RCD tripping because of lighting circuit

[Thomasdh]

I've got a Wylex 100A split load consumer unit. Layout is as follows (from left to right):

Attic Lighting (6a MCB), Upstairs Lighting (6a MCB), Basement Socket (20a MCB), Living Room Sockets (32a MCB), Kitchen Sockets (32a MCB), RCD (80a 30Ma) / Basement Light (6a MCB), Downstairs Lighting (6a MCB), Attic Socket (20a MCB), Bedroom 1 Sockets (20a MCB), Bedroom 2 Sockets (20a MCB), RCD (80a 30Ma), Isolator Switch.

The problem is I'm having tripping of the RCD which I've located to the upstairs lighting circuit. I had been moving boxes in the attic before it happened, so this seemed to make sense. I currently have the upstairs lighting MCB switched off and the neutral cable pulled from the neutral bar, as if the lighting circuit isn't there at all, and I'm having no problems with tripping now. To try and sort this, I've followed through a fair few posts on here, got some great advice, but am still stumped. The advice seemed to point to an earth/neutral fault, but I can't locate it.

The tripping only happens when I switch on a light on that circuit - when all light switches are off no tripping occurs. I've checked all the junction boxes (the circuit is wired with the junction box system as opposed to through ceiling roses) and light switches but can find no touching cables. The only thing I did find was a face plate screw pressing on earth - I've moved the cable out the way but it still trips. My lighting set up is as follows: Bedroom 1 - ceiling rose with single switch, Bedroom 2 - ceiling rose with single switch, Bathroom - 2 mains halogen spots with single switch, Stairs/Landing - 2 mains halogen spots with 2-way switches at the bottom and top of stairs.

Please help!

 

[flameport]

Cable in the loft crushed between a joist and a board on the floor, or crushed under a box.

 

[Thomasdh]

Thanks for quick and definitive response. I'll go up now and ensure they're all free from anything pressing down on them, then report back.

 

[Thomasdh]

Still trips. Checked the whole lighting run in the attic, moved everything off the cables (even if on boarding), then reconnected the circuit in the CU and turned an upstairs light on. It tripped again.

 

[sparkwright]

Look closely where cables pass through any switch drop conduits, if you have conduits.

 

[Thomasdh]

I don't have conduits - what would I be looking for though?

 

[londonboy]

Cables chaffing on the rough edge. Could there be a loose connection at a ceiling rose in one of the rooms?

If you can roughly work out how it is wired you could split it in half (disconnect part of it) and try and narrow down the fault that way.

 

[BSBS]

Has this problem started since you put the upstairs lighting on the RCD?
If it has only started since the RCD has been on the circuit and it only happens when you use a switch on that circuit is it any switch or have you narrowed it down to just a couple of switches on the circuit?

The reason I ask is that I had a similair fault after fitting a dual 30mA RCD board at my parents place. Every time you turned the landing lights on the RCD would trip. The cause of the fault was a borrowed neutral, it the landing lights had been wired from the downstairs circuit taking a live from the switch at the bottom to the switch at the top of the stairs but then used a neutral form the upstairs lighting circuit.

Something you may want to check. Good luck in sorting it

 

[flameport]

moved everything off the cables (even if on boarding)

If stuff has been put on top of the cables, it is quite likely that the cable is damaged and will stay that way, even if the objects on top have been removed.

 

[Thomasdh]

The ceiling roses don't have any loose connections in them. It's wired like this - feed from CU to attic, junction box 1 (feeds bedroom light and switch), junction box 2 (feeds bathroom light and switch), junction box 3 (feeds stairs/landing light and switches), junction box 4 (feeds 2nd bedroom light and switch).

Londonboy, I'm off to follow your suggestion - how about if I remove the 'out' from the junction box starting at the end of the circuit and working back toward the CU. After each light feed has been disconnected I'll test by turning on a light and seeing if the RCD trips. This should identify which light has the problem, then it's down to finding where it is at that particular light. Does that sound like a plan?

Hints for what to look out for that I may miss would be helpful - by cables chaffing due you mean that this could act like the crushing that flameport suggested?

 

[Thomasdh]

BSBS - the lighting circuit hasn't changed since the CU was installed. Thanks though.

Flameport - if a cable has been damaged by crushing, would I be able to tell by looking at it?

 

[Thomasdh]

Interesting... I disconnected junction box 4, reconnected the circuit inside the CU, then flicked a light switch and it tripped. So the fault is closer down the circuit to the CU... So, I disconnected junction box 3, reconnected the circuit inside the CU, then flicked a light switch and it tripped again. Time to move to junction box 2... I disconnected this, then went to reconnect the neutral inside the CU and it tripped immediately - without even having to flick the light switch.

Off into the attic again, will report back. Any help is much appreciated.

 

[Chri5]

BSBS - the lighting circuit hasn't changed since the CU was installed. Thanks though

Might have.....

You mention a loft lighting circuit AND a 1st floor lighting circuit, is there any chance that you have some how added either LOFT lighting <-> 1st FLOOR lighting.

Smells like a shared neutral / neutral cross between circuits.

If you were to temporarily place both circuits on the same mcb and neutral at the CU and then the rcd doesn't trip it would indicate a neutral issue.

 

[flameport]

if a cable has been damaged by crushing, would I be able to tell by looking at it?

Possibly, although it is just as likely that the inner insulation could be damaged and not show much or anything on the outside of the cable.

 

[Thomasdh]

I've disconnected junction box 2 (nearly all the way back to the CU now!) and so the only lighting on the circuit is to bedroom 1. Reconnect inside the CU, and voila, no tripping and it's so good to see a light come on!

Seems like it is fault with the bathroom lighting...


Chri5, I'll give that a go too.