crabtree rccb nuisance tripping.

10 Feb 2004
Reaction score
United Kingdom
Hi my brother in law has a crabtree split load board, the installation is fairly good, all bonded correctly and the cabling is tidy. Everything in the right place and tight.

The lights are on the non protected side and the sockets are on the protected side as they should be....

However, when the lights are turned on (this can happen on 2 separate light circuits) the rccb trips, and the protected sockets lose power. The lighting circuits however stay energised.

They have lived in the house for 2 years and have had builders in the whole time the board was installed just after they moved in by a sparks - family friend who is no longer a family friend (for other reasons...).

Having had a good look at the installation, the only 2 things I can see as causing this are a shared neutral or earth between the lighting circuits and a socket ring, or a faulty RCCB.

Anyone got any other possibilities? I'm going to pop round with some test equipment over the weekend to have a better look, but I'm having a hard time trying to work out what the poss causes are.


Sponsored Links
This sounds like a bit of a tester!

1, firstly when you say lights are switched on, do you mean switched on at the breaker or any light switch?

2, you say the rccb trips on upper and lower lighting circuits?

3,apologies if insulting your inteligence but you sure everything is as it seems? nuetrals in correct nuetral bar? no strange accsessories in the house that you cant identify?
Check lighting neutral as indus implies - where is it connected to?
Sponsored Links
All the neutrals are in the correct slots and nice and tight. Nothing crossed, the rccb can trip when a light switch on either circuit is switched, not necessarily the mcb but the actual switches.

The only thing I can think of other than a dodgy rccb is that somewhere in the ceiling void there is a junction box that has a neutral or earth dragged from one of the rings.


The more I think about it the less I can see a specific cause.
And there's no strange accessories, I've unplugged it all and the problem still occurs.

Nothing on timers, no immersion, no underfloor heating, no external lights.

Is there anything plugged into the socket circuit?
It is possible if the supply is TN-CS that there could be a neutral to earth short on a circuit on the RCD protected side of the board.
disconnect the socket ccts from the mcbs and see if the lights trip the rcd.if they do rcd fault if they dont N-E cross fault on the lights and sockets which believe me is a nightmare.Who needs rcds anyway get rid
Yeah, I will bear out what thingy says. Sorry, I mean S123!!

I had a situation where energising a security light on the non protected side tripped the RCD. It was PME, and there was a N/E short at a S/O.
paulmassam said:
disconnect the socket ccts from the mcbs and see if the lights trip the rcd.if they do rcd fault if they dont N-E cross fault on the lights and sockets which believe me is a nightmare.Who needs rcds anyway get rid

A*s of your talking you are out.
paulmassam said:
Who needs rcds anyway get rid

BS7671 says everyone who has socket outlets which may supply equipment for use outdoors.

Thats who.

For what possible reason would you want to get rid of them?
Becuase they are always tripping whenever there is a fault, and removing the RCD saves actually repairing the fault?
It is at this point I troll out my oft-used horror story. My parents-in-law had a spa bath on a stand-alone RCD. The rest of the install was on a standard incomer on a PME supply.

The RCD kept tripping so for some mad reason known only to the nutter who called himself a spark, he disconnected the MEC.

This cured the RCD tripping problem, until the dishwasher went out of service. When the service guy came round, he could not believe his eyes!

There had been a problem with the d/w, and the element had arced against the casing, cutting a huge hole in the base of the appliance.

All because the guy decided he could not be ar*ed looking for the fault.

Who knows what could have happened?
Yeah, sorry, I was typing lazily... Yes, that's what I meant!!
There's only one earthing conductor, so there's no 'main' EC.

Have a look at the lovely diagram on page 25 of the bible! :D

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links