Tripping MCB after 20 mins

mbow · Apr 7, 2011

Hi everyone,

I have a crabtree consumer unit with the old Crabtree C50 MCBs in it. The lighting circuit is a riing on a 7.5A MCB, 1.5mm cable and services 6 bathroom spots, 6 lights and 2 outside security lights.

The problem is that the mcb trips after 20 minutes or so. this is when only a couple of lights are used so demand isnt high. It even trips when no lights are on!! I have tried isolating the problem to specific lights but it trips randomly.

One thing I have noticed is i have rock wool insulation over the cables in the ceiling void. Can this be heating the cables up??

Can anyone give me an idea of what could be causing it to trip?

Thanks

Deleted member 9648 · Apr 7, 2011

Have you IR tested the circuit to eliminate a fault?.....if not that would be the obvious first step.

morph · Apr 7, 2011

unusual circuit there.
if it aint overload then its eart fault developing over time,
have you tested the cables , removed fittings and done a visual?

mbow · Apr 7, 2011

Ive done a visual and it all looks good, and IR test results are good too. Its stumped me a bit!

Deleted member 9648 · Apr 7, 2011

If it's not overloaded and IR tests are good it may be a worn out MCB.....try changing it for one of the same rating and type....a clamp meter will also be useful to check the current being drawn.

ian · Apr 7, 2011

I have had similar problems a few times before, its always been a problem in one of the lights, generally insulation breaking down after a while, or a fault in a PIR, or a transformer.

Ian

mozzo · Apr 8, 2011

There could also be moisture ingress in the outside lights, so if after checking internal light fittings and changing MCB the fault still exists then you could isolate the eternal lights as a test to determine if the problem is indoor or outdoors. Also another thing to bear in mind is that you should make no assumptions about the wiring of a property especially old wiring. It is not unheard of for DIY and other trades to tap off from lighting circuits to use as a socket outlet somewhere in the house.