Author Topic: Why Can Not Using a Differential Probe Blow Up Scope  (Read 8321 times)

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Offline bostonmanTopic starter

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Why Can Not Using a Differential Probe Blow Up Scope
« on: August 18, 2022, 06:05:55 pm »
I'm sure this has been asked many times, but I'm confused about probe grounding and how a scope can get damaged.

The ground lead on a scope is Earth ground. If I measure a battery or a circuit referenced to a battery, the probe ground lead would go to circuit ground. This would tie battery ground to Earth ground.

Now if I measure a circuit all referenced to the secondary of a transformer, maybe a capacitor on a full wave bridge rectifier, normally I just stick probe ground onto any bare metal I can find.

If the secondary is all referenced to the transformer, why does this not cause the measurements to be incorrect?

Also, how does a scope blow up just because someone takes the probe ground, uses it on the secondary, and measures the filter capacitor?

I'm thinking of getting a differential probe, but I'm uncertain when to use it, why I need to use it, and whether using it will be as accurate as using an expensive scope probe.
 

Offline james_s

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Re: Why Can Not Using a Differential Probe Blow Up Scope
« Reply #1 on: August 18, 2022, 06:15:42 pm »
You should watch Dave's video on exactly this topic.

In a nutshell, probing a battery power circuit or something on the secondary side of a transformer does NOT require a differential probe. Consider though something like a SMPS where the transformer is in the middle of the circuit rather than right on the input. In this case "ground" on the primary side can be floating at mains voltage above earth ground so if you connect the scope ground (which is tied directly to earth) to this point it should be obvious why you're going to get a flash & bang.
 

Offline Siwastaja

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Re: Why Can Not Using a Differential Probe Blow Up Scope
« Reply #2 on: August 18, 2022, 06:49:38 pm »
There is no magic, it's really simple. The "ground clips" of the channels just are internally connected together, and further to the protective earth of the mains plug.

Whenever you would connect the ground clips (or the mains protective earth) to different circuit nodes so that there exists a path for current, you will create a short circuit through the scope. That's the problem.
 
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Offline bostonmanTopic starter

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Re: Why Can Not Using a Differential Probe Blow Up Scope
« Reply #3 on: August 19, 2022, 02:51:14 pm »
Quote
You should watch Dave's video on exactly this topic.

Good suggestion, I just watched the video.

The visual was a big help as I somewhat understood the concept, but seeing was better. Floating grounds, placing DC voltage on a ground to change the reference point, etc.. have always been confusing. I like to think ground is 0V, and everything is referenced to just plain old "ground".

Anyway, two questions. First is Dave mentioned that providing the device doesn't have Earth ground on the IEC connector, then the scope ground lead can be connected anywhere. Just to confirm, I think that is assuming it's connected "anywhere" on the secondary side of the transformer. I'm not going to try this, but thought if I touch HOT on my AC receptacle to Earth ground, then I'll pop the breaker. I believe this is true because Earth ground is isolated at my house (it's just a rod that goes several feet into the ground), but, neutral is tied in with Earth ground back at the power station. Obviously correct me if I'm wrong.

The other question is regarding somewhat my original question. How does the scope physically blow up by touching the Earth ground lead to a high voltage/current source? I understand the physical path current takes (more now after watching the video), but I imagine any current is just pulled to Earth ground bypassing all the electronics the scope until (hopefully) a circuit breaker or fuse pops or the scope probe blows up in your hand because the thin ground wire couldn't handle the high current thus immediately heating until it melts off.

In other words, I touch the scope ground lead to a mains, and the probe can blow up in my hand and/or the ground wire on the probe can melt until it's an open circuit. Simple enough to understand and certainly not a good position to be in because of injury risks. Assuming the ground lead and/or probe doesn't blow up in my hand to open the circuit (and ignoring whatever damage can happen to the unit I'm measuring. i.e. opening a trace, popping the fuse, etc...), I'd assume the only "damage" to the oscilloscope would be a ground wire opening inside the scope which would (hopefully) be a simple fix (open the scope, solder a new wire from the BNC to Earth ground, and remove the old burnt wire).

