BigWallop wrote:

Well, I'm sorry too, because I can't find anything, anywhere, that says a
standard RCD in a normal domestic installation will trip the supply unless
there is a fault from Phase to Earth.

Well, you are completely and utterly wrong.

A common question here goes along the lines of "I isolated the circuit
at the fuse/MCB to do some work on it and when I cut a cable the RCD
tripped. Why?"
Answer: because you created a neutral-earth (strictly neutral-CPC) short
with your wire cutters.

If the appliance is open circuit, all switches open, then the phase
should not be connected to neutral through any load, and so this
can't cause an imbalance in the phase to neutral loop which the RCD
is there to detect.

But there are always other loads to think about. These are passing
current through neutral conductors somewhere. A N-E short creates an
opportunity for the neutral current to take two parallel paths, one of
which is through your RCD. The RCD then sees current in the neutral but
not the phase(s); if this exceeds the threshold the RCD will oblige by
tripping.

So, for arguments sake, we'll say that neutral and earth are in the
same potential state. The phase is at higher potential than both
neutral and earth. Phase and neutral are "not" connected through any
load (open circuit). A straight short occurs between a neutral and
an earth conductor. What happens at the RCD?

It probably trips. As soon as you short the neutral to earth the
neutral is no longer isolated -- unless its isolated upstream at an
all-pole isolator (not usually the case unless you've opened the main
switch) -- current can now flow in the neutral and trip the RCD.

Therefore, the fault must lie between phase and earth for the RCD to trip
the supply, because neutral is already at, or so close to, earth potential
that the RCD will not detect any difference on this side of the supply.

"At or so close to earth potential" is missing the point. It's easier
to think about where current is flowing, and how it will be shared if
there are parallel paths. If you want to think in terms of voltage
though, consider the fact that you've got low impedances (copper wires)
and that 30mA (or whatever) to trip an RCD is not much current. If the
neutral is, say, only 0.1 volt 'above' earth then a N-E fault loop
impedance of 3 ohms will cause over 30mA of residual current and trip
the RCD. Typical Z-E shorts in domestic wiring will have a loop
impedance of well under that. Even with the fault on the end of 50
metres of 1 mm^2 T&E cable you'd see under 2 ohms.

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 hope you can now see the flaw in that argument. It's not highly
unlikely at all; it's highly likely.

The type of earthing of the installation makes a difference. Let's work
through the 3 usual cases:

1. TN-S (earth comes via supplier's cable sheath)
-------------------------------------------------
Here there is always a voltage difference between N and E -- even if
*everything* in your house is switched off. This is because other
consumers' loads are causing a voltage drop in the supply cable neutral
between the point where your service cable is connected and the earthed
point at the transformer. A N-E short in the house diverts a little bit
of their neutral current through your neutral, your RCD and back to the
transformer via the cable sheath. The RCD will almost always trip in
these circumstances.

2. TN-C-S (PME)
---------------
Here your earth comes from the supply neutral entering the premises. If
there are no loads on in your house there is no neutral current to
divert and the RCD will not trip on a N-E short.

However if you are working on one RCD protected circuit (with its phase
isolated) and there's load current flowing in another circuit protected
by the same RCD, a N-E short in the circuit you're working on will
divert some of the other circuit's load current through the earth,
causing an imbalance and tripping the RCD.

3. TT (no supplier earth, own earth rod)
----------------------------------------
This is essentially the same as the TN-S case, except that the impedance
of the parallel earth path is much higher and the risk of tripping
somewhat lower. Nevertheless if your earth electrode is good and the
incoming neutral is a volt or two above the local ground (again because
of other people's loads) then a N-E short will trip the RCD -- e.g.
neutral 1 volt above earth, earth electrode resistance 30 ohms (neglect
other impedances): residual current = 33 mA, so a 30mA RCD will trip but
a 100mA one won't trip.


BTW-1: RCD tripping on a N-E short is a Good Thing because it alerts you
to the presence of a potentially dangerous problem. Neutrals and earths
are not fused. If there's a parallel path and the main desired neutral
path happens to go o/c then lots of load current could end up going
through a bit of 1 mm^2 wire in your house, which might respond by
catching fire...

BTW-2: None of this is relevant to the original subject of this thread.
From the original article we know that the washing m/c does not trip
the RCD when plugged in with the circuit MCB open. Therefore a N-E
short can be ruled out and phase-earth leakage is the problem. Probably
the EMI filter, or water leakage, as someone said a while ago.

--
Andy