Connecting an Oscilloscope to the primary side of a SMPS

[tggzzz]

This thread is troubling me because I have been directed in the past by a very experienced EE to float my scope for certain measurements. However in my case, the DUT was not powered by mains. It was powered by a supply giving it 48V. Is floating the scope still ill-advised in this case?

In your case, if your DUT was battery powered, why was it (a) necessary and (b) beneficial to float the scope?

More generally, there can be significant capacitance between a scope's case and other conductors. Some nodes don't tolerate excess capacitance, which could lead to erroneous traces and/or DUT damage.

Very generally, it is reasonable to walk out into a quiet cul-de-sac road at night without looking. That doesn't mean it is sensible to do it on other roads at other times, does it?

Once you have answered that, read this https://groups.io/g/TekScopes/message/10754 Key sentence: "This guy was WELL AWARE of the danger and took a lot of precautions to prevent shock . . . Bottom Line: He died anyway."

[shapirus]

Once you have answered that, read this https://groups.io/g/TekScopes/message/10754 Key sentence: "This guy was WELL AWARE of the danger and took a lot of precautions to prevent shock . . . Bottom Line: He died anyway."

But what exactly did he do that caused his death? That message doesn't tell the details, which leaves the question unanswered. Did he take a lot of precautions and then touch a metal knob on the scope? Did he take a lot of precautions and then insert two nails with bare hands into the wall outlet? Did he take a lot of precautions and then walk out of the window?

[coppice]

Once you have answered that, read this https://groups.io/g/TekScopes/message/10754 Key sentence: "This guy was WELL AWARE of the danger and took a lot of precautions to prevent shock . . . Bottom Line: He died anyway."

That doesn't seem to say what killed him. I assume he used the oscilloscope's controls, which is pretty hazardous with an, old mostly metal, oscilloscope. It would be interesting to know, though. I've only seen someone hurt by a grounded oscilloscope, when a probe touched something it shouldn't, and he suffered metal vapour deposition on his hand and arm. As I said earlier, safety is a process, not a product.

[tggzzz]

Those questions are asking for overly specific details that distract from the general lessons. Similar question: what caused the loss of an empire? Answer: a nail was missing from a horseshoe. Accurate, but not helpful.

On one level what caused it is that he went into work that morning. On another perhaps his supervisor had set unrealistic expectations, or he had had an argument with someone. Maybe one end of a wire fell out his pocket and touched the scopes' case.

None of those would have mattered if he hadn't floated the scope. That's the key point.

If the answer to the question is that he touched the controls, some bright spark would latch onto that and claim everything would have been OK if he hadn't touched the controls (and that scope controls shouldn't be touched when the power is on!) Don't believe me? Look at some chains of "reasoning" elsewhere on this forum!

[bdunham7]

Those questions are asking for overly specific details that distract from the general lessons. Similar question: what caused the loss of an empire? Answer: a nail was missing from a horseshoe. Accurate, but not helpful.
...
None of those would have mattered if he hadn't floated the scope. That's the key point.

Your method would make aviation safety investigations quick and simple...

[Sighound36]

For all SMPS work I use a fibre optic full isolated voltage probe with 150Mhz BW, when dealing with small voltages that float on top of larger ones ALWAYS isolate the probe (or use a fully isolated probe) or use a scope with a fully isolated input usually a battery powered scope.

[shapirus]

Those questions are asking for overly specific details that distract from the general lessons.

They don't. The rules are only useful when it is clearly explained what exactly causes a given consequence, so much so that it makes any doubt impossible.

The reason why it may be dangerous to do what that guy did is vague, unless it is explained how exactly he got himself killed.

In other words, knowing the rules is only half the job of staying safe. Understanding them and the physical effects and processes which are behind those rules make it complete.

[tggzzz]

Those questions are asking for overly specific details that distract from the general lessons. Similar question: what caused the loss of an empire? Answer: a nail was missing from a horseshoe. Accurate, but not helpful.
...
None of those would have mattered if he hadn't floated the scope. That's the key point.

Your method would make aviation safety investigations quick and simple...

