I have a separate garage that has power supplied from my house RCD equipped
CU. In the garage there is a separate two way CU (lights and sockets). The
sockets in the garage are all twin plastic apart from one twin metalclad, a
total of 4 twins outlets. They are all daisychained off the previous one
i.e. ----Feed from
house----[CU]-----[Skt]--------[Skt]-------[Skt]-----[Skt]. These were all
in when we moved in.
I have lived here for 12 years and over the last couple of months the garage
MCB in the house has tripped intermittently. The only thing in the garage
that is permanently plugged in is a freezer (and it has been for 9+ years).
I tried plugging the freezer into different sockets to try to eliminate it's
'normal' socket but it still kept tripping. Today I plugged in my socket
tester and found one socket didn't have an earth, this socket isn't the
freezer's normal socket. I traced this back to the previous socket and
reattached the earth wire. I then looked inside the freezer's plug top and
found the live conductor was loose. The copper strands were still inside
the hole but the screw wasn't tight. The plug top has never been apart
before so either it was supplied like this from new or it has worked loose.
I have now tightened this up. Neither of the loose wires were able to touch
any other conductors.
My question is am I likely to have found the problem? I can keep checking
but this means me having to stick my head in a cupboard and cricking my
neck. I will check periodically but have I sorted it? I can also check on
most days when I go in the garage and switch the lights on.
PS. Thanks Geoff the stamps are in the post tomorrow.
Thanks for the info but knowing when it has tripped won't tell me what is
causing it to trip. As I said in my OP the freezer is the only thing
constantly plugged in so it is not tripping when something else is calling
Not uncommon for freezers to be a source of leakage so test that.
Connect a low powered mains light bulb in series with the freezer earth
lead. hopefully the bulb will not light but you can measure the voltage
across the bulb. It ought to be zero.
Far better for things like freezers to be powered by a non RCD protected
supply to prevent nuisance defrosting.
Is it feasible to run a temp extension cable from the house (different
circuit) to run the freezer, and see if that trips? Also, you can then see
if garage trips when there's no demand on it.
Incidentally, I don't claim to be an expert but 4 twin sockets off one spur
alarms me a bit - are the trips sensibly sized...?
The circuit in question is a radial - not a spur. With a radial
circuit the cable is sized such that it can safely carry the full
Hence there are no restrictions on the number of sockets in can service,
although the suggested maximum floor area served by the circuit is 50m^2.
 A spur is a special case of a single cable leaving a ring circuit
where the responsibility for fault and overload protection becomes split
between the protective device at the head end of the circuit providing
just the fault protection, and the selection of equipment on the tail
end enforcing the overload protection - hence the need for specific
limitations on what can go on the end of an unfused spur.
My first answer:
If you mean MCB (Miniature Circuit Breaker) and not RCD (Residual Current
Device) then no, you have you not found the fault.
An MCB opens when there is more than a specified current flowing in a
circuit. That could be a traditional overload (too much stuff drawing power
at once) or a short from Live to Neutral or Earth. Neither of your found
faults would have caused this.
MCBs can become faulty so it you have another circuit with an *identical*
MCB (same rating and type) you could try swapping them to aid investigation.
And then after a little more thought my second answer:
I guess if the Live in the plug was loose and making intermittent contact
then it could produce all sorts of spikes on the mains which could upset the
MCB. If the freezer was taking a few amps because it was on a "go" bit of
its cooling cycle and the Live opened then the back emf could be substantial
and I've no idea what the MCB might make of it.
Sorry, each of those two answers would probably have been useful on their
own but together they don't help at all do they!
You mean the RCD is not tripping? so its current overload and not earth
leakage that is happening?
The only thing in the garage
It is JUST possible that a continuous high current arc was overloading
the MCB, but you would have found evidence of extreme arcing and burning
in the plug.
I can keep checking
Frankly if you are overcurrenting the thing, you have a wiring short or
a faulty freezer, and its likely to go bang sooner or later and take out
Happened to me when I painted a kitchen in a rented house. the paint got
into a hole with a rawlplug in left over from some previous shelf, and
as it happened, the thing had been neatly drilled through a ring main.
The landlord blamed me and refused to give my deposit back when I left.
I got the last laugh though, because my deposit was three months rent in
advance, and then I paid monthly in advance after that. There was no
deposit to give back ;-)
Yes it is the individual 16A MCB that is tripping NOT the RCD. The one in
the garage doesn't trip only the one in the house CU. It has only started
happening within the last couple of months we have lived here for 12 years
and the freezer has been in there for 9 years plus (from new). So far
(touch wood) the MCB hasn't tripped since I repaired the two faults
I have looked again at my house CU and I think I may have lead certain
people up the garden path due to my lack of terminology/knowledge with
regards to MCB's and RCD's.
The CU is 17 years old and is an MK Sentry. The trip switch in question is
actually a RCBO. The details on the breaker are:
I believe the 6930s is the Old MK part number but it now appears to be
superseded by 6934s. Only my house sockets, (1 upstairs and 1 downstairs)
and my garage are protected with this type of RCBO. All other circuits are
just MCB's (2 x 6A, 1 x 16A, 1 x 32A) and a 100A big red switch for
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.