Terry Pinnell wrote:

OK, but this is where I'm still obviously being dense. Given a normal,
properly working setup, can you please summarise what *should* happen
and which switches in the main CU should open for each of the
following events:

1. A device on the LH side (non RCD) develops a short.

My assumption: it opens that MCB and the main LH switch.)

2. Leakage to earth (say 30 mA) occurs in one of those non-RCD
circuits.

My assumption: nothing obvious happens.

3. Leakage to earth occurs (say 30 mA) in an RCD circuit, such as if I
accidentally touched a live wire while standing on damp ground.

My assumption: I'd jump a bit but within a few ms the main RCD would
trip, so all power to *all* RCD protected circuits would be lost, the
individual MCB for that circuit would open, so I could identify it,
but power would remain on for the rest of the non-RCD circuits.

4. A device on the RH side (RCD) develops a short.

My assumption: it opens that MCB and the main RCD switch, but power
would remain on for the rest of the non-RCD circuits.


Not always. The various voltage spikes and surges associated with that
OFTEN trip the RCD AS WELL. Or sometimes they don't. They may alos trip
other non associated RCD's


There is always *some* earth leakage, often of a capacitative nature (if
nothing more than a ****load of T&E running together)and my experience
is that a nasty spike on the live caused first by a short, and then by a
trip opening up, often trips an RCD. ANY RCD.




You have
an undesirable bit of wiring here, in that the RCD socket in the garage
is itself powered from a RCD protected feed. Unfortunately when you have
cascaded RCDs like this there is no "discrimination" between them, a
leakage current large enough to open a RCD can open either or both, and
you don't know which one will get there first (it will also usually be
inconsistent possibly giving different results each time you test)[1].

Hence the garage socket with RCD should be powered from the non RCD side
of the CU. (As it stands you could remove the RCD socket in the garage
and replace with a normal one, since its own RCD is in effect redundant)

Thanks for that advice, I'll ask my electrician for a quote. The
garage RCD was presumably installed when the house was built, about 18
years ago, before I bought it some 6 years later. Until only a year or
so ago we still had the old fuse-type CU, so presumably that was OK.


You may want to consider e.g. a whole house 100mA RCD and 30mA RCD's for
where there is a genuine possibility of shock.




The fact that the MCB opened would suggest that the immersion is faulty
or there is a wiring error / damage on its circuit. These things don't
last forever so a faulty element would be my first guess. As a first
step I would suggest you need to isolate it from its supply and test it.

OK, that will be done next.


Good advice.

But don't discount a chewed through cable either.