i had a chat with the plasma supplier, told them of my woes and the only suggestion they had was to drive an earth rod, connect the terminal on the cutter to that and then connect the CNC table likewise. Unfortunately I couldn't drill a hole through the factory floor so I just connected the terminal to the table frame.
I tried that and managed three pierce tests this time but then locked up again.
Then I disconnected the earth from star point to PC case and it worked! Passed repeated pierce tests, then I run a simple cut-line test of six 70mm lines at reducing speeds, table was still responding so I ran another test pass followed by a more detailed test and it still responded afterwards!
here's the results...
numbers are mm/min at 30A.
i like the result at 4000mm/min, it's very fine and about 0.75mm kerf, this is 1.5mm sheet so very thin.
i gather the point where it closes up at the end is due to deceleration?
anyway, it was getting late so I quickly dialled in 4000mm to my next test in sheetcam and compiled the g-code.
It looked ok, showed up some signs of serious axis wobble but ok. However on turning it over it appears not to be cut very well. ...
Now, I'm not too worried as I did rush the test and the changes to sheetcam so I may have bodged it up. The main thing for this thread is that it's working and surviving extended cuts.
many thanks to all for the helpful suggestions, much appreciated.