It takes several seconds before it trips out. so I meansured the current before it tripped - it was taking only 10 amps. The I noticed the neon inthe DP switch was rather dim so switched that off/on a few times and the neon came on brightly. Measure the shower current now, and it was 37amps - and did not trip. So, looks like a fault in the switch (resistive contacts or whatever). Tomorrow I'll replace the switch with a new one.
My question is though, how come the RCD tripped? It must be sensing I(in) not equal to I(coming back) - so that is an earth fault (isn't it?). But should this not have "just" resulted in a (dangerous) hot DP switch? I want to be sure that there is no real earth fault I am missing somehow. Thanks
ps Many years ago when I installed that switch (sold for purpose) I remember it was a right pig to connect due to the large TW/E conductors used - 9kW I belive the shower is. maybe there is mechanical damage to the switch.