My in-laws have a problem where the TCD on the distribution board
> trips occasionally when the electric shower is on. They have had the
> shower replaced and the new one also does the same thing. I assume
> this discounts the shower as the problem. I have asked them to
> disconnect all appliances and try again and again it sometimes is OK
> and sometimes trips. Any idea how I can investigate further? I come
> from a computing hardware background so have basic electrical skills
> and have a good multimeter!
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I would guess that there is a neutral earth fault somewhere on one of the RCD protected circuits (not necessarily the shower circuit) and the high current that the shower draws is exposing the fault.