DSO powered through isolation transformer, is it safe?

[Chriss]

Dave thank you for this post. This clarified absolutely all my questions and doubt.

We are now at the same point (I mena, you and I are completely agree) does connecting any earth lead to ground (scope or dmm etc. test leads ) to any device (isolated or not) is dangerous.
I use scope regularly every day but I'm using it most of time, maybe 99% of time on low voltage battery powered DUT's like
when I repair car ecu's, working on car, checking some signals on some uC controlled devices, arduino, R.PI dev. boards etc.

I really rare have contact with HV repairing like PC PSU, some other sort of SMPS etc. maybe that leftower 1% but that 1%
from my side of view is a really huge number of possibility to kill myself and/or kill some device, tools etc.

When I deal with HV stuff I always use my isolation transformer what I have on my bench more then 20 years.
I can say, many times during my life this device in combination of the "bulb in series" saved my devices or maybe my life too, even if I have a GFCI installed in my house, I really don't trust them so much.
I like my life much more. 

This 1% drive me always to talk about this situation with friends.
In my area, people talking about the isolation transformer as a 100% safety for his/her life and the life of the tool, but that is not true and I can't just accept this because when something is connected directly to ground, that could go wrong.

I can remember when I accidentally shorted out my usb scope exactly because of the earth and ground clip of that crap usb scope through my desktop pc which was of course grounded to earth.

That was a big mess, it drived the sh_t out of me when it sound BANG! and after several hour of investigation everything was binned. 

I don't know, that 1% is always in front of me whenever I power on my bench scope.
I also never use my scope when I'm nervous or deconcentrated, a small problem can cos a really big mess.

Conclusion:
Measuring around the car electronic with a bench scope which is earthed is not the best option but it is "safe" to some point.
I mean until everything works like it should.

Working on the bench with the same scope on a uC development board which is connected to the PC usb port until we take measurements that can end up in a huge mess.

Using the same scope on the same uC dev. board but battery powered that is ok.

Using the same scope on a DUT where the DUT is powered through a isolation transformer that is "safe" but must count
does everything is earth grounded and can end up in a mess.

But, what's going on if I power my scope or bench dmm through the same isolation transformer where the DUT is connected?

Then the ground clip from my scope or dmm are no more earth connected and floating.
The complete circuit is floating compared to the main (the primary side of the isolation transformer).

In this case, actually a big chance is to make a mess if the measurement tool goes bad and the HV touch the
chassis or earth point in the scope or dmm and in that case HV would travel through the GND clip and I could short out the DUT.
But how big is the chance for this scenario actually? . . .

[DDunfield]

It's amazing how often this subject comes up.

We are now at the same point (I mena, you and I are completely agree) does connecting any earth lead to ground (scope or dmm etc. test leads ) to any device (isolated or not) is dangerous.

That's an over-generalization. I will try to explain my opinion in more detail.

The problem occurs only when you are attempting to connect your scope ground lead to something that is NOT ground.

When working with a DUT that is grounded (especially if as in most testbenches it is on the same circuit as the test equipment), or even if it is not grounded (ie: battery powered, isolated by it's own or external transformer). It is perfectly acceptable and in fact normal to connect your scope lead to ground on the device.

When you decide to connect the grounded scope clip it to something that is NOT ground, there are two distinct but related issues that you have to deal with.

1) If the device is grounded, and you are connecting to something NOT grounded, then you are effectively shorting whatever you are connecting to ground. This will cause current to flow, which depending on the setup may a) blow a fuse, b) vaporize a trace in the DUT or your scope c) cause a major failure in the power supply of the DUT d) get hot/start a fire e) etc...

This is easily circumvented by placing either the DUT or the scope (see below before complaining) on an isolation transformer thereby cutting it's connection to ground.


2a) If you isolate the scope and connect it's ground clip to something NOT ground on the DUT, then any exposed grounds on the scope now sits at whatever voltage offset you clipped the lead to. This can make your scope hazardous to touch.


