suffered third-degree burns in her pelvic region when she accidentally spilled hot coffee in her lap after purchasing it from a McDonald's restaurant was awarded $640,000 ( at first the amount was 2.8 million but later was reduced to $640,000). And this list is huge. And it even includes some urban legends like "Winnebago driver case" when the driver set the cruise control at 70 and went into the back to make coffee and later sued Winnebago for not advising him in the owner's manual that he couldn't actually do this ;-(
So is it really a technical problem being discussed here or something else?
PS: edited 10 minutes later - even my MacBook pro has two adapters - 2 and 3 pronged plugs (N.A. style) And whatever you connect through USB (a scope, a logic analyzer etc ) to your laptop will be floating too in case of 2 pronged plug. What is a big deal here?
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Regrettably I think that Electro Detective's reply 86 in https://www.eevblog.com/forum/testgear/floating-scopes/ (https://www.eevblog.com/forum/testgear/floating-scopes/) is the appropriate attitude, including ...
"I'm jumping ship (for once) and following the majority.. to recommend LEAVE WELL ALONE if not 100% sure,
or best practice is to dip into the piggy bank for a differential probe as front line measurement cannon fodder :-+
There are TOO MANY VARIABLES that are almost IMPOSSIBLE to document and break down here for the casual user wanting a 'fast fix'
which usually means a good chance for an earth/ground related BANG! "
Quite so, ED's reply is the 'modern' philosophy in today's litigation mad society where practices like floating scopes could never be taught for fear of culpability. It was by necessity back then as the vast bulk of scopes didn't have differential plugins nor were differential probes as widely available as they are now.
Ach. I don't think ED's statement supports the contention that the fear of culpability is a central issue. His "conversion" seems to be based on a realisation of the reality that there are subtle dangers that cannot be quickly and easily taught to newbies.
That floating was done in the past is, as you say, because the equipment wasn't (widely) available.
What I would offer as comment is a Tek 24** wasn't a cheap scope back then and any that would set it up as a dedicated floating scope would've known the risks to both themselves and instrument.
ISTR an anecdote on TekScopes about someone that realised it was dangerous, took precautions - and still died.
The likely extreme voltages it ever saw in floating mode would most likely be just rectified mains of which it could handle with comparative ease.
That could add 350V (240*sqrt(2)) across some components.
I get that if this scope is to be shared, culpability again rises it's ugly head where really any use of a scope and in fact any test gear exposes the operator to risk.
If this is too much to bear along with usage of a scope with uncertain history.....then buy a new scope !
The "it might hurt someone else" is the key consideration, as opposed to the Darwin Award.
If someone is prepared to float an old scope, there's nothing preventing them floating a new scope - but that's outside the scope of this thread.
Ways of increasing confidence in lack-of-danger is the topic of this thread.
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In the end, I think future failure modes are unknown and can't possibly be known. ...
If the risk is perceived to be high, junk the scope. Personally, I would replace the cord to ensure that the chassis and BNC connectors are grounded and put it back in service. I can't possibly predict some inter-winding anomaly and I doubt that I would worry so long as all exposed non-current carrying metallic surfaces were grounded. Even if the scope went into hand grenade mode, I doubt the damage would be all that extensive. A few pops, a few sizzles and that would be about it.
Those were my initial thoughts, until I did some research and came across the Tek statement. That raised doubts that my inaction might result in a newbie being hurt.
But in a group environment I wouldn't be so cavalier - I would let somebody else buy a different scope. I wouldn't want to be involved with the purchase either. Not my circus, not my monkeys!
Quite; that's part of the dilemma :(
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I have been reading this topic and silently wondering how far this thoughtful investigation about possible damages because the ground was disconnected would go. I am trying to understand the magnitude of this problem and somehow all what I see or can explain as a former el. engineer is that it is mostly a liability related issue (versus common sense) than anything else.
...
So is it really a technical problem being discussed here or something else?
I've tried to frame it as a technical problem which can, hopefully, be resolved technically.