The only situation I can imagine is that the spark arcs to the electronics and blows up the scope, but, as quickly and easily this can happen, I'm questioning a theoretical situation where high current simply flows to Earth ground until a breaker/fuse opens.
 

Offline TimNJ

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Re: Why Can Not Using a Differential Probe Blow Up Scope
« Reply #4 on: August 19, 2022, 04:10:43 pm »
I haven't watched the video in a long time, but regarding this...

First is Dave mentioned that providing the device doesn't have Earth ground on the IEC connector, then the scope ground lead can be connected anywhere.

It doesn't matter if a device physically has an earth pin or not. If you are trying to measure a mains circuit (i.e. on the primary-side of a transformer, or a circuit with no transformer at all), you cannot place the ground clip at any point which is not actually the building's earth. That's the only rule.

This is because earth and neutral are galvanically tied together somewhere in the building.

I think what he was saying was: If you are supplying your DUT from a power supply (i.e. wall wart) with only two AC input blades, the output (secondary-side of the transformer) is guaranteed to be galvanically isolated from mains. In this particular case, if your DUT is powered by this isolated supply, you can put your ground clip wherever you want in your DUT's circuit.
 

Offline Faringdon

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Re: Why Can Not Using a Differential Probe Blow Up Scope
« Reply #5 on: August 19, 2022, 04:40:54 pm »
Just noting there is a serious electrical warning in this post.......in italics (i dont want  anyone to come to grief)

If you remove the earth from your scope's mains plug, then you can connect the scope ground anywhere (well, anywhere on a typical mains circuit with voltages below ~450v from earth potential) and you wont get blow up.
However, that gives you a "floating scope".
I have worked in places where they do just this.
However, if you do it, then remember that your scope ground connects to earth "normally".........and that earth connection , in your scope's power supply, conencts to the y capacitors which also connect , at their other ends, to mains line and mains neutral.......so in other words, if you cut off the earth from the scope.........then do not touch the scope's ground clip while the scope is plugged in....because you will get a capactive shock through the y caps.......do not touch anything metal on the scope.......if you want to change the scope probe position, you have to unplug the scope, and move the probe, then plug the scope back in..

If you are like me, you have a micsig dp10007 diff probe, which is good, but cannot be used to well  probe signals , as the "dangley" probes pickup loads of common mode noise......so if i need to see a signal  nicely, then i sideline the micsig....and i  feed the scope from an  100W mains isolation transformer....and cut off the earth from the scope plug.....and then use a home-brew coaxial probe to probe the signal....and i make heck sure that i dont touch the coax probe whilst the scope is plugged in
« Last Edit: August 19, 2022, 04:49:35 pm by Faringdon »
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Offline james_s

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Re: Why Can Not Using a Differential Probe Blow Up Scope
« Reply #6 on: August 19, 2022, 04:52:36 pm »
"Ground" is an arbitrary concept, it means a point in the circuit from which all voltage measurements are referenced, this can actually be any point in the circuit but usually the node that connects to the negative terminal of the battery or bridge rectifier is used. It may or may not be tied to earth ground, in fact "ground" in a circuit can be the live wire coming out of the wall, it just depends on the circuit. Ground of the scope is earth ground, if you try to connect this to a ground that is at some other potential then current is going to flow.

The scope is not going to literally blow up as in explode, but it can be damaged. The reason should be obvious, if you create a short circuit through the ground clip of your probe a large current is going to flow until the weakest link fails. This could mean a trace on the scope PCB is vaporized, or the probe cable burns up or any number of other things depending on the path the current takes.

Until you understand this concept intuitively like the back of your hand, don't connect the scope to anything that plugs into the wall unless it uses an external power brick or wall wart.
 

