MCB consistent delayed tripping - no load

[trebles]

First post so please be gentle. I am no sparks but am ok with basic wiring.

Looked through forum but can't find exact problem we have.

Have a 6 amp MCB in the CU feeding 2 lighting circuits - kitchen and utility room. The MCB trips after 10 minutes or so. Until then all lights work fine. It will trip even if all the lights are switched off.
MCB can be reset but trips again after 10 minutes. If you keep resetting it, the time taken to trip is reduced gradually.

I have isolated the problem to the utility circuit by removing its connection to the MCB. The kitchen circuit then works fine, no tripping. So the tripping is caused by the utility room circuit.

On the utility circuit are four interior 60 watt incandescents, and three outside lights. I thought it may be a problem with the oldish outside lights so I have removed the fittings and (carefully) taped up the wire ends. No change - it still trips.

I am confused why there is such a delay before tripping. I guess some kind of short would cause it but why the delay? Is it likely to be some sort of earth leakage caused by damaged wiring? What can I do to fault-find this? I have a good multimeter.

It has been like this for some time (years!) as I disconnected the utility room and we have been using a table lamp in there. Now I have received orders to sort it out or no more clean clothes....

Please help!

 

[JohnD]

1) when you cut or disconnect a cable, put the ends in a junction box which will cost you 50p or so. Don't just tape up the ends.

2) look at the Watts rating of all the lamps and tell us what they are. Be wary of halogens.

3) observe if anything else is on that circuit, for example a frost heater or shed.

4) have a look at your electricity meter so you can tell how much power is going through it (dial turning or LED flashing)

turn off everything at the consumer unit (fusebox) and observe that current drawn is zero. Now turn the suspect circuit back on and identify what is the current drawn and tell us.

Look for any signs of damaged cables or accessories, or water penetration. Mice like chewing cables.

Consider replacing the cable to the utility room if it seems to be damaged, and fitting new switches and ceiling roses. You will have to study how to do it as mistakes are common on lighting wiring. Or try to get recommendations for a local electrician, and verify that he is a member of a self-certification scheme.

 

[Chri5]

Replace the MCB, there only £4-£5.

6a = 6 x 230v or 1380w in total, if the outside lights were 300w x 3 plus the 4 x 60w plus the litchen lights (you haven't mentioned what you have in this room) you may have overloaded the circuit.

 

[trebles]

Cheers for your advice johnd

1) OK - but cable ends are outside - how do I waterproof them? Those traditional circular brittle-plastic boxes are not waterproof. Should I use one of those new soft plastic squarish boxes with chocolate block connector inside? (as featured in dragon's den).

2) one 60 watt central ceiling light. Two 40 wall lights high up. One overhead 60 watt in toilet in utility room - simple incandescent bulbs that all work fine - until it trips - no halogens

3) nothing else on circuit - there are 2 outside circuits fed from 2 MCBs protected by RCD - all in the CU. (BTW, the CU and mains connection and meter are in the utility room).

4) Will do that and report back

5) Have had mice in the kitchen (before I blocked entry hole!). Utility room is an old Devon coachhouse connected to the kitchen by enclosed passageway and was refurbished and rewired 8 years ago. The roof space is open with plasterboard fixed to rafters in between exposed purlins. Cabling for lights runs from CU on wall in coachhouse above plasterboard. Cables for light switches embedded in mortar-rendered walls. The mice were in the house - not the coachhouse - which is a separate building. Difficult to see how they could have got up there - however, I wouldn't put anything past the little bu...s! What cables I can see are not damaged and can't see any water penetration but will check thoroughly and report back. Is it possible to check cable integrity without ripping out the plasterboard and hacking into render?

Could I try to isolate the problem by removing every light fitting/switch one at a time and seeing what happens?

I read in another post you mentioning something about using a multimeter
(on resistance) to check if there was cable damage and by the ohms reading how far it was from multimeter connection? You didn't explain exactly how to do it though. Is that right or am I dreaming?

There is no cable as such to the utility room as the lighting circuit is connected to the CU which is in the room.

