Lighting circuit tripping

edrobbo8 · Aug 14, 2011

Just been to a call out, 2 x 6A lighting circuits, one tripped out last night which triggered the RCCB on the main switch, so pretty sure it is a L-E fault rather than dead short as MCB switched on fine.

The circuit that caused the problems is one of the 6A light circuits, however the house was wired 10-15 years ago and additions have been made all over the place.

Took the live, neutral and earth out of MCB, neutral and earth bars, and there is ~216V L-N and L-E on the ends of that cable. This made me think that someone has borrowed a live or neutral from the other lighting circuit. Switched off other 6A MCB and tested - nothing, so switched it back on and tested ends of cables again, nothing.

Very strange, any thoughts on this?

Also, I.R test was low at around 3 M Ohms. Re-terminated cable into MCB that had caused problems and it all works fine, checked in a few switches to see if there was a borrowed N or L but nothing.

What would you do? Leave it until it trips again?

Thanks for the help!

malcolmsanford · Aug 14, 2011

When you had the 216v on the disconnected cables was any lights on the other circuit on?

When you isolated the other lights MCB, tested and there was nothing, then reactivated the other circuit MCB, and there was still nothing, was all the lights off on that over circuit, or perhaps the 2 way light off?

This does seem like a borrowed neutral. As it was wired 10-15 yrs ago it would have been to the 16th and most likely the lights are on the non protected side, are they?

Did you find a fault, that tripped the RCCB/RCD

edrobbo8 · Aug 14, 2011

Thanks for the reply Malcolm.

The lights on the other circuit were not on even when their MCB was turned on, so it was as if the circuit was open while the cables were disconnected from the tripping MCB.

There was no obvious fault that tripped the RCCB, and they are all protected by the RCCB on the main switch, only 5 circuits in total and all protected. I think it will require more inspection but as it was a Sunday the guy just wanted his lights working, but think I will get in touch to sort it out as there is obviously problems somewhere, but not much access as loft has been converted in last 2-3 years, and I think that is where the borrowed neutral might be.