Looking for some advice on what to check next....

welder is an older (20 yo) CIG buzz box transformer style arc welder. max current of 130A, and stepless adjustment down to 50A.

in the new shed (new RCDs) the RCD/MCBO trips whenever I try laying a long bead of weld - but the RCD is quite happy if I pause halfway through a rod.
rods are 2.5mm 7018s, running at around 90A.
If I dial the current down, I can run a full rod without tripping, but there are times when I want the higher current. It definitely is linked to time at full or high current, but it is tripping the RCD, not the MCB

I cleaned out the welder, blow the dust and rap out of the windings, and did a megger test between the windings and case - all good.
all that is inside the welder is the transformer, a thermal cutout, and a cooling fan. Each part tested fine for the megger and conductivity. The windings are enamelled copper.


thoughts on the cause of my observed problems?
anyone else experienced this?

Thanks,
Des


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