VFD (?) tripping RCD

[Jonathan]

I know it's not the most ideal way to test, but ammeter in the earth wires to see if there's any measureable leakage?

Should at least be able to isolate if it's the VFD, or motor causing problems.

I did that, second to last bullet point. But thanks anyway...

If the problem persists I could try putting a shunt resistor in the earth wire and leaving the oscilloscope (DSO) monitoring it. Set the trigger to whatever voltage corresponds to say 10mA, or just under 30mA (something like that) and see what I get when it trips.