Offline Ice-Tea

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Re: Why Can Not Using a Differential Probe Blow Up Scope
« Reply #7 on: August 19, 2022, 05:03:45 pm »
1) I personally feel floating of a scope should not even be mentioned, especially not to someone who is slowly learning about "earth" "zero volt" and the like. The idea should not even be enterained untill those concepts are clear as day.
2)
However, if you do it, then remember that your scope ground connects to earth "normally".........and that earth connection , in your scope's power supply, conencts to the y capacitors which also connect , at their other ends, to mains line and mains neutral......

This is an underestimation of the risk. One would float a scope to be able to probe mains, mostly, so all exposed metal (probe clip, BNC shell, screw on thousing, whatever) will be live. Connecting to line and neutral through Y caps is the least of your worries as those connections would be, by design, quite safe.

3) In addition, it is my personal belief scopes should not be floated period, not even with an isolation transfo. That gear is designed to work correctly (ie conform to safety and EMC standards) with earth connected.
 
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Offline rsjsouza

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Re: Why Can Not Using a Differential Probe Blow Up Scope
« Reply #8 on: August 19, 2022, 05:25:05 pm »
1) I personally feel floating of a scope should not even be mentioned,
I personally always disagreed with this. Anyone in this field with two neurons can think of isolating the earth to measure something. The subject must be mentioned at all times and the proper disclaimers must be emphasized.

Not to mention there are test equipment that are earth floated by design - especially the older models of oscilloscopes, function generators, etc. Knowing the pitfalls is very wise. 

2)
However, if you do it, then remember that your scope ground connects to earth "normally".........and that earth connection , in your scope's power supply, conencts to the y capacitors which also connect , at their other ends, to mains line and mains neutral......

This is an underestimation of the risk. One would float a scope to be able to probe mains, mostly, so all exposed metal (probe clip, BNC shell, screw on thousing, whatever) will be live. Connecting to line and neutral through Y caps is the least of your worries as those connections would be, by design, quite safe.
Indeed. The oscillloscope or any other test gear will be fully isolated if powered by an isolated power supply, evem with the neglectful leakage of the Y capacitors in case a switching supply is used (power transformer designs have even more neglectful currents). The real problem is you (the operator) becoming the wire that  closes the circuit between the equipment being tested and ground.
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Offline james_s

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Re: Why Can Not Using a Differential Probe Blow Up Scope
« Reply #9 on: August 19, 2022, 05:28:31 pm »
I personally always disagreed with this. Anyone in this field with two neurons can think of isolating the earth to measure something. The subject must be mentioned at all times and the proper disclaimers must be emphasized.

Not to mention there are test equipment that are earth floated by design - especially the older models of oscilloscopes, function generators, etc. Knowing the pitfalls is very wise. 

You're entitled to an opinion, but just remember that competent, experienced professional engineers have been killed by floating a scope to take a measurement. We're dealing with a beginner who doesn't fully understand the concept of ground here, it would be negligent to even suggest floating a scope to them. Yes there is equipment that is designed to be floating, that isn't what we're discussing here.
 
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Offline Faringdon

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Re: Why Can Not Using a Differential Probe Blow Up Scope
« Reply #10 on: August 19, 2022, 05:44:08 pm »
Quote
Also, how does a scope blow up just because someone takes the probe ground, uses it on the secondary, and measures the filter capacitor?
...Aaaah, this may be something different....
I once unplugged a mains circuit, and there was residual voltage left on  a capacitor, without me realising. I had the scope ground clip connected to the capacitor ground.
I then took my soldering iron (which obviously has an earthed tip)....and  then soldered the positive terminal of the capacitor......so basically i shorted the capacitor out.
Maybe thats what you are referring to?

Anyway, another method for you to probe, without having to cut off the earth from the scope mains plug...would be to supply your DUT via a mains isolation transformer, (MIT) and be sure that this M.I.T   doesnt connect earth to  either of the secondary outputs. Then you can put your probe ground anywhere on that DUT,  and it doesnt have to be a diff probe in that case. Obviously still dont touch the high voltage isolated outputs.
« Last Edit: August 19, 2022, 05:49:38 pm by Faringdon »
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Offline bdunham7

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Re: Why Can Not Using a Differential Probe Blow Up Scope
« Reply #11 on: August 19, 2022, 06:09:12 pm »
You're entitled to an opinion, but just remember that competent, experienced professional engineers have been killed by floating a scope to take a measurement. We're dealing with a beginner who doesn't fully understand the concept of ground here, it would be negligent to even suggest floating a scope to them. Yes there is equipment that is designed to be floating, that isn't what we're discussing here.

