electrical - shower trips RCD - but DP switch looks the problem

This morning our elec. show started tripping the RCD even at low shower settings. The shower is feed is via a pull-cord DP "shower" switch.
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.
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On 28/01/2012 14:21, dave wrote:

Sounds like the switch was arcing over rather than making proper contact. The result would end up feeding a vary noisy electrical supply to the shower. The shower probably has a mains filter on its input (to prevent the large current switching it does injecting noise back into the mains). Its probably wired with a couple of capacitors between L&E and N&E. At normal mains frequencies, these will pass negligible current to earth, however the noisy supply from your arcing switch may well contain enough higher frequency content to pass more current to earth. That is before you even start looking at the RCD itself. These are not invulnerable to nuisance trips caused by spikes and surges.
So replace the switch, and conduct an insulation resistance check on the circuit if you can. Failing that, use a multimeter on a high ohms range to check for (hopefully) no continuity between earth and either of the live wires...
--
Cheers,

John.

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On 2012-01-28, dave wrote:

That's exactly what makes an RCD trip: difference in current in the live & neutral wires ("residual current"). The difference doesn't have to be much --- I'm thinking 30 mA is typical (but ICBW, and I think it varies between countries).

The residual current has to be going somewhere, eventually to earth. Fuses & breakers prevent fires (by tripping before the current gets high enough to cook the cable), whereas RCDs prevent electrical shocks (by tripping when the some of current is going somewhere other than back through the neutral, such as through you).
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I changed my old circuit breaker to a RCD and found that our power shower tripped out. On investigating we found that the shower had been wired incorrectly with the neutral connected to the earth and vice versa - did not affect the shower until RCD fitted.

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