In article ,
BigWallop wrote:
[big snip]
This would only happen if there was a closed circuit between
phase and neutral, and a neutral conductor passed a fault current
to earth at such a low impedance, which should be highly unlikely
because neutral is already, or very nearly, at earth potential.

I don't think that is correct and in fact we have had
an RCD trip caused by an electric kettle. Both the
wall and kettle switches were Off and it required the
kettle to be unplugged to stop the tripping.

An RCD trip from an N-E short is quite possible in this
house when you look at the sums. We are on a two-wire
supply from a transformer that is a few hundred yards
away. Neutral is connected to Earth at that transformer.
The N-E resistance measured at our house is 33 ohms, and
is a measure of the resistance of that few hundred yards
of clay soil, between our Earth rod and the Earth rod at
the transformer.

Our N-E voltage, measured at the house, averages 0.7Vrms.
This is probably due to the loadings of the five other
houses upstream of us.

I= V/R, so an N-E short would result in 0.7/33 (21mArms)
of unbalance current through the RCD.... even when there
is nothing connected to the Line wire through the RCD.

If the N-E voltage or the ground resistance varies (long
hot summers?), then the RCD unbalance current (and the
chance of an RCD trip) would also vary.

--
Tony Williams.