In article , roger
writes

It is possible to have an appliance [or eg a shower] which works fine
but has a leak of a few ohms between neutral and earth.
This may not normally trip the RCD.
However, when a nearby kettle and the dishwasher are switched on and
draw 20Amps or so, the neutral-to-earth voltage increases enough to
cause a trip.


Thanks for the suggested explanation. However as I was the only one in
the house at the time, I can say with a high degree of certainty (*),
that nothing else got switched on either whilst the shower was trying to
be used, or the hob.

(*) the possible exception was the thermostat on the fridge cutting in.


Adrian
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