MCB Tripping On Lighting Circuit - Out Of The Blue?

[TMess]

Firstly, no alterations to any lighting circuits have been undertaken in years. Nevertheless, yesterday, the MCB on our Consumer Unit for our upstairs lighting circuit tripped. I switched it back on and a couple of seconds later, it tripped again, we once again I flicked it on and it stayed on all night. This morning, it tripped once again, and then again after I turned it back on.

Before calling out a real sparky, is there anything obvious I can check that could be causing this (light bulbs, 12v transformers, etc.)?

 

[TMess]

See my additional post #3! It will do..... you have to let it cool down for a couple of minutes first else you'll go round in circles.

I have followed your advice and let it cool for 15 minutes and I once again swapped it with another MCB from another circuit. Still trips.

 

[SPARTYKUS]

We haven't undertaken any work which would have affected the upstairs lighting circuit. Unsure about whether rodents could nibble cables upstairs?

In all honesty, is an electrician likely to find this fault much quicker than I could (i.e. using their testing equipment)? Or is it purely a case of ripping floor boards up?

Yes a competent sparky with test equipment WILL find a fault. Just randomly ripping up floorboards will not !

 

[polo1]

Yep, follow the advice have been given and get in a spark. You clearly have a short in the circuit. If above the affected lights is a loft, there may be no floorboard lifting required! But it does require a structured approach to fault find, so don't expect miracles in the first 5 minutes of the visit.

Regards

 

[SPARTYKUS]

Tmess

where in the country are you based

 

[piz]

Do you have an outside light ? Is it cracked filling with water?

 

[TMess]

Had a sparky come along, fellow member of this forum, he couldn't find the fault as we have hidden junction boxes in the ceilings (loft) covered by flooring. So, I had a proper go at finding the fault myself. It turned out to be a length of cable supplying a bathroom light above a cupboard.

Some numpty, many years ago before us, drilled a hole into the tile right in line with the cable. Accidents happen mind, but not when the hole is drilled only 50mm above where the cable emerges from the wall!

So, I've disconnected this cable and pulled it out of the wall. However, it's impossible to slide a new length in and as this is right next to the eaves, I can't get a 500mm drill bit in to enlarge the original chase. l'll have to remove the tile to get a new length of cable in, so that's a job for another weekend. Unless anyone has a better idea of how to get a new cable down from the loft and only 500mm down the wall?

Finally, how is it that this fault has only just caused the circuit to short? The light (and length of cable) has been working for over 6 years at the very least and the circuit hasn't tripped once.

 

[Octopus]

How do you know that "this cable" is in fact the fault?

 

[Richard Burns]

I echo Murdoch's question.

However if you can get a rod or wire to push up the chase from below into the loft attach the new cable to it and pull back down, being careful not to abrade the new cable this may be a way of pulling in the cable.

IF this is the fault then it may be that the drill hole initially tripped the circuit and then once the drill was removed it was no longer shorting but now perhaps damp has got into the cable recently and caused a low level fault.

 

[Skyba]

Seems strange how what you'd expect to act like a short circuit fault has acted like on overload fault or am I missing something.

I suppose if they only partially damaged the cable it could cause a high resistance

 

[TMess]

How do you know that "this cable" is in fact the fault?

I narrowed it down to this light fixture by systematically splitting up the upstairs circuit into rooms. I then re-connected until the MCB started to trip again. This narrowed it down to the bathroom. I then disconnected the ceiling light transformers and the extractor fan and the MCB still tripped. Which only left one more light, the cupboard light. Disconnecting this and reconnecting the transformers and extractor resulted in the MCB playing ball.

I then reconnected the wall light cable, but removed the light fixture. This resulted in the MCB tripping and a small spark and black stain emerging from the wall where the wall plug was some 20 - 30 seconds after the MCB was reset. I tested the light fixture on a different length of cable and it didn't cause any shorts.

So that's the long-winded explanation of how I found the fault to be with the cable.

- - - Updated - - -

However if you can get a rod or wire to push up the chase from below into the loft attach the new cable to it and pull back down, being careful not to abrade the new cable this may be a way of pulling in the cable.

IF this is the fault then it may be that the drill hole initially tripped the circuit and then once the drill was removed it was no longer shorting but now perhaps damp has got into the cable recently and caused a low level fault.

Tried getting a rod up there, it won't go higher or lower than around 60mm into the chase .

 

[Deleted member 26818]

Get some cotton thread and a hair dryer, blow the thread up or down the chase untill it comes out the other end.
Tie on some string, pull through, then a single conductor then some T&E.
Will be easier with someone to help guide in.

 

[SPARTYKUS]

Spin, that deserves a mention , really like that amazing practical solution.


Respect

 

[Deleted member 26818]

It's how we internally wire handlebars and frames, except that we use compressed air rather than a hair dryer. Although I have used the hair dryer method before on handlebars.

 

[DNS1]

That really is a brilliant idea!

Although I might have to set my hot air gun to cold and use that, I'm not sure walking around on site with a Nicky Clarke hair dryer will look too good (and certainly wouldn't go down well with the missus...)

 

[tony mc]

Did the electrician narrow the fault area down for you just out of interest?