True

Nonetheless, the AAIB reports I've read highlight generic lessons such as that. They also mention any other contributory causes.

Example: the glider which was hit by lightning near Dunstable Downs and exploded. That lead to improvements in "plastic" airliners w.r.t. lightning strikes.

Or all the airspace infringements by power planes relying on electronic maps that choose not to include significant hazards

[tggzzz]

Those questions are asking for overly specific details that distract from the general lessons.

They don't. The rules are only useful when it is clearly explained what exactly causes a given consequence, so much so that it makes any doubt impossible.

From that we can deduce you aren't a parent.

"Don't run into the road without looking" is sound advice. It really isn't necessary to discuss which bit of the car impacts the body, nor what happens to the brain when it is subjected to rotational acceleration. Unless, that is, you have some blancmange handy and are prepared to sacrifice it in the name of science, safety, and fun.

[shapirus]

From that we can deduce you aren't a parent.

Wrong.

"Don't run into the road without looking" is sound advice.

It is. It's not sufficient to be effective enough, this is why I add: "...because you can be hit by a car", because otherwise the child will have a "why?", which, if not asked, will leave the question unanswered, which can lead to ignoring the advice and, worse, maybe trying experimentally what would happen. This short explanation is both simple and sufficient even for a child: it is obvious that getting hit by a car is going to hurt. No further details will normally be requested (in my case, however, sometimes they will be).

In times past children were taught using rods and other equipment to follow the rules, so even if they did not fully understand the real reasons why the rules must be obeyed, they at least understood that they would be beaten if they break them. Nowadays those methods are considered inappropriate, therefore children need to get and understand a reasonable explanation, at least, in as many cases as possible.

It's the same with electricity. There is no need (in this case) to explain the mechanism of how exactly electricity affects the internal organs and precisely what of that can cause death, but it is essential to explain the possible ways in which a dangerous potential can come into contact with the human body, causing a potentially lethal current passing through it, when the oscilloscope is used in an unsafe way.

[nctnico]

Those questions are asking for overly specific details that distract from the general lessons.

They don't. The rules are only useful when it is clearly explained what exactly causes a given consequence, so much so that it makes any doubt impossible.

The problem is that some are incapable of explaining clearly or comprehending the reasons why so there can be a lot of mismatch between the transmitter and receiver of the message. Adhering to 'Do as I say, not as I do' is a good default assumption.

[tggzzz]

From that we can deduce you aren't a parent.

Wrong.

"Don't run into the road without looking" is sound advice.

It is. It's not sufficient to be effective enough, this is why I add: "...because you can be hit by a car", because otherwise the child will have a "why?", which, if not asked, will leave the question unanswered, which can lead to ignoring the advice and, worse, maybe trying experimentally what would happen. This short explanation is both simple and sufficient even for a child: it is obvious that getting hit by a car is going to hurt. No further details will normally be requested (in my case, however, sometimes they will be).

In times past children were taught using rods and other equipment to follow the rules, so even if they did not fully understand the real reasons why the rules must be obeyed, they at least understood that they would be beaten if they break them. Nowadays those methods are considered inappropriate, therefore children need to get and understand a reasonable explanation, at least, in as many cases as possible.

It's the same with electricity. There is no need (in this case) to explain the mechanism of how exactly electricity affects the internal organs and precisely what of that can cause death, but it is essential to explain the possible ways in which a dangerous potential can come into contact with the human body, causing a potentially lethal current passing through it, when the oscilloscope is used in an unsafe way.

You snipped relevant context from my message, and proceeded to a straw man argument.

But then you reversed that position and started agreeing (in your last paragraph) with the point I was making. Strange.

For amusement I'll note that I taught my daughter to be careful of electricity with an electric fence. I included sufficient information that she understood how and why to do and not to do the test.I

Fundamentally you have to try to pitch the lesson so that it expands understanding. Pitching too much detail prevents that.

[bdunham7]

The reason why it may be dangerous to do what that guy did is vague, unless it is explained how exactly he got himself killed.

