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 of the circuit?

[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.