I'd say that you could measure the resistance from earth pin on the plug to each of the other pins with a multimeter, and see if there's anything significant after it settles down. It may not tell you anything if it needs a high voltage to break down though.
I know this is an old thread, but non of the sparks have mentioned that a big arc welder, with it's big transformer, will need a C or D curve breaker!
The inrush current of the welder when an arc is struck will be massive, and will easily trip out the B curve breakers usually fitted to domestic consumer units.
I am assuming that this problem has long since been sorted, but for future reference of those wishing to use big welders at home (or at work), an appropriate (16 or 32 amp) socket, wired directly back to the distribution board where it is fed from a C/D curve breaker is required.
Keep your extension leads for welders as short and as heavy as possible.
Yes it is a very old thread and will have been well sorted by now.
It's not as simple as just slapping a C or D type breaker in. It's not uncommon for a domestic Ze to be too high to open a D type breaker, never mind the Zs.
Having said all that, and now read the old thread, the op has a problem with BS1362 fuses blowing too, which the circuit breaker type will have no effect on.
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