This morning we had the lights on and the MCB tripped after 10 minutes as usual. I just tried to switch it on now and it tripped immediately with a snap - so the fault is time-based and seems to build-up and get worse.

Cheers
Jon

 

[trebles]

Chri5
outside lights were only 60 watts each .. small incandescents again. they are now removed. Kitchen lights are two 60 watt spots. Total load of both circuits is well under 6 amps I think.

MCB trips even with nothing switched on anyway. Also, I didn't mention - I swapped the MCB with another that was working fine and that one still tripped. The old MCB is now on another circuit and working fine - so I don't think its the MCB.

It must be an earth leak somewhere? But to find it......
Cheers
Jon

 

[bongos]

Is the MCB that keeps tripping also protected by the RCD?

It is unlikely to be an earth leak. An MCB tripping generally suggests a L/N short circuit. Can you get access to any of the fittings to check for loose wires or poor connections?

Have you put up any new shelves or nailed anything into walls recently? Often a nail through the cable can cause annoying intermittent faults. Has there been any water leaks that could have got the circuit wet?

If you are still having no luck you are probably going to have to call an electrcian. They will be able to run some tests including insulation resistance which will show up broken/damaged cables.

 

[electronicsuk]

Is there any chance something could be wired into the circuit that shouldn't be? Ideally you would have a clamp meter and be able to measure the current on that circuit when all lights are off and see if there is just cause for the MCB tripping.

If there really is an overcurrent situation then you may have an intermittent fault as described above, or someone has done something silly like feeding a 13A socket off the lighting circuit. Might also be wise to test this theory by isolating everything except the problem circuit and check that everything you wouldn't expect to be on the lighting circuit doesn't work any more.

 

[trebles]

hi bongos
No. I have a split CU. On the left of the busbar are 2 MCBs fed from an RCD to the right of them. These feed outside circuits. In the middle are about 6 MCBs to interior circuits. On the right of the busbar is the twin pole master switch which switches power to the middle MCBs and the RCD.

THe MCB in question is in the middle and not fed from the RCD.

Sounds like I will be taking off every switch and light fitting/rose tomorrow to see if I can spot anything. There haven't been any new shelves etc (I think!) but this has been wrong for several years (see below). Is a 200 year old building so there may be moisture somewhere. However, it happens even in a dry spell so I don't think its moisture related. But I disconnected the outside lights in case there was damp in there. I'll have a good look tomorrow.

hi electronicsuk
I am sure there are only lights on that circuit. I was involved with the wiring 8 years ago in that I ran a lot of the cables but the CU was fitted by a sparky mate (no longer around unfortunately). It started going wrong several years ago and I gave up trying to find the fault and just disconnected the utility room circuit - we lived with a table lamp as I mentioned. At the time I was broke and couldn't afford an electrician.

Its a weird fault in that the trip is normally delayed. However if I keep resetting the MCB it gets shorter til the trip is finally immediate - like this morning. Any one have an idea what would cause this particular symptom? It can't be a dead short all the time because it would always trip immediately surely? It tripped immediately last time I tried it so if that continues I won't be able to test for current draw. Maybe this is one for an electrician but I will take everything apart first to see if I can spot anything obvious. Gut feel says it's damaged wiring and will need an insulation test but we'll see.

Thanks for input everyone - I'll keep you posted

 

[JohnD]

the delay will be due to the way the MCB works.

They have two tripping mechanisms

A magnetic trip, which pulls against a spring, and trips on a high overload

A thermal trip which heats up under lower overloads and trips when it gets hot enough. This takes longer to trip the less the overload is. In your case, it takes a while to heat ujp enough to trip, but, once it is warm, it takes less time to reach the critical temperature next time.

 

[Taylortwocities]

....... An MCB tripping generally suggests a L/N short circuit. .

I don't think so.

An MCB is an over-current device. An MCB tripping could be

a Live/Neutral short circuit (as you say)
but also
a live/earth short circuit
or
any load exceeding the tripping current of the MCB.