2b) If you connect your grounded scope clip to something NOT ground on an isolated power system, you are grounding that system at a point where it was not intended, which sets any exposed grounds on the DUT to whatever voltage offset you clipped the lead to. This can make your DUT hazardous to touch.


In your example, with the 300+V power supply, I would not want something with 300+V exposed "grounds" sitting on my bench, therefore I would consider "floating" to be hazardous.
If it was however a 5 or 10v power supply I would not have a problem with the arrangement, because accidentally coming into contact with 5 or 10v would not present a hazard.
In other words, issue #2 usually doesn't present a major problem with low-voltage devices.
(It should be noted that dropping a metal tool in the wrong place - say between the floating DUT and another grounded piece of test gear can still cause sparks and destruction).

In days of old, when differential probes and scopemeters were generally unobtanium, many people decided to "float" their scope on an isolation transformer, and then start happily measuring non-ground referenced things in the 300+V secondaries of classic tube televisions and the like ....

The scopes they were using looked like Photo#1 (OldTek.jpg). Note the large metal box, metal trim around the front panel, and many of these vintage scopes had one or more metal knobs. All now sitting at the 300+V that they had clipped their ground lead to. (Disclaimer - I don't have any scopes like this any more - photo snarfed from the web).

Not surprisingly some of them died!

Thus was born the adage "float the DUT not the scope".
Or course, this caused the problem that the large metal chassis of the TV was now sitting at 300+V relative to ground (as described above), but since many of these old sets had "hot" chassis which repair shops were used to, it was still the best solution - best to have one large hazardous hunk of metal on the bench instead of two. (and the metal scope was REALLY BIG).


Photo#2 (Abomination.jpg) shows something currently sitting on my bench. This (among other things) is a high power speaker amplifier. It is quite high power and has some fairly unpleasant voltages inside. Since it's a bipolar driver, there are times when I want to look at things not relative to ground. It is also attached to speakers, input sources (signal gen. etc.) and debug devices (ie: a PC). While I could try and float "everything" it's not really practical to float this DUT.

Photo#3 (TDS210) shows a more modern scope (still more than 20 years old). You can see that things have changed a lot. There is no longer metal case, there is no longer metal trim. No metal knobs, the plastic knobs do not have metal setscrews. The only exposed connections are the BNC connectors and the probe calibration output. As you can see in this case, I have attached a probe with an insulated BNC, and capped off the other BNCs and compensation output. It's pretty hard (but not impossible) to come into contract with a ground in this configuration.

For a while (before I decided it was still not a great solution) I used this on an isolation transformer when I had to look at non-ground referenced things up to a couple hundred volts,  above that since the scope front panel does not have an insulation rating, I wouldn't do it. And I just didn't like fondling a device that I knew was improperly connected at unpleasant voltages.

Just for the record, I AM NOT recommending you float a scope like this - but (and I know lots will disagree with me), I do not think this particular arrangement is more hazardous than floating a DUT (which will often be something new that you are not familiar with), and in many case can less hazardous (but still hazardous).

Photo#4 (ScopeMeter.jpg) shows what I currently use if I need to look at something not ground referenced. The Fluke is great for probing like a scope, the UT-81 is handy as a graphical DMM.
EDIT: You might think that these being just "scopes that run on batteries" are not much different than the transformer isolated bench scope. There is however one BIG difference, which is that these are rated for HV application. They are much better insulated than a bench scope (eg: the BNC's on the Fluke are insulated on the outside so it's hard to contact them even if not capped), and that insulation has been tested/certified for the device rating.

Photo#5 shows the other good solution, a differential probe. (I also don't have one of these, photo snarfed from web).

Whew!

Dave

[bitseeker]

I opted for the belt-and-suspenders solution: isolation transformer for the DUT and HV differential probes for the scope. Of course, there's still the potential of zapping yourself by touching two points in the DUT.

It's a common lament that differential probes are very expensive (until Micsig released theirs) and that they "cost as much as my oscilloscope." I got the Sapphire/EEVblog ones before Micsig came out with theirs, but my perspective was that buying an inexpensive scope enabled me to have budget left over for a differential probe. It was a worthwhile tradeoff to me over a higher-grade scope.