Lighting Fault

[keithbr13]

Customer has been experiencing tripping of mcb for light circuit, during the night when everything is turned off. I have replaced the mcb but the fault is still happening. Called back last night, they had come in from work and the trip had gone out during the day, the trip wouldnt reset and i noticed a flash from the nearest light fitting. I couldnt see any thing wrong but changed the fitting, then reset the circuit, everything seem ok as I was leaving it tripped again ! but rest ok. I am going back to test out the circuit, but any ideas would be greatfully received.

 

[ProDave]

Pinched cable probably, intermittently shorting out.

IR test should find it.

 

[Doc Hudson]

Without doing any formal test, IR, Earth continuity, Polarity etc just looking rarely achieves much. These sort of problems I would be assuming a cable fault somewhere. Little or no point changing MCB's as a first diagnosis check, as in most cases the MCB is operating correctly due to some fault symptoms on the circuit.

Doc H.

 

[Sidewinder]

Well Doc, obviously correct, however, one needs to remember an MCB has a finite number of operations in anger, so once the fault is found and rectified, it may pay to replace the breaker?...

 

[Theorysparky]

If it trips while everything is switched off ?????

for the price of a few quid change the breaker while doing your tests

if the tests are ok you dont need to go and get a breaker !!!!!!!!

 

[Riggy]

My understanding was that OP had already changed breaker?

If it trips when everything is off IR tests can be done with everything off as well? - watch out for two way switches, test both strappers.

 

[Adrian]

I had this once and found out it was the cable, it had a nail in it and as they walked on the bedroom floorboard it pushed the nail into the cable and tripped the downstairs lights

 

[SPECIAL LOCATION]

With every thing off there is still 230v going to every switch, and possibly to most light fittings if using the loop @ fitting method...

So plenty of room for faults to tip a breaker IMHO....

First thing I would do with any intermittent tripping problem is IR test the circuit...

ALSO I doubt I would change the breaker per-say...

But I may swap two breakers over..

e.g. Swap a known good breaker from the same CU onto the suspect faulty ciruit...

and put the suspect dodgy MCB onto a circuit that has had NO problems...

The fault will either stay with the breaker OR stay with the circuit!

 

[NozSpark]

also check for L-N faults... I had one the other day, for some reason a ELV transformer was shorting out!

 

[spinlondon]

I've had the nail through the board before now.

Someone notched the joist and then the carpet fitters had put their nail through the board into a cable.

Only tripped if you stepped in one particular place.

 

[ProDave]

Yes last intermittent light circuit trip I had turned out to be a pinched cable in a roof space. It was the cable feeding an outside light on a flat roof extension. The flat roof had been replaced 1 week before the circuit started tripping. I just disconnected that cable and rewired the outside light.

 

[keithbr13]

Spent the afternoon chasing this problem, I did IR test LN, LE. NE reading .01 meg ! The lighting circuit is wired using a junction boxes Found the feed cable to the loft box to be a lead cable connected to pvc with choc block ! Can I take it the lead cable will be giving the low IR reading ? All I need to do now is replace the cable and find the box it feeds from !

 

[Zeespark]

Strip out that part of the circuit, install new wiring and try to do away with junction boxes all together.

Andy

 

[Sidewinder]

Can someone explain how an low IR on N-E will trip an mcb?...

 

[Zeespark]

It wouldnt snakehips, only an RCD/RCBO/DP mcb would?

Andy

 

[Sidewinder]

Exactly mate, but the OP says it is an MCB?

A DP MCB may not either!

 

[Zeespark]

I apologise if the DP MCB doesnt, I know they exist but I am not 100% sure of the exact mechanics of it.

Andy

 

[SPECIAL LOCATION]

Can I take it the lead cable will be giving the low IR reading ?

Isolate both ends and test it would surely answer this..?

 

[Sidewinder]

Zee, its just a 2 pole version of a 1 pole or a 2 pole version of a 3 pole!

Think 2 1 pole breakers with a bolt through the operating levers to link them together, thus if either sees a fault current then both trip, same as a 3 pole.

 

[Zeespark]

Thanks bud, I apologise if seems like a silly convo to be having, covered VERY little of this in college and not alot of this as an apparentice.

Andy