Big Ohmic Problem ???

PCW wrote:

What you do not want is any alternate path to ground via _any_ sense wiring

Oh, ok, thank you for putting that in laymen's terms for me That makes perfect sense now.

How do you feel about fuses on the sensing wires?

Ok, so here's my interpretation of where we ended up:



Correct?

PCW wrote:

My feeling is that it should not be needed, except in the event of component failure
(which seems unlikely)

Isn’t that kind of the point of fuses? You don’t need them until you do, and if you don’t have them, you start letting the smoke out of things...

Just one last comment on the fuses and then I rest my case:

Anytime you attach a small gauge wire to a high energy potential you have a possibility of fire due to thermal overload in case of a fault. That's why in electrical installations you are required to fuse the wiring according to the possible short circuit current in case of a fault.
In this case I would agree that a ground fault in the isolated power supply is unlikely. IF you use a double insulated supply that is.
Here you need to have two faults. An unwanted ground loop in the sense wiring and a bad ground at the table or work piece. As we have already seen this can happen due to a misunderstanding of the circuit diagram and forgetting to attach the ground clamp.
I'm not a safety freak but there was already at least one case of fire and another where the potential was very real by grounding the sense loop. So clearly some thought should be given to protection.