lighting circuit tripping for no reason.

[barrata]

Hi all

Can I ask for some advice please? About 3 or 4 days ago the RCD for downstairs lighting tripped. Nothing too strange you might think.

The strange thing is that the RCD tripped during the day when no lights where on. I reset it and it went back on fine only to trip again about an hour later.

I turned it back on and tried all the lights on the circuit and they all worked fine and didn't trip the RCD when I turned them all on.

This has been the pattern for the last few days, it trips, I turn it on and then it goes again half an hour later even though nothing has been switched on anywhere.

No work has been done on the circuit for at least 6 months when we changed a light fitting but the circuit worked perfectly after that was changed up until now.

It an old wylex CU and the fuse is 6amp.

Does anyone have any thoughts as to why this might be happening.

thanks

andy

 

[JohnD]

You say RCD but I think you mean MCB. Does the thing that trips have a button on it, marked "T" or "Test"?

What letters and numbers are engraved on the thing that trips?

Does this lighting circuit include an outside lamp?

have you any signs of mice or water leaks in the house?

 

[barrata]

Hi JohnD

thanks for the swift response. It's actually the fuse for the downstairs lighting circuit that trips (ie it flicks from on to off) - sorry about my lack of knowledge of the terminology.

There is an outside lamp but we haven't had rain for weeks if that was possibly any issue. And we haven't had a water leak ever. Pretty sure there aren't mice that could be affecting it either.

thanks again

andy

 

[mo2]

when you say 'trips for no reason' you really mean that it trips for a reason, but you have no idea what this reason is.

And yes, it sounds like you have a 6 amp mcb

 

[barrata]

when you say 'trips for no reason' you really mean that it trips for a reason, but you have no idea what this reason is.

And yes, it sounds like you have a 6 amp mcb

Fair enough - "....no obvious reason"

andy

 

[JohnD]

Have you nailed down any floorboards, or nailed up any pictures recently?

Are there any loft lamps?

Have a walk round the loft with a strong torch, looking for any sign of damaged cables (vermin or something put on top of it) especially around junction boxes or ceiling roses, or water leakage. Do the same with the light switches and ceiling roses in the rooms.

If you house has any metal conduit that the cable has been run through during rewiring, there is a possibility that a cable has been chafed or pressed against a sharp edge.

If you have two lighting circuits, and therefore two identical 6A MCBs, you will find that the MCBs can be unplugged and swapped. It is most unlikely that you have a faulty MCB but this is an easy check. Turn them off before plugging and unplugging, and turn off the main switch as well.

Meanwhile, ask your friends and neighbours if they recommend a good local electrician, as it is possible you might not be able to trace it yourself.


edited to say: Downstairs lights, sorry, not likely to be a problem in the loft. Might be water from the bathroom though, or a nail. I am guessing you are not equipped to do resistance testing and measurement yourself?

 

[viewer]

Hi Barratta,
Don't know if any help, but had an interesting problem a few weeks ago. Went to deal with a flooded (due to bath left running) house where semi enclosed lighting circuit fuse had blown. Dried etc and put back lights. All worked. 30 minutes after leaving it blew again. Decided water still in fitting so put up temporary lamp. 30 minutes after I left - you guessed it. No problems showing with an insulation test. Decided to slowly go through everything. Kitchen fan was on lighting circuit. Nothing apparently wrong and it was switched off. Disconnecting the fan solved the problem. I've no real idea what was wrong, why any problem took 30 minutes to appear and why the insulation test showed nothing wrong (surely a damaged / shorting motor or switch should have given a low reading?). Anyway. Do you have a fan or burglar alarm (etc) on the lighting circuit which you can disconnect to narrow the field of investigation?
HTH V

 

[barrata]

Hi viewer and JohnD

thanks for your responses. I really appreciate it.

I got home last night and checked all the light fittings and as you suggested John, the cable had been rubbing against a sharp edge in the metal conduit into one of the switches. Our cables are fed through metal conduit which are chased into the walls.

Thanks again for all you help guys.

Just out of interest, why would this happen intermitently? Surely if it was shorting on the conduit it would trip all the time?

andy

 

[JohnD]

Might have been the fairies, might have been vibration, might have been that the cable and the conduit were expanding diferently in this hot weather.

How did you fix it? The sharp edges need to be covered or it will happen again. Sounds like a good reason to earth metal conduits even when they are just short bits in switch drops.

 

[barrata]

It must have been the fairies - that would explain how my house gets messy when I leave my wife and four year old at home for the weekend !!

I filed / sanded down the offending conduit, put insulation tape on it aswell and luckily had enough surplus cable to cut out the frayed bit.

thanks again

andy