Have I Found It!

[John]

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.

Cheers

John

PS. Thanks Geoff the stamps are in the post tomorrow.

[Harry Bloomfield]

John brought next idea :

Probably not. You can get freezer supply failure alarms. Could you perhaps plug one of these into a socket, so that you would know when it tripped? It might offer some clues as to when it is tripping.

[John]

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 for power.

Cheers

John

[Martin]

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...?

[Bob Minchin]

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.

hth

Bob

[Calvin Sambrook]

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!

[The Natural Philosopher]

You mean the RCD is not tripping? so its current overload and not earth leakage that is happening?

The only thing in the garage

Probably not.

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 the wiring.

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 ;-)

[John Rumm]

Do you actually mean MCB or do you mean the RCD in the house?

You have found a problem (well two in fact). But whether they are connected will depend a bit on the answer to the above.

[John Rumm]

The circuit in question is a radial - not a spur[1]. With a radial circuit the cable is sized such that it can safely carry the full circuit load.

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.

[1] 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.

[Martin]

So I was right - I'm not an expert ...!

Many thanks for the explanation, John.

[John]

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 yesterday.

Cheers

John

[John]

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:

[Test Button] [Switch] RCBO LN 6930S 16A 30mA Type 2 240V

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 everything.

Cheers

John

[John Rumm]

OK, that changes things a bit. In which case I would wait and see if you get another trip. An arcing connection in the plug could trip a RCD in the right circumstances.