I'm coming back to this design again after some time away from it. Previously I was trying to design the circuit to be completely passive, but that hasn't wound up being very reliable, so now I'm resigned to attempting it with an isolated power supply.
This little excerpt might be a little hard to read, but the "relay test" inputs each come from the load side of a contactor that switches North American split-phase 208/240 VAC (so each leg is 120 VAC from ground 180 or 120 degrees phase separated). The 150k resistor is a 1W flame-proof resistor, followed by a 1 mA current limit and a 1k resistor to ground. The voltage across the 1k resistor is compared to 1V. The output of the comparator isn't shown, but it lights an optoisolator that passes the status on to the logic circuitry.
The questions I have here are about the portion of the circuit shown.
Is the 1 mA regulator serving a purpose here or should I just remove it? My own paranoia led me to put it in to insure that if the path to ground was poor enough that any potential residual current would be limited to 1 mA, but the big series resistor is supposed to be guaranteed to fail-open, so there should be no way for its value to reduce by enough... right?
As designed, the comparator and its optoisolated output are completely isolated (one detail missing from the screen grab is that pin 8 of the comparator - its Vcc - is connected to Viso - the isolated +5 supply from DC1). Does this have any implications for the potential voltage the comparator could see if the ground impedance rose to infinity? Should I ground the isolated supply output? It's an isolated supply because I'm trying to insure that there's a good and proper "moat" completely around all of the HV circuitry. The one ground connection allowed is from the low side of the two 1k sense resistors in the GCM circuit. There are no other "exits" from the HV playpen other than components rated for many thousands of volts of isolation.
The goal here is to detect when the ground impedance rises above no more than 100Ω/V, or in this case, 24kΩ. The secondary side of the optoisolator has a peak-hold, so it's ok that only the peaks rise above the comparator threshold.