First of all, thanks for the great contribution MachinistSF.
I built a cnc plasma cutter and i am having the same problem with HF interference (I suppose that is the problem), when I activate the relay that turns the plasma on, the usb communication with the computer is lost and the screen blinks. My question is,  if it is caused by HF interference, does the ground wire (that would generate the pilot arc) have to be connected to the plasma ground or does it have to be connected to the ground rod in earth?. Sorry if the question is quite obvious but i am not a specialist. Another question i have is , do i need an additional circuit to support the pilot arc wire in order to cut contact when it begins to cut the metal?. I am working with a chinese plasma, named NEO P8014
Thanks!