Intermittent lighting mcb triping

[Soulman steve]

Initially was obviously a hard fault because mcb would not reset. Unscrewed and checked all light switches for obvious fault (some metal) no visual problem, then found possible issue with disconnected wire in ceiling. Rose. Repaired this and initially mcb stayed in. Connected each light switch and checked light in turn and all was fine. Then tripped again after few minutes. Swapped circuit with another 5a mcb to see it was the mcb but fault transferred to new breaker. Removed some boards from loft to see if appeared to be a trapped squashed wire but nothing obvious. Reset mcb and left a light on to see if it tripped again but did not trip and after about 3 hours turned light off so no lights on in circuit, on after a while mcb tripped again. REset breaker successfully.
Looking for ideas to try to find problem which has only been occurring since yesterday

 

[SJD]

Any lights that get hot? Such as recessed halogen 50W downlights that are cooking the wiring, I've seen plenty of charred cabling, connector blocks and sometimes insulation burnt to cause a short, possibly intermittent. Similarly inside light fittings with tungsten or halogen lamps, overheating especially if fitted with lamps higher than the manufacturer's rating.

 

[Dartlec]

Is the circuit on an RCD as well? If not, then the tripping MCD might be down to water getting into JB or fitting somewhere - either outside, or under piping/plumbing fittings...

If there was an RCD then I'd expect that to have tripped first with water, but even then it's not guaranteed...

Otherwise the only way other than trial and error will be a case of testing IR between live and earth and narrowing it down to a faulty section....

It may be a fault within a cable, so not visible even if you visually check it all, so testing with the right tester may be the only way to narrow it down further...

The obvious things to check are a cable caught at a switch (sounds like you may have tested this), or rubbing on a metal back box - or perhaps a faulty metal light fitting...

 

[Soulman steve]

Any lights that get hot? Such as recessed halogen 50W downlights that are cooking the wiring, I've seen plenty of charred cabling, connector blocks and sometimes insulation burnt to cause a short, possibly intermittent. Similarly inside light fittings with tungsten or halogen lamps, overheating especially if fitted with lamps higher than the manufacturer's rating.

I do have recessed lights but these are on a different circuit which is not a problem. I have. Chandelier with a metal ceiling cap but this is rarely on and has not been on for some weeks now. I also do not use large wattage lamps anywhere and am puzzled when it trips when no lights are on

 

[Soulman steve]

Is the circuit on an RCD as well? If not, then the tripping MCD might be down to water getting into JB or fitting somewhere - either outside, or under piping/plumbing fittings...

If there was an RCD then I'd expect that to have tripped first with water, but even then it's not guaranteed...

Otherwise the only way other than trial and error will be a case of testing IR between live and earth and narrowing it down to a faulty section....

It may be a fault within a cable, so not visible even if you visually check it all, so testing with the right tester may be the only way to narrow it down further...

The obvious things to check are a cable caught at a switch (sounds like you may have tested this), or rubbing on a metal back box - or perhaps a faulty metal light fitting...

The circuit and mcb is protected by an RCD and that does not trip, that is why I am puzzled as an earth fault would be more likely to have tripped the RCD first. Also puzzled while it has seemed to trip even when there is no load on circuit, after leaving a light on for some time and not tripping, and as of today it has not tripped over night even with metal cased outdoor lights being on a dawn to dusk setting.

 

[7029 dave]

The circuit needs testing with an IR tester.

 

[static zap]

Is it upstairs with loft ad-hoch boarded over.
( seen wire in conduit , trapped and cut by conduit ends. )
As 7029 Dave says .
..For safety sakes - IR testing is needed ..

 

[Soulman steve]

The circuit and mcb is protected by an RCD and that does not trip, that is why I am puzzled as an earth fault would be more likely to have tripped the RCD first. Also puzzled while it has seemed to trip even when there is no load on circuit, after leaving a light on for some time and not tripping, and as of today it has not tripped over night even with metal cased outdoor lights being on a dawn to dusk setting.Have just checked fuse board again

 

[Soulman steve]

Just checked fuse board again and noticed that the circuit is NOT protected by an RCD just the mcb but luckily has not tripped for 2 days now.

 

[Soulman steve]

Is it upstairs with loft ad-hoch boarded over.
( seen wire in conduit , trapped and cut by conduit ends. )
As 7029 Dave says .
..For safety sakes - IR testing is needed ..

I live in a bungalow and loft is ad hoc boarded over. As said earlier i lifted some of boards and checked for damaged cabLes but did not notice any. It has not tripped since sunday after I checked around, however I have a normal Fluke multimeter tester with resistance testing. would this be suitable to test the insulation of the live to earth of would it need a specific ‘megger’ type tester, and if so would you suggest removing the main feed from fuse board and testing here to earth be enough and what resistance reading would be acceptable

 

[timhoward]

You would need a purpose-made insulation tester. A normal mulitmeter wouldn't tell you much unless there's a dead short (which is unlikely given the lights at least sometime work).
It would be cheaper to have someone spend <1 hour checking it it than to buy an IR tester. This sort of work also requires care and experience to avoid damaging anything while testing.

 

[Soulman steve]

You would need a purpose-made insulation tester. A normal mulitmeter wouldn't tell you much unless there's a dead short (which is unlikely given the lights at least sometime work).
It would be cheaper to have someone spend <1 hour checking it it than to buy an IR tester. This sort of work also requires care and experience to avoid damaging anything while testing.

Yes I guessed it would need a proper insulation tester like a megger tester.

 

[DPG]

Yes I guessed it would need a proper insulation tester like a megger tester.

Not just the tester, but also the knowledge of how to use it correctly and how to interpret the results.

 

[static zap]

Can you tell what gauge wire it's wired in , as I was amased how many amps must have flowed in my MCB to have not caused a fire.
(Fresh new dry wood in close proximity --oops)
Typically 1.00 or 1.5mm.
Yep was my "new" .. Old dusty cobweb filled house !
22 years ago.
(Also any external lighting -attached / modern LEDs left on )

 

[LastManOnline]

? If not, then the tripping MCD might be down to water getting into JB or fitting somewhere - either outside, or under piping/plumbing fittings...

Agree. This intermittent fault has all the hallmarks of a "water fault". They can be a right nuisance. Forensic visual inspection of all circuit accessories is one of your best weapons in this situation