OK, help me with this odd requirement for a fuse/circuit breaker: 5mA but 20KV!!

[cvanc]

Yup... need to find some kind of circuit interrupter/fuse/breaker that trips at 0.005A and can interrupt 20,000 volts.  Any hope of something like this existing as a commercially available item?  Thanks...

[wraper]

You should shut down generation of that HV, not break HV directly.

[Gyro]

The simple answer would be a vacuum relay but it's way more complicated than that!

The major question is what is the prospective fault current - you might want to trip at 5mA, but what will be the available current into a short circuit? That pretty much decides whether there is any sensible possibility of breaking at high voltage.

The second issue I see is current sensing, if you can't do that in the ground return then there are big issues in current sensing a line that it is sitting at 20kV.

Edit: The other question is AC or DC. Breaking 20kV at DC is a bigger problem still. You need to supply much more information before anyone can have a reasonable chance of answering it.