High side current monitor on 500V (insulation monitor)

High side current monitor on 500V (insulation monitor)

Need to control the insulating state line with Potential 500V.
I am interested decrease insulation under 10M ohm, ideally with the possibility to measure a decrease in the range 100k
Just with a resolution of 100k Ohm

One possibility is to measure the current on High side over 1Ohm resistor,
but I can not think how to implement cheaply, the equipment is a processor with 12bit ADC, but can not move it to the potential of 500V

The second option is to use the fact that the potential on the line retains 60uF capacitor charging from the source.
But I'm not sure if this is a good idea equipment is outside (-20 +50C), the capacity will change with temperature and time and for measurement have only 600ms time windows between
two operation action on line.

In short, I would appreciate an idea for a cheap robust solution

Thanks
Sheldon

P.S.
What cheapest MOSFET can switch on 500V into discharged 60uF foil capacitor ( from 30W power supply)

Use an opto isolator with the LED current calibrated against the collector current over the range you want. Wire the LED so it is in forward conduction.
Frank