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Hypothetical question about Isolated Supply, GFCI & Oscilloscope use

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bsodmike:

--- Quote from: nctnico on October 03, 2020, 08:41:47 pm ---Don't think about using isolation transformers. In order to use these safely you need your entire bench to be setup properly and you'll need training as well.

--- End quote ---

All the devices on my bench are tied to mains-earth reference, through my APC UPS (which powers everything).

The isolation transformer I'm planning to use shortly (it's a DIY build), runs a Variac into an Isolation transformer.  The output is wired in series through 2x 60W lamps (in parallel) to act as current limiting devices.

Furthermore, none of the other bench devices (Oscilloscopes, Signal generator) will be floated, only the DUT will be floated by powering it via the Isolated supply.

You've mentioned a "proper" bench configuration and training; in terms of the bench setup, what else is needed, and also with regards to training, roughly what does this cover beyond the points I've raised?

bdunham7:

--- Quote from: bsodmike on October 04, 2020, 09:48:32 am --- of course at 10X probe impedance; thanks for the info on the 1X probe/50R scope input configuration.. interesting!!

--- End quote ---

Just so we're clear, that was intended as an example of improper setup.  What I was getting at is that your GFCI won't trip when you probe the circuit anywhere with a 10X probe because the probe impedance is too high.  With isolation, you have two separate reasons the GFCI won't trip.


--- Quote ---"Of course, this defeats the touch safety feature of isolation." -- Do you mean that the Oscilloscope frontend circuitry ground-points (especially the BNC connectors) are raised to a higher potential, and this could be a hazard or... ???

--- End quote ---

No, sort of the opposite. Floating the scope can result in unsafe touch voltages on the scope. If the DUT is floating, then the scope stays grounded.  When people tell the cautionary tale of a skilled engineer being killed this way, apparently they are referring to a situation where this person ungrounded the scope and connected to some sort of high voltage, then was electrocuted while operating the floating scope.  You actually can float a scope or a signal generator b/t/w, but I'd only consider it when you are dealing with lower voltages, like the output of an audio amplifier. 

With your isolation transformer, you are theoretically safe if you touch either output (but not both) if it isn't connected to anything because there isn't any ground reference. The other side from where you touch has very high impedance to ground and very little current can flow through you.  However, if you ground one side, the other side can now zap you because there is a complete path.  So if you want to maintain touch safety and take a measurement, you have to use a method with at least two leads but without a low-impedance path to ground anywhere--and that could be a device with completely isolated, floating input section like a typical DMM or isolated scope, a handheld battery-operated device, etc, or you can use a system that is not truly isolated but has a path to ground that is of high enough impedance that the leakage current won't shock you.  This can be two separate scope probes without any ground leads, or it could be a differential probe.

nctnico:

--- Quote from: bsodmike on October 04, 2020, 09:53:03 am ---
--- Quote from: nctnico on October 03, 2020, 08:41:47 pm ---Don't think about using isolation transformers. In order to use these safely you need your entire bench to be setup properly and you'll need training as well.

--- End quote ---
All the devices on my bench are tied to mains-earth reference, through my APC UPS (which powers everything).

The isolation transformer I'm planning to use shortly (it's a DIY build), runs a Variac into an Isolation transformer.  The output is wired in series through 2x 60W lamps (in parallel) to act as current limiting devices.

Furthermore, none of the other bench devices (Oscilloscopes, Signal generator) will be floated, only the DUT will be floated by powering it via the Isolated supply.

You've mentioned a "proper" bench configuration and training; in terms of the bench setup, what else is needed, and also with regards to training, roughly what does this cover beyond the points I've raised?

--- End quote ---
Basically floating everything and constructing your bench and furniture from non-conductive materials.

If you connect a grounded scope to a floating DUT the DUT becomes grounded again at and it is uncertain at which points the DUT carries dangerous voltages. Worse; the GFCI will be defeated as well in this situation. Clip the ground leads of your probe to two points in the DUT; you might short something. Buying CAT rated differential probes will be cheaper and safer to work on mains voltages. Get rid of the isolation transformer if you love your life.

bdunham7:

--- Quote from: nctnico on October 04, 2020, 07:23:18 pm ---Basically floating everything and constructing your bench and furniture from non-conductive materials.

--- End quote ---

Seems a bit over the top, the sort of thing that happens when one lets the imagination run wild.  Why is it OK not to do this without an isolation transformer?


--- Quote ---Get rid of the isolation transformer if you love your life.

--- End quote ---

Rather than debate the various technical arguments that have been thrashed out before, let me ask this.  Are you aware of any instance, ever, anywhere in the history of the entire world, where someone was killed while using an isolation transformer on a DUT?  I'll settle for anything, even if their bench was outside in the rain, or they caught a stray bullet, or even if the user was on the bomb squad and the DUT was a bomb.

bsodmike:

--- Quote from: nctnico on October 04, 2020, 07:23:18 pm ---If you connect a grounded scope to a floating DUT the DUT becomes grounded again at and it is uncertain at which points the DUT carries dangerous voltages. Worse; the GFCI will be defeated as well in this situation. Clip the ground leads of your probe to two points in the DUT; you might short something. Buying CAT rated differential probes will be cheaper and safer to work on mains voltages. Get rid of the isolation transformer if you love your life.

--- End quote ---

You've definitely got loads more experience than me, but I certainly would be uncomfortable plugging in a faulty DUT into mains directly without any Isolation.  I'd much rather have an Isolated supply and use that with caution (60W bulbs etc), when probing at the start with DMMs.  What I like about this approach is that any shorts cause the bulbs to turn on fully, and even if you intentionally (or otherwise) cause a short, they just go on fully with minimal sparking at all. 240VAC at max ~1A goes through (see YouTube link below):

https://youtu.be/51mjt9nFoeA?t=526


For any Oscilloscope work: I agree to the point that differential probes are the way to go, so I'm taking your advice one that front :)

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