Thanks for all the replies so far.

It's the RCD in the house that they are tripping- no heaters operating.

I took one of the Mac's out to my parents place yesterday and got the same results after a couple minutes- had booted up and was justing sitting there doing nothing.

Plugging in other known good Macs to the same power outlets don't give any trips.

I will have another play today and see what I can find.

Isolating the ground pin? Will have a look.

Thanks