I won't disagree with the overall notion that this is not an intro-level subject, but do keep in mind that the one known example (AFAIK) of an experienced, professional engineer getting electrocuted this way involved the apparently common but obviously dangerous method of simply breaking off or disconnecting the ground pin using regular mains power.  Or course even an isolation transformer wouldn't have helped in that case since I believe it was the high-potential DUT that energized the scope.

Another good reason not to go floaty-scope is that it may not work very well--you may end up looking at a whole bunch of mains power artifacts.  In every case I've ever used isolation, isolating the DUT is the only reasonable way to do it.  The only exception I can think of outside of a high voltage laboratory--and I haven't run into it--is if you might need to float the scope with a low-voltage DUT that cannot easily be isolated from ground.  But that would be a more advanced topic with its own hazards, so I'll agree that in the current context, the answer to the idea of floating the scope should be a firm no.
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Offline james_s

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Re: Why Can Not Using a Differential Probe Blow Up Scope
« Reply #12 on: August 19, 2022, 07:57:16 pm »
I won't disagree with the overall notion that this is not an intro-level subject, but do keep in mind that the one known example (AFAIK) of an experienced, professional engineer getting electrocuted this way involved the apparently common but obviously dangerous method of simply breaking off or disconnecting the ground pin using regular mains power.  Or course even an isolation transformer wouldn't have helped in that case since I believe it was the high-potential DUT that energized the scope.

Another good reason not to go floaty-scope is that it may not work very well--you may end up looking at a whole bunch of mains power artifacts.  In every case I've ever used isolation, isolating the DUT is the only reasonable way to do it.  The only exception I can think of outside of a high voltage laboratory--and I haven't run into it--is if you might need to float the scope with a low-voltage DUT that cannot easily be isolated from ground.  But that would be a more advanced topic with its own hazards, so I'll agree that in the current context, the answer to the idea of floating the scope should be a firm no.

I would bet that it has happened more than once, and there have almost certainly been many more shocks resulting in various degrees of injury that were not fatal. I have floated a scope before and lived to tell the tale but I've also had a few close calls that were frankly too close for comfort and very easily could have been my last. It's one of those things that can have valid use cases but it should NEVER be attempted by anyone who doesn't fully understand what they are doing, and the OP clearly has not yet reached that level of understanding. It's best to just not even bring it up.
 

Offline rsjsouza

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Re: Why Can Not Using a Differential Probe Blow Up Scope
« Reply #13 on: August 19, 2022, 08:40:28 pm »
I personally always disagreed with this. Anyone in this field with two neurons can think of isolating the earth to measure something. The subject must be mentioned at all times and the proper disclaimers must be emphasized.

Not to mention there are test equipment that are earth floated by design - especially the older models of oscilloscopes, function generators, etc. Knowing the pitfalls is very wise. 

You're entitled to an opinion, but just remember that competent, experienced professional engineers have been killed by floating a scope to take a measurement. We're dealing with a beginner who doesn't fully understand the concept of ground here, it would be negligent to even suggest floating a scope to them. Yes there is equipment that is designed to be floating, that isn't what we're discussing here.
Perhaps my writing was not entirely clear; I don't condone the practice of floating a test instrument. What I disagree is to prevent the subject from being discussed entirely for the sake of protection. This is not protection but instead an irresponsible way to leave the gap open for careless experimentation by newbies.

