Lights tripping RCD

[wired]

I’ve got an old (10-15yrs) CU and all circuits are protected by a 32ma RCD. My upstairs lighting circuit is tripping the RCD.

If I switch off the 6amp MCB that protects the lights the RCD resets. I can then switch the MCB on and everything is OK but then the RCD trips again after a couple of minutes.

(SOMETIMES it will actually reset if the MCB is left switched on; everything is then OK for a couple of minutes and then it trips again.)

Could anyone give me any idea what the most methodical way to trace the fault is, please? We’ve got quite a few metallic light fittings so I’m guessing this may be a good place. I’m a trainee at the moment so have some knowledge but would like to know how you qualified and experienced guys would approach a fault like this. Thanks

 

[breezer]

Get the IR tyester out, or in your case, take the last fitted metalic light down, see what happens

 

[wired]

Thanks Breezer.

As it's just started doing this 'out of the blue' i.e the circuit's not been added to or altered any ideas what the most likely cause would be? We've been in the house for about 2 years and it was rewired about 10 years ago. We've still got the original mettallic fittings and dimmers (the previous owners were big on dimmers!) that were in the place when we bought it but not had any problems up untill now. Thanks

 

[breezer]

something is "leaking" question is what?

I would suspect fitting expanded a tiny tiny bit and is now in contact with something it should not be

 

[Spark123]

Could also just be something that has gone crusty with the heat of a lamp over the years.
Do you have an outside light on this circuit?
Any signs of water getting in?
Do you know how to use an insulation resistance tester (AKA Megger)?

 

[wired]

Thanks Spark123. Yes I know how to use an IR Tester, (just about!) and no there isn't an outside light on the circuit. I haven't had a proper chance to poke around yet to see what's going on.

Is the fact that it will reset for a couple of minutes and then trip significant? I'd have thought that a P/E fault would just trip instantly - but as I say I'm still training and have little practical experience.

Thanks again.

 

[wired]

Found the problem!! Several cables in the loft looked like they've been gnawed through; carried on investigating only to find a pile of Rat S**t!