I'd agree it is at least worth discussing, but do we need to know exactly what part of the scope he touched?  IIRC, that isn't actually known.  What I remember from the original story is that it was determined that the electrocuted engineer was aware of the danger and took some precautions by installing some shielding or whatnot, but didn't do an adequate job and somehow, whether from forgetfulness or clumsiness or very bad luck, came in contact with the high voltage.  If your scope is still sitting on a cart, as I believe it was in this case, you likely have not done a good job of shielding and safely floating it.

You actually can float a scope safely and it has (and probably still is) done in special circumstances using isolated power supplies and putting the scope out of reach behind adequate shielding.  I vaguely remember some story about old Tek 500-series scopes being floated up to very high voltages for some high energy experiments.  The problem is that none of this is appropriate in the vast majority of circumstances, certainly not a home lab or even a commercial repair shop.  The effort required to do it properly is simply way beyond what it takes to do something simpler and safer.

[shapirus]

I'd agree it is at least worth discussing, but do we need to know exactly what part of the scope he touched?

That's the problem! We do not even know if it's touching the scope that killed him. Maybe he touched the hot wires that he was going to probe? Maybe something else? There's more than one way to get yourself killed.

[ogden]

This thread is troubling me because I have been directed in the past by a very experienced EE to float my scope for certain measurements. However in my case, the DUT was not powered by mains. It was powered by a supply giving it 48V. Is floating the scope still ill-advised in this case?

Still not advised even if considered safe. You can't trust fast transient/signal waveforms you measure such a way, not to mention that circuit may function differently when connected to scope ground "antenna" which is capacitively coupled to mains/earthing.

[tggzzz]

The reason why it may be dangerous to do what that guy did is vague, unless it is explained how exactly he got himself killed.

I'd agree it is at least worth discussing, but do we need to know exactly what part of the scope he touched?  IIRC, that isn't actually known.  What I remember from the original story is that it was determined that the electrocuted engineer was aware of the danger and took some precautions by installing some shielding or whatnot, but didn't do an adequate job and somehow, whether from forgetfulness or clumsiness or very bad luck, came in contact with the high voltage.  If your scope is still sitting on a cart, as I believe it was in this case, you likely have not done a good job of shielding and safely floating it.

You actually can float a scope safely and it has (and probably still is) done in special circumstances using isolated power supplies and putting the scope out of reach behind adequate shielding.  I vaguely remember some story about old Tek 500-series scopes being floated up to very high voltages for some high energy experiments.  The problem is that none of this is appropriate in the vast majority of circumstances, certainly not a home lab or even a commercial repair shop.  The effort required to do it properly is simply way beyond what it takes to do something simpler and safer.

Just so.

It is possible, but very difficult to do correctly, very easy to miss something, and the consequences are very serious.

The picture I posted is intended to emphasise how difficult it is to do correctly. The chance of an inexperienced amateur doing it safely are close to zero. That doesn't mean they will come to harm, any more than walking into a road without looking means you will be knocked down.

This is a common problem, so common that safe solutions are readily available - so use them!

[Jim from Chicago]

This thread is troubling me because I have been directed in the past by a very experienced EE to float my scope for certain measurements. However in my case, the DUT was not powered by mains. It was powered by a supply giving it 48V. Is floating the scope still ill-advised in this case?

In your case, if your DUT was battery powered, why was it (a) necessary and (b) beneficial to float the scope?

My DUT is not battery powered. When I said my DUT is not powered by mains, I guess I meant that it's not directly powered by mains. I have a variable DC power supply which is plugged into mains, and that power supply is set to 48V which then powers the DUT. So there's nothing on the DUT itself which is over 48V. Within the DUT there is isolation so that the return for the 48V is a floating ground, and I needed to capture waveforms which are referenced to the 48V return. Originally when I was first doing this testing, I was not floating the scope, but then my mentor directed me to float it because he thought that earth-grounding the 48V return could be causing erroneous results. I'm not sure what his reasoning was behind that. It doesn't make much sense to me. But regardless I just want to make sure I'm safe because I'm actually supposed to take more measurements with the exact same setup in the near future.

