wrote:

Andrew Thelwell wrote:


Hi all, I'm the OP,

First off thanks for all the replies -- great to get so much feedback.

Let me clarify a few points... this should help with the discussion:

1) The house is newly-purchased, not a new build. It's a 1930's
semi-detached

2) I may have misled when I referred to RCDs (sorry!). We have two
large rotary switch type things on our consumer unit labelled
"Earth-Leakage Circuit Breaker".


Ohhhhhhhhhhhhhhhh. Those. In that case we know a couple of things
straight away. This is oldish wiring, and uses TT earthing, meaning a
local earth rod with high earth impedance.

Yup...

In fact at that age you may find that there is no dedicated earth at
all. Our house was wired in a similar way once, and the only earth was
provided by the gas pipe. It may be worth checking where the earth wires
from the CU (or ELCB if they are external to the CU) go to.

It is these that have tripped
ELCBs rarely trip in error. I think I know whats going on now.

They can however be sensitized by leakage from a faulty applicance such
that they are prompted to trip when a non faulty appliance with a small
legitimate earth leakage (such as a bit of computer kit with RFI filter
on the mains input) is connected.

With a ELCB the liklihood of a trip will also increase as the weather
gets wetter, since this will tend to loawer the earth impedance of
whatever you are using as an earth.

3) The problem with the meter when the guy came to change it was that
he was as follows: He had one of those 'mains tester' screwdrivers with
the LEDs in the end that should only light up on when touched to a live
terminal. However, he got it to light up on ALL FOUR terminals attached
to our meter (+ in, - in, + out, - out, yes?). I don't think there was
an suggestion as to the 'size' of the current though. He also got it to
light up when he pressed it to the big grey block (neutral block) in
the meter box, which apparently it shouldn't do. This happenend both
with our consumer unit main isolator turned ON and OFF. It stopped,
however, when he pulled the main fuse out of the meter box.


OK, then we can safely ignore anything he says

Yup... I am supprised that somone doing this type of job is trusting his
life to one of those toy screwdrivers!

Two mains engineers from Central Networks came out that night and
tested thoroughly for almost 2 hours in our meter box and around our
roof (we have overhead supply). They found nothing out of place at all.


And we can safely conclude nothing is out of place at all with your
supply as far as the CU or fusebox.



They asked if we'd experienced any oddities inside the house, which we
hadn't. However, since then we have seen our Earth-Leakage Circuit
Breakers (not RCDs, sorry!) trip three times. Two of these times have
been due to turning on the one socket in the spare room. One other time
the other ELCB tripped when my girlfriend turned on the microwave. All
sockets in the house test out fine using a normal B&Q socket tester,
except the one in the spare room, which shows only one light,
supposedly "L/N Reverse".

Andrew, what sort of wiring is there in the house? Modern PVC clad stuff
or old rubber or rubber/fabric coated stuff?

OK, I think I see the problem clearly. An ELCB will trip when it sees
large earth currents (unlike RCDs which trip on small earth currents).
This means you have a major fault in whatever appliance caused it to
trip when the socket was switched on. So the microwave and whatever was
plugged into the spare room socket should be taken out of service now,
and some checks done on them later to confirm or deny theyre the
problem.

From everything you say, the only problem with the installation itself
is the swapped wires behind a socket, which is a trivial matter easily
fixed in 2 minutes.

I would tend to agree - the socket polarity fault sounds like it is not
directly related to the tripping problem.

Your 2 faulty appliances would not have tripped anything at your last
house, which I can conclude had low impedance earthing and no RCD.
However on a high impedance earth, which you have here, such faulty
appliances are not safe to use at all.

Note that the faulty appliance could be a fixed one like an immersion
heater, electric shower, cooker element etc - i.e. things that came with
the new house rather than one brought with.

(things with heating elements are prime candidates for high earth leakage)

--
Cheers,

John.

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