Help please, rcd tripping when light circuit is connected

[martlewis]

Can anyone advise what might be the problem please. We have a split load cu, sockets, boiler etc are rcd protected, lights are not.

Neutrals are seperate, rcd protected to one block, non rcd to other. For some reason whenever I turn on the lighting circuit it trips the rcd circuits. Any advice? I've asked an electrician to call but it won't be until monday so no lights.

Thanks.

 

[hairyben]

is this a new board, or an existing board thats just developed this fault?

sounds possibly like a borrowed neutral, but could be many things.

 

[martlewis]

It's an existing board that previously had all circuits on non rcd side except the shed. The board was put in when the shed was to protect it. Moved the sockets over to rcd side and moved socket neutral to rcd neutral bar too but now light trip the rcd. I can have either lights or sockets and shed but not both!

 

[rebuke]

Either someones miswired in the CU so that the neutral from the lighting circuit is wired to the RCD side (the imbalance would then cause the RCD to trip), or one of your lights is using a borrowed neutral (i.e. it's using a neutral from one of the ring mains instead of the lighting circuit).

I'd suggest (obviously with the power isolated etc) carefully identify all the neutrals on the RCD side, and make sure none of them relate to the lighting circuit (i.e. if it's all T&E follow the neutral back to the outer sheath, then trace the live back from there if you see what I mean). If that is the case, then you've got a borrowed neutral, which will be harder to find...

 

[martlewis]

In that case it's a borrowed neutral as I've traced and double checked the wires in the cu. That'll be fun to find.

On a lighter note I suppose I could move the lights to the rcd side (I know it's not the best idea) but our alarm system is combined with the fire system throughout the house and there are emergency lights linked in at top and bottom of stairs.

 

[hairyben]

In that case it's a borrowed neutral as I've traced and double checked the wires in the cu. That'll be fun to find.

On a lighter note I suppose I could move the lights to the rcd side (I know it's not the best idea) but our alarm system is combined with the fire system throughout the house and there are emergency lights linked in at top and bottom of stairs.

Start by disconnecting the lighting circuits one-by-one (MCB+N) to identify which one has the borrowed neutral. Be carefull that the "disconnected" neutral doesn't go live...

 

[martlewis]

Start by disconnecting the lighting circuits one-by-one (MCB+N) to identify which one has the borrowed neutral. Be carefull that the "disconnected" neutral doesn't go live...

There's only one lighting circuit. Only 9 lights in the house in total. Moved the lights to rcd side and all is well, except if the rcd trips.