Anyone involved with this field is most probably curious, inventive and industrious - it doesn't take much from a newcomer to think that it is a practical idea how to circumvent something so simple without considering all the risks.
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Online tautech

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Re: Why Can Not Using a Differential Probe Blow Up Scope
« Reply #14 on: August 20, 2022, 12:40:35 am »
Anyone involved with this field is most probably curious, inventive and industrious - it doesn't take much from a newcomer to think that it is a practical idea how to circumvent something so simple without considering all the risks.
Exactly and for that reason alone we must do anything we can to discourage such dangerous practices....even talking in any detail about this sets curious minds off to investigate.   ::)

If we are to be all properly responsible in these days of cheap differential probes we should all be shouting in unison........... DON'T FLOAT YOUR SCOPE !
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Offline magic

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Re: Why Can Not Using a Differential Probe Blow Up Scope
« Reply #15 on: August 20, 2022, 07:52:07 am »
Nice, another scope floating war :popcorn:


So back to the topic,
I haven't watched the video in a long time, but regarding this...

First is Dave mentioned that providing the device doesn't have Earth ground on the IEC connector, then the scope ground lead can be connected anywhere.

It doesn't matter if a device physically has an earth pin or not. If you are trying to measure a mains circuit (i.e. on the primary-side of a transformer, or a circuit with no transformer at all), you cannot place the ground clip at any point which is not actually the building's earth. That's the only rule.

This is because earth and neutral are galvanically tied together somewhere in the building.

I think what he was saying was: If you are supplying your DUT from a power supply (i.e. wall wart) with only two AC input blades, the output (secondary-side of the transformer) is guaranteed to be galvanically isolated from mains. In this particular case, if your DUT is powered by this isolated supply, you can put your ground clip wherever you want in your DUT's circuit.
Emphasis added and this is right.

The part about neutral and earth being tied together is true, but you shouldn't put scope ground clips on neutral. Firstly, it may cause a fraction of neutral currents to return through the probe and scope's earth connection and trip the RCD in your building. Secondly, if you have no RCD or a broken one and neutral connection fails somewhere upstream, all your mains return current will flow through the probe and scope, possibly damaging them. Lastly, if both the neutral and earth disappear but live doesn't, you will have full mains voltage on all exposed metal of the scope.

You certainly can't connect the grounding clip to any arbitrary point of a circuit running directly on high voltage from mains. That's almost guaranteed magic smoke.
 

Online tggzzz

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Re: Why Can Not Using a Differential Probe Blow Up Scope
« Reply #16 on: August 20, 2022, 08:21:03 am »
You're entitled to an opinion, but just remember that competent, experienced professional engineers have been killed by floating a scope to take a measurement. We're dealing with a beginner who doesn't fully understand the concept of ground here, it would be negligent to even suggest floating a scope to them. Yes there is equipment that is designed to be floating, that isn't what we're discussing here.

I won't disagree with the overall notion that this is not an intro-level subject, but do keep in mind that the one known example (AFAIK) of an experienced, professional engineer getting electrocuted this way involved the apparently common but obviously dangerous method of simply breaking off or disconnecting the ground pin using regular mains power.  Or course even an isolation transformer wouldn't have helped in that case since I believe it was the high-potential DUT that energized the scope.

Another good reason not to go floaty-scope is that it may not work very well--you may end up looking at a whole bunch of mains power artifacts.  In every case I've ever used isolation, isolating the DUT is the only reasonable way to do it.  The only exception I can think of outside of a high voltage laboratory--and I haven't run into it--is if you might need to float the scope with a low-voltage DUT that cannot easily be isolated from ground.  But that would be a more advanced topic with its own hazards, so I'll agree that in the current context, the answer to the idea of floating the scope should be a firm no.

"One known example"? I think you probably mean "one example posted on this forum". Absence of evidence (or other fatalities) is not evidence of absence.

Just to remind the OP, and others...