There are indeed non-technical issues that also need to be addressed. The technical issue may be a good way to force other people to deal with the non-technical issues - but that's a different argument.
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Regrettably I think that Electro Detective's reply 86 in https://www.eevblog.com/forum/testgear/floating-scopes/ (https://www.eevblog.com/forum/testgear/floating-scopes/) is the appropriate attitude, including ...
"I'm jumping ship (for once) and following the majority.. to recommend LEAVE WELL ALONE if not 100% sure,
or best practice is to dip into the piggy bank for a differential probe as front line measurement cannon fodder :-+
There are TOO MANY VARIABLES that are almost IMPOSSIBLE to document and break down here for the casual user wanting a 'fast fix'
which usually means a good chance for an earth/ground related BANG! "
Quite so, ED's reply is the 'modern' philosophy in today's litigation mad society where practices like floating scopes could never be taught for fear of culpability. It was by necessity back then as the vast bulk of scopes didn't have differential plugins nor were differential probes as widely available as they are now.
Ach. I don't think ED's statement supports the contention that the fear of culpability is a central issue. His "conversion" seems to be based on a realisation of the reality that there are subtle dangers that cannot be quickly and easily taught to newbies.
That floating was done in the past is, as you say, because the equipment wasn't (widely) available.
Yes but if any of us on the forum suggest floating scopes, aren't we in some way culpable ?
The dangers aren't subtle, they're deadly !
What I would offer as comment is a Tek 24** wasn't a cheap scope back then and any that would set it up as a dedicated floating scope would've known the risks to both themselves and instrument.
ISTR an anecdote on TekScopes about someone that realised it was dangerous, took precautions - and still died.
As per above and user error cannot be overlooked.
Drive a car at highway speeds on back roads without a seatbelt and the result can be similar.
The likely extreme voltages it ever saw in floating mode would most likely be just rectified mains of which it could handle with comparative ease.
That could add 350V (240*sqrt(2)) across some components.
Exactly but it's an offset that the whole scope is subjected to not so much a stress on individual componentry across the whole scope. The PSU is where the greatest stress will be.
I get that if this scope is to be shared, culpability again rises it's ugly head where really any use of a scope and in fact any test gear exposes the operator to risk.
If this is too much to bear along with usage of a scope with uncertain history.....then buy a new scope !
The "it might hurt someone else" is the key consideration, as opposed to the Darwin Award.
If someone is prepared to float an old scope, there's nothing preventing them floating a new scope - but that's outside the scope of this thread.
Ways of increasing confidence in lack-of-danger is the topic of this thread.
The safest move is to restore the mains Gnd, period. That assumes the supply ground is sound and in perfect order.
In the end, I think future failure modes are unknown and can't possibly be known. ...
If the risk is perceived to be high, junk the scope. Personally, I would replace the cord to ensure that the chassis and BNC connectors are grounded and put it back in service. I can't possibly predict some inter-winding anomaly and I doubt that I would worry so long as all exposed non-current carrying metallic surfaces were grounded. Even if the scope went into hand grenade mode, I doubt the damage would be all that extensive. A few pops, a few sizzles and that would be about it.
Those were my initial thoughts, until I did some research and came across the Tek statement. That raised doubts that my inaction might result in a newbie being hurt.
And actually how old is that Tek statement ?
You can be quite sure their documentation has been updated to make users fully aware of the risks of floating scopes so not to expose themselves to today's litigation mad world.
Many Tek documents that were once on their website are now gone without trace unless you can find them stashed on another website.
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I've tried to frame it as a technical problem which can, hopefully, be resolved
There is no technical resolution. As stated, there are basic checks that can be done on any old piece of test equipment. Beyond that, common sense measures like GFCI outlets and basic safety rules is all you can do. If that doesn’t satisfy your safety and litigation fears, then removing yourself from the environment (or position of responsibility) is likely the only way to resolve your concerns.
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MODERATOR: User analias has been banned for operating multiple accounts.
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I just now noticed what the OP's username is. I wonder why?