Hello all,
I would like to ask you guys a question on how to measure the potential drop across a resistive in a high voltage loop.

I am measuring the potential drop across pairs of bars on a commutator of a large DC motor. 400V at 300A is beond applied to the commutator and I wish to use op amps to read the analouge voltage and imput it to an ADC.

My problem is, although the volotdrop across 2 bars may be a few volts, the potential at which the bar is sitting at with resect to the 400V ground is in the region of 300V.

My question is, does the input circuitry that I want to build "see" this 300V potential difference between the input pin and the 400V ground? I wish to use a seporate 24V DC supply that runs from the 220V ac mains, to power my circuit. I there know that the 400V test supply ground is seporate from the 24V DC ground. But is the 24V supply ground sitting at a 300V potential, relative to "True ground" due to the fact that it is floating at 300V due to the 400V test supply?

How can I use op amps to read the small voltdrop that is sitting
on a 300V potential difference between the reading pin and "true ground"?

Thank you for your time.
sun