"Tony Williams" wrote in message
...
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.


So, I'll start again. A neutral / earth short circuit will not cause an RCD
to trip. If a fault or high impedance earth loop is used, then the neutral
becomes higher in potential than it should be, so it isn't at neutral
potential any more because neutral potential is the same as earth potential.
What has happened is, the conductor that is used as the neutral bond has now
risen to phase potential because earth hasn't removed the residual current
from it. The neutral is now a phase. If the phase then leaks to proper
earth potential through a lower impedance path, then an RCD is designed to
detect this leak and trip open. Again. Neutral and earth are at the same
potential, so a straight short across them will do nothing. If an induce
current of any kind lifts the neutral conductor to phase potential, then it
no longer is at neutral potential. This can be caused, as you have
demonstrated yourself above, when the earth bonding does not remove the
residual current properly and causes what should be the neutral conductor to
rise in potential energy, and so it becomes more to phase potential. Now
create a short circuit across the neutral conductor which is now sitting
well above earth potential and you cause an imbalance between phase
potential and earth.

An RCD will not trip open with a fault between neutral potential and earth
potential, because both neutral potential and earth potential have exactly
the same potential. An RCD will only trip open circuit if it detects an
imbalance between phase potential and earth potential.