I am of course talking complete and utter twaddle. And not for the first time. The pilot arc earth connection needs to be maintained until the main arc (to the workpiece) is established, whereas if I controlled it with the HF start relay, I'd kill off the pilot arc as soon as it established. It would probably just buzz on and off until something broke.

The only sensible way to enable and disable the pilot arc would be to monitor the main arc ie the current in the ground lead. So a DC current controlled relay (Hall effect sensor with comparator and relay) would sound like the best solution, unless I want to get really invasive and risk blowing something up.

Something like this perhaps https://www.amazon.co.uk/dp/B0CX52TP...fed_asin_title No idea how low the current can be adjusted but I could always put a couple of turns through the hole to increase the sensitivity.

So having had a good poke about inside, it's time to put the unit back together and start behaving again.