Example fatality from https://groups.io/g/TekScopes/topic/7632418#10795 with my emphasis
Quote
What makes it so dangerous to float the scope is that it is very easy for you to come in accidental contact with the floating scope chassis and receive a very bad shock, possibly lethal. One of my first customer contacts as a Sales Engineer for Tektronix was to call on the Sylvania Lighting Center in Danvers, MA and investigate a rumor about an engineer working there that was killed while using a Tek scope. I found it it was true. During lunch, one of the engineers was working alone in the lab on a lighting experiment that was using some 3 phase, 220 volt power. He needed to make some measurements between points none of which were at earth ground. So, he floated the scope . . . He even has the scope sitting on a scope cart with a sheet of insulation material between the bottom of the scope and the metal tray it normally sits in so the scope cart would not be "hot" with the scope. He also had a "tunnel" of plexiglas on both sides and over the top of the scope in a crude attempt to prevent anyone from accidentally touching the hot scope. The back was not covered with plexiglass in order to allow the fan to do its job and the front was not covered so the engineer could access the scope controls. This guy was WELL AWARE of the danger and took a lot of precautions to prevent shock . . . Bottom Line: He died anyway.
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Online tggzzz

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Re: Why Can Not Using a Differential Probe Blow Up Scope
« Reply #17 on: August 20, 2022, 08:26:47 am »
I'm thinking of getting a differential probe, but I'm uncertain when to use it, why I need to use it, and whether using it will be as accurate as using an expensive scope probe.

Make sure it is the right type of probe (or scope) for the job - and that includes the right type of differential probe; yes there are several.

FFI at
https://www.eevblog.com/forum/projects/isolation-transformer-for-scope/msg2259465/#msg2259465
https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/
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Offline magic

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Re: Why Can Not Using a Differential Probe Blow Up Scope
« Reply #18 on: August 20, 2022, 08:33:38 am »
He also had a "tunnel" of plexiglas on both sides and over the top of the scope in a crude attempt to prevent anyone from accidentally touching the hot scope. The back was not covered with plexiglass in order to allow the fan to do its job and the front was not covered so the engineer could access the scope controls. This guy was WELL AWARE of the danger and took a lot of precautions to prevent shock.
Emphasis inverted ;)
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Offline Psi

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Re: Why Can Not Using a Differential Probe Blow Up Scope
« Reply #19 on: August 20, 2022, 08:58:56 am »
I'm sure this has been asked many times, but I'm confused about probe grounding and how a scope can get damaged.

Usually it's the device/circuit you're testing that gets damaged from probe ground errors, not the scope.

The device gets a dead short across some of its circuitry.
The scope gets an unusually large current flowing through its earth connections. but that's normally BNC terminals to the shielding case and the case to the mains earthing wire. Not through chips or components so it usually doesn't damage the scope. There are of course exceptions if the current has to go through PCB traces to get to mains earth. But usually they can handle a fair bit of fault current, more than the device your testing can.
And even when it does blow out a trace on the scopes PCB it's an easy fix for anyone who has the skill to use a scope in the first place.
« Last Edit: August 20, 2022, 09:05:53 am by Psi »
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Offline magic

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Re: Why Can Not Using a Differential Probe Blow Up Scope
« Reply #20 on: August 20, 2022, 09:20:17 am »
There are of course exceptions if the current has to go through PCB traces to get to mains earth. But usually they can handle a fair bit of fault current, more than the device your testing can.
That's true until you come up with something like grounding the negative of an SPMS primary, which basically goes straight to mains live for half of each power cycle. Then it's a race for survival between the rectifier, SMPS PCB and scope PCB.

The scope input may have an advantage of ground planes ::)
 

Offline Psi

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Re: Why Can Not Using a Differential Probe Blow Up Scope
« Reply #21 on: August 20, 2022, 09:21:37 am »
yep, there are exceptions.
But if the scope is built properly and has a metal cage around it with the BNCs attached and a earth wire terminal to the mains cable then it's pretty unlikely to take out your scope.
« Last Edit: August 20, 2022, 09:23:09 am by Psi »
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Re: Why Can Not Using a Differential Probe Blow Up Scope
« Reply #22 on: August 20, 2022, 09:29:46 am »
yep, there are exceptions.
But if the scope is built properly and has a metal cage around it with the BNCs attached and a earth wire terminal to the mains cable then it's pretty unlikely to take out your scope.

