Fault Finding A Lighting Circuit

[thomp1983]

Hi all,

I got up in the middle of the night and noticed the downstairs lights weren't working, looked in the consumer unit and found the mcb tripped.

Flicked it back on and the lights resumed working.

Got up this morning and the mcb had tripped again, put it back on, went round all the rooms checking there were no blown bulbs which there isn't.

Mcb tripped again at some point during the day and now will only stay in for about 20 seconds before tripping again.

Only thing that's changed is the cat managed to get himself shut in the new extension overnight and I've known him to crawl all the way under the flooring to the eaves the other side of the house so it's possible he's disturbed something or wee'd somewhere he shouldn't of.

I'm off on Wednesday and the house will be empty so need to use the time to fault find the issue and could do with some advice re. my thoughts on how to proceed.

Tools wise I have a standard fluke multimeter and a uni-t multi tester that I've rarely used.

All the downstairs lights are wired via junction boxes for each light so, the radial feed comes from the consumer unit in to the first junction box, there is then a cable from the jb to the switch, a cable to the light and a 4th cable feeding the next light on the radial, all the other lights carry on in similar fashion.

There are no outside lights on the circuit or reason to think water ingress is the cause, unless it's the cat as above.

So my plan is to proceed as follows,

Draw a circuit plan,
Switch off the mcb for the downstairs light circuit
Open the first jb from the consumer unit, visually check connections then remove the cable feeding the next light on the circuit.
Switch the mcb back on and see if it holds.
If it does then switch mcb off, reconnect feed to second light, open jb for second light remove feed to third,
Mcb back on to see if it fails with 2 lights connected.
Repeat the above until I find the light which causes the mcb to trip.

Once narrowed down to a particular light visually check all the connections to that light, jb and switch, if not visually obvious then further testing of the wiring will be required.

Does the above sound the right way to proceed or is there a better way?

Thanks
Chris

 

[terryplumb]

Is it definitely an MCB that trips ,not an RCBO ?

 

[thomp1983]

Yes definately a 6a mcb, 2nd from the right.

 

[terryplumb]

Are the lights in new extension on the same circuit breaker ?
Has any work been done recently on the light fittings,or anything else on the circuit ?
Anything else besides lights on the circuit ,smoke alarms, burglar alarm, loft light or socket ?

 

[sparkymarka]

What type of bulbs do you have in your property?…..

 

[thomp1983]

Downstairs light circuit is solely lights, nothings been touched in at least 6 months, the extension lights are on the same circuit but they've been fitted for quite awhile.

I'm pretty sure all of the bulbs are leds and as I said when the mcb stayed on they were all working.

Thanks
Chris.

 

[terryplumb]

If every light on the circuit is switched off ,does MCB still trip ? If so ,there is either a short circuit or the MCB has gone faulty ( less likely but possible)

 

[thomp1983]

Yes all the lights are currently off and it still trips.

I might, given the ease of doing so swap the 2 6a mcb for the lighting circuits over in the c/u and see what happens, certainly an easier start than going around all the j/b's and it turning out to be a faulty mcb.

Thanks,
Chris.

 

[terryplumb]

Worth trying first. Are you competent to work safely inside the consumer unit ?

 

[333rocky333]

Regarding your plan rather than the first light start at the middle light then work either way dependant on if it trips.
Not ideal way to do things using the mcb to trip. May be better removing the lamps and using the continuity setting on your meter..
As someone mentioned earlier, seems like a L to N issue which is rare, is there any extracter fans on the circuit.

 

[thomp1983]

No extractor fitted, the downstairs light circuit only supplies the lights for 9 rooms and the usual bottom/top of stairs lights.

 

[SUNRAY]

One of my test methods for a short cct is to wire a large load in series with the whole circuit, such as a 500W floodlamp.

 

[thomp1983]

One of my test methods for a short cct is to wire a large load in series with the whole circuit, such as a 500W floodlamp.

Could you explain why please?

 

[SUNRAY]

Could you explain why please?

If a lamp is wired in series (in the live wire), a short between L & N after the lamp will make the lamp light rather than trip the MCB. MCB's are suposed to be replaced after so many trips and the lamp will reduce the tripping count as well as give a clear indication of the existance of the fault every time the circuit is re-energised during fault finding.

 

[securespark]

Another possible split would be to disconnect the extension lighting from the circuit.

Then if it doesn't trip, you know the issue is in the extension.

If it still trips, you know the issue is on the original circuit.

But rocky is right: best not keep re-energising a faulty circuit.

In the 90s, I remember searching for a lighting fault on an old house. The breaker held for a bit, then tripped. On the landing near the loft hatch, me and the customer heard a small noise.

I went up into the loft to do a visual search for the issue. Unknown to me, the customer went to reset the breaker.

Well, I found the fault! About 15 seconds after the breaker was turned on, there was a massive flash and a strong smell of burning wood.

I quickly crawled over to where it was and found a small fire starting. There was a rats nest of VIR cables with the conductors just twisted together, laying in a corner made by two joists surrounded by sawdust, which had just caught fire.

Looking back, it was quite comical. I yelled something like:

"Oh deary deary me, there appears to be a slight amount of combustion happening here, please disconnect the electricity supply at your earliest convenience..."

I grabbed a load of Rockwool to try and smother the flames, then yelled for the chap to bring his kitchen fire extinguisher. Luckily, he was ahead of me and he chucked it through the loft hatch.

A bit of a squeaky bum moment....