Upstairs lighting circuit - troubleshooting

[omniflow]

Our upstairs lighting circuit has tripped the circuit breaker. We've just got back from 2 weeks away, so weren't here when it happened and I have no idea what might have caused it. It's mostly standard switches, bulb holders and bulbs. There are a few 50W halogens with transformers and a couple of bathroom mirrors with inbuilt lighting.

I have tried to reset the circuit breaker, but it immediately trips again.

I don't really know where to start looking or how best to troubleshoot. Do I remove light bulbs, or would it be best to start with anything that's not a lightbulb?

[Mr Pointy]

Is it tripping the 6amp overcurrent circut breaker or some protective device like an RCD/ELCB or RCBO? If it's the circuit breaker & it won't stay in then it's a hard short & should be easier to find. Start by removing all of the bulbs, then get at the halogen transformers & bathroom mirrors. If it's not these then it's possible a wiring issue (mice?) & you can split the loop-in/loop out of each ceiling connector to break the chain & see if the fault goes, in which case the issue is downstream from the break (lighting circuits are commonly radials not rings). If fact you could start out doing this rather than chase transformers & mirror connections.

Of course the MCB could be faulty as well but you'd have to mess about in the CU to test that.

[megaphone]

Any outside lighting running off the circuit?

Edited by megaphone on Monday 16th June 08:11

[OutInTheShed]

Start by inspecting the loft for 'visitors'.
Tree rats chewing cables?

[Typhoo]

Try isolating the bathroom / en suite extractor fans, then try resetting the upstairs breaker.
A neighbour once had this problem and condensation had settled on a circuit board inside a fan housing.

[omniflow]

No outside lights and no sign of any gnawed wires (yet). I've eliminated about 40% of stuff but then got fed up. I'll start with the extractors in the morning.

[omniflow]

Mr Pointy said:

Is it tripping the 6amp overcurrent circut breaker or some protective device like an RCD/ELCB or RCBO? If it's the circuit breaker & it won't stay in then it's a hard short & should be easier to find. Start by removing all of the bulbs, then get at the halogen transformers & bathroom mirrors. If it's not these then it's possible a wiring issue (mice?) & you can split the loop-in/loop out of each ceiling connector to break the chain & see if the fault goes, in which case the issue is downstream from the break (lighting circuits are commonly radials not rings). If fact you could start out doing this rather than chase transformers & mirror connections.

Of course the MCB could be faulty as well but you'd have to mess about in the CU to test that.

It's an MCB that pops immediately when I reset it.

Would it be quicker to isolate one room at a time by totally disconnecting the light switch? As far as I am aware this would get everything on the lighting circuit in that room, apart from the extractor. This would be quicker and easier than crawling about in the loft looking for transformers, and would at least allow me to pin it down to a single room.

Or, is it possible for something in the room to be causing a hard short (on the lighting circuit) even after I've disconnected the light switch?

[clockworks]

Older style MCB with the pop-out button? I had a couple of those fail in my last house.

[omniflow]

clockworks said:

Older style MCB with the pop-out button? I had a couple of those fail in my last house.

The MCB is probably around 25 years old. No pop-out button. It's a switch - up is on, down is "popped". I use the term "popped" because it makes a popping noise when it trips.

I presume it could be the MCB, but I'll try and eliminate everything else first.

[Mr Pointy]

omniflow said:

Mr Pointy said:

Is it tripping the 6amp overcurrent circut breaker or some protective device like an RCD/ELCB or RCBO? If it's the circuit breaker & it won't stay in then it's a hard short & should be easier to find. Start by removing all of the bulbs, then get at the halogen transformers & bathroom mirrors. If it's not these then it's possible a wiring issue (mice?) & you can split the loop-in/loop out of each ceiling connector to break the chain & see if the fault goes, in which case the issue is downstream from the break (lighting circuits are commonly radials not rings). If fact you could start out doing this rather than chase transformers & mirror connections.

Of course the MCB could be faulty as well but you'd have to mess about in the CU to test that.

It's an MCB that pops immediately when I reset it.

Would it be quicker to isolate one room at a time by totally disconnecting the light switch? As far as I am aware this would get everything on the lighting circuit in that room, apart from the extractor. This would be quicker and easier than crawling about in the loft looking for transformers, and would at least allow me to pin it down to a single room.

Or, is it possible for something in the room to be causing a hard short (on the lighting circuit) even after I've disconnected the light switch?

Are you familar with how lighting circuits are usually wired in the UK - the loop in/loop out arrangement? The cable from the CU is dasy-chained around the lights with the connection usually made above the ceiling or in the ceiling rose but sometimes in newer houses it's made in the light switch. You can usually tell which because if it's the old type there will be only one cable in the switch back box while with the newer type there will be about three (power in, power out to the next light/switch & switched power to the light).

https://ultimatehandyman.co.uk/how-to/light-fittin...

If you can identify the loop in & loop out cables you can break the chain & isolate the downstream section of the circuit which should help track the issue. If the breaker holds in the the problem is after where you broke the circuit so re-join that & move onto the next point in the chain. Just make sure you know which cable is the incoming power from the CU & which is the ongoing circuit or you risk have a live cable floating round. If the looping is done in the switch it's much easier as you won't have to get the ceiling roses down.

[omniflow]

Mr Pointy said:

Are you familar with how lighting circuits are usually wired in the UK - the loop in/loop out arrangement? The cable from the CU is dasy-chained around the lights with the connection usually made above the ceiling or in the ceiling rose but sometimes in newer houses it's made in the light switch. You can usually tell which because if it's the old type there will be only one cable in the switch back box while with the newer type there will be about three (power in, power out to the next light/switch & switched power to the light).

https://ultimatehandyman.co.uk/how-to/light-fittin...

If you can identify the loop in & loop out cables you can break the chain & isolate the downstream section of the circuit which should help track the issue. If the breaker holds in the the problem is after where you broke the circuit so re-join that & move onto the next point in the chain. Just make sure you know which cable is the incoming power from the CU & which is the ongoing circuit or you risk have a live cable floating round. If the looping is done in the switch it's much easier as you won't have to get the ceiling roses down.

It's probably safest to say that I have a passing familiarity with it. I used to understand it about 30 years ago, but haven't had to do any lighting wiring for at least 25 years, so it's all a bit of a hazy recollection.

[omniflow]

Closure for this one - albeit a bit late.

Gave up on my investigations and called an electrician. Chap was fantastic and came round the next afternoon. After a fair amount of very warm investigation in the loft, he tracked it down to a chewed cable. The house is "chalet style", so the some of the wiring was quite hard to access.

As pointed out (by Mr Pointy), the correct way to isolate parts of the lighting circuit is in the ceiling roses / junction boxes in the ceiling, rather than at the light switches. This meant that my investigations were a total waste of time. The whole thing was made more complicated by the fact that single light fittings had been replaced by downlighters in two of the rooms - not as part of the initial installation.

Overall costs were extremely reasonable.

[Actual]

omniflow said:

he tracked it down to a chewed cable.

Our house has evidence of previous mice in the loft (just mouse poo).

There was a creeper which grew from ground and into roof that might have been the cause and has been removed.

For peace of mind I have placed a Ring Camera in the loft and it will send an alert if it detects anything up there.

[omniflow]

We've lived here 25 years and it's the first problem of this type that we've had. We're in South Bucks so Glis Glis are everywhere.