Have you tried that, or is that statement based solely on assumptions?

What are your other implicit assumptions? If made explicit, would they change the outcome? I suspect they would :)
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Offline bostonmanTopic starter

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Re: Why Can Not Using a Differential Probe Blow Up Scope
« Reply #23 on: August 20, 2022, 01:34:08 pm »
Quote
The scope gets an unusually large current flowing through its earth connections. but that's normally BNC terminals to the shielding case and the case to the mains earthing wire. Not through chips or components so it usually doesn't damage the scope.


I think my original confusion and why I initially posted the question was due to always hearing the scope can blow up. Normally I attach the ground lead to chassis ground, and, if it's obvious that is an isolated ground, I seek the negative/return. Most of my high voltage measurements have been at most AC input, but the majority are always low voltage secondary transformer side measurements. I have measured AC directly across the plug, but always make sure I know which side is neutral before doing so (I actually found a receptacle in my house that had a reverse HOT and NEUTRAL by way of one of those cheap LED testers that plug in and give you a sequence of lights that show the state of the receptacle - so it's not safe to assume that NEUTRAL is actually correctly connected).

My confusion came because I know the ground lead is Earth ground and touching a "hot" will cause a short circuit, but, the more I kept hearing that the scope can blow up, the more confused I got about the so called ground loop. Ignoring the scope probe blowing up in my hand, and blowing up the DUT (or hopefully the fuse first), I visualized a scenario where touching the ground lead to the wrong point will cause the scope to float so high that the internal electronics blows up due to nothing more than it floating. This doesn't seem to be the case. From my understanding, providing I keep the voltage below the maximum input voltage (400V ?), the scope isn't going to simply blow up because the ground lead floated (ignoring that I'm shorting a point to Earth ground and something will blow up).

As usual, maybe I over thought this and didn't need to ask the question. A very common situation is a power supply with three prongs, the scope with three prongs, scope probe ground lead touches a "hot" point, the fuse blows on the DUT, and nothing bad happens. A theoretical situation I imagine is that the instantaneous current is so bad that it blows open ground etches and pathways to ground in the scope causing the electricity on the point inside the scope that just opened to seek a path to ground and finds it through an adjacent IC chip thus "blowing" up the scope.

As for floating the scope, electronics that have ground points floating above ground, high voltage power supplies, etc... these are most things I stay away from because I don't grasp enough about grounding to feel safe gambling with anything (or being around high voltage especially if I'm alone). Sometimes "Earth" ground makes sense to me, other times I get confused. In some equipment, such as a weed wacker, Earth ground is a secondary grounding source to avoid being electrocuted should the neutral open but you won't get a shock should the third prong go missing. In other stuff, such as maybe a refrigerator, with the third prong broken off, the device will give you a shock. Rhetorically asking, why does one pose a shock risk and the other doesn't?

A silly question: why not install a PICO fuse in the ground lead so it opens before (hopefully) any injuries occur or damage to the scope?

Regarding differential probes, I assume they are not bullet proof meaning measuring something you shouldn't can cause an arc thus taking out the scope. In any case, it seems using them is certainly the safest method, however, are they reducing signal quality/bandwidth?

If my scope probe is some high end HP going into a high frequency scope (my four channel is 600MHz), and I use a diff probe, then I'm reducing signal quality.

What about measuring voltages without the ground lead connected, can this cause the scope to blow? Sometimes I've measured signals where it didn't make any difference whether ground was connected or not, but I assume the connection was through Earth ground.



 

Offline bdunham7

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Re: Why Can Not Using a Differential Probe Blow Up Scope
« Reply #24 on: August 20, 2022, 02:07:45 pm »
"One known example"? I think you probably mean "one example posted on this forum". Absence of evidence (or other fatalities) is not evidence of absence.

The example you've repeated is commonly repeated and is the only one I'm aware of.  Are there others?  Perhaps, but I don't know and nobody else seems to know of any, so they're unknown to me,  you and the rest of us.  Unless you know of another, in which case please enlighten us.
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 


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