After reading through the various threads/articles on this issue, I do realize that floating the scope is a generally bad practice, so I will bring this up to my mentor soon.

[Jim from Chicago]

Well I brought this issue up to my mentor (an EE with ~30 years experience) and he said he doesn't have a problem with floating scopes. I showed him the documentation from Tektronix which warns against the practice but he basically thinks they're being overly cautious and trying to sell more of their differential probes. As I type this, I have a floated scope in front of me. My only solace is that the voltages on my DUT should not exceed ~48V. Regardless...is this a red flag that I should I be looking for a different job?

[TimFox]

You can be injured with 48 V:  see the electrocution literature.  https://www.merckmanuals.com/professional/injuries-poisoning/electrical-and-lightning-injuries/electrical-injuries
I have seen CROs floated by much less than that, when a differential voltage waveform was needed with respect to a lower DC voltage, but I would never recommend it to anyone.
I also knew employees of an electrical utility who claimed they could hold a 120 V wire, but I definitely don't recommend that!

[tggzzz]

Well I brought this issue up to my mentor (an EE with ~30 years experience) and he said he doesn't have a problem with floating scopes.

I'm an EE with >40 years experience. No, it wasn't 1 year repeated 40 times

Does that make my statements better than those from a 30 year EE? Of course not.

A lot will depend on the details of the nodes to which you attach the shield. A 48V 1Mohm node is probably less problematic than a 48V 1mohm node. But if that node is slewing fast w.r.t. the bench/ground, then the capacitance between the scope case and bench/ground might destroy the UUT.

In other words, "the devil is in the details", and nobody here can provide a definitive answer.

[Faringdon]

is this a red flag that I should I be looking for a different job?

No, it means you are well suited because you think about this stuff.

[tggzzz]

is this a red flag that I should I be looking for a different job?

No, it means you are well suited because you think about this stuff.

Yes. But with the proviso that you are allowed to think, question, and refuse to do something.

If that is "career limiting" in that company, you should consider it to be an opportunity rather than a problem.

[nctnico]

Well I brought this issue up to my mentor (an EE with ~30 years experience) and he said he doesn't have a problem with floating scopes. I showed him the documentation from Tektronix which warns against the practice but he basically thinks they're being overly cautious and trying to sell more of their differential probes. As I type this, I have a floated scope in front of me. My only solace is that the voltages on my DUT should not exceed ~48V. Regardless...is this a red flag that I should I be looking for a different job?

You better refer your mentor to the user manual of the oscilloscope which likely says it must be grounded. Not just for safety but also to return HF noise into ground. IOW: your oscilloscope likely isn't designed for work without a ground connection. So ground your scope. Both for safety AND for optimal measurement results. Personally I'd be extremely wary about any information coming from someone who suggests to float a scope (or any piece of test equipment). I'd follow the manual for the piece of test equipment to the letter where it comes to safety. One of the things you should imprint in your mind is that experienced people leave out important details in their instructions because they (wrongly) assume that everyone is aware of those details.

Regarding your measurement: if you measure a floating DUT (powered from an isolated bench supply), then it doesn't matter which ground point you choose as long as you use the same ground point for ALL test equipment connected to the DUT.

[jonpaul]

float the DUT/SMPS on an isolation xformer.

Scope is grounded.

Done this since 1970s no issues

J

[vk6zgo]

On a related topic, what has happened to the transformer market? Has the dominance of switched mode supplies driven 50Hz/60Hz mains transformers to boutique status and prices? Its years since I've bought such a transformer. I've just been looking for one to use for bench top isolation and the prices make me wince. There is quite a bit of copper in them, but I don't remember every recoiling at the price before.

They might cost a bit more now due to inflation and particularly copper prices, but I don't remember that decent transformers were ever all that cheap other than surplus items.

Just about any sizeable city back in the day had at least one transformer rewinder, so if a relatively obscure OEM transformer failed, it was often cheaper to get it rewound, than try to get one from the other side of the world.
Some of these rewinders also produced their own line of new transformers.

All that expertise had disappeared now, & the only cheap transformers are generic ones from the PRC.