Board change (RCBOs) Lights trip sockets

[teacherspark]

Evening all,

I changed a board today in a ground floor flat. 3 circuits with separate RCBOs (Shower 40A, sockets 32A, and Lights 6A)

Switching on any light switch causes the both RCBOs for the lights and sockets to trip.

Insulation resistance results are in the 100s between conductors on the sockets. They are low between conductors on the lights (under 10Mohms) but still a 'pass' and nothing I'd expect to trip a RCBO.

Tested between circuits and get 0 ohms between neutrals.

A borrowed neutral from the sockets for the lights?

The main feed for the lights seems to go straight up from the board and the sockets straight down, I can't see an obvious place where the two might meet! There is a cellar so a lot of the wiring for the sockets can be observed.

The lights have not been looped in and out of the light fittings or switches so there must be JBs above. However, 'above,' is someone elses flat!

Split the ring and see if I can pin point the connection?

Or is there something else that anyone has experienced before in this situation?

The RCBOs only trip when a load is applied and the a light switch is turned on, they don't trip if the circuit is energised and all the lights are turned off.

There is a single bathroom light fitting fed on it's own circuit from the board which can be energised in the 6A RCBO and used with no issues without tripping. So the RCBO should be fine and it all points to the lighting circuit and possible borrowed neutral.

I've disconnected the lights for now as it's currently vacant and I'll be back tomorrow afternoon to continue.

Thanks for reading

 

[Nebu1a]

I had this once.
Kitchen fitter had taken the under cabinet light feed from a light switch and the neutral from a socket.

 

[Deleted member 105166]

Clamp L&N on the lighting circuit at the board with all lights off, then with one on, two on, etc, that should indicate if current is taking an alternative path.

 

[Nebu1a]

If all of the lights trip the RCBOs then the neutral from the consumer unit to the lighting junction box must be borrowed somehow. If it is a plasterboard ceiling then the maybe cut a smale hole and use a camera to look in the void.

 

[shaun1]

Have a look in the light switches, especially doubles and 3 gang etc, to see if someone has taken the line across to feed a second switch which maybe then runs to a wall light/outside light etc and then down to a socket for the neutral. If you can't gain access to the wiring in the ceiling void, then someone else wouldnt be able to either to pick up the neutral.

 

[buzzlightyear]

possible a borrowed neutral from the lighting and the sockets .
it might be bridged some where.

 

[teacherspark]

I had this once.
Kitchen fitter had taken the under cabinet light feed from a light switch and the neutral from a socket.

There is a unswitched fused spur, which from a brief investigation, supplies a few things, the extractor, under cabinet lights and cooker ignition and maybe something else

The under cabinet lights still work when the lighting RCBO is switched off so they are definitely fed from the sockets and nothing trips when I switch the cabinet lights on.

But it is definitely a starting point.

Thanks for the reply.

 

[Yozza19]

Wander lead between n on light and n on sockets with n disconnected in dB

 

[Wilko]

Hi - yes likely to be a borrowed neutral, what a pain. Just thinking out loud, do any of the light switches have an N in them?

 

[buzzlightyear]

disconnect the neutral from the lights rcbo and put the tester on it and see if you a have a borrowed neutral to line on the socket circuit .

 

[DPG]

And let us know what it is once you find it!

 

[teacherspark]

Fused Connection unit above the counter top in the kitchen supplying ignition for a gas oven. Twin and Earth from the FCU, down to behind the oven and then the flex from the oven jointed to the twin in a JB, screwed to the wall. However this JB seems to have been originally for a lighting circuit running behind the oven right to left. 'They' seem to have removed the CPC from the middle terminal, connected that in terminal block, freeing up the middle terminal to connect the switch live for the oven ignition....oh but where have they connected up the Neutrals?? They have just connected those all together! Also from the same FCU there was another 1mm twin and earth going up, connecting the neutral from that also tripped the RCBOs. I just disconnected that and informed the customer, as that couldn't be traced. Possibly an old feed for under cupboard lights.

FYI, how did I find it....
I split the ring at the board and at the 'furthest' point away from the board to roughly get two halves of the ring. Connected up one of the legs and worked out which leg was the 'culprit' and then kept splitting that leg down until I got to the point before and after the tripping stopped; the FCU!

Winner winner chicken dinner!

 

[buzzlightyear]

nice tidy job their , did they write on the capping what is what .

 

[MFS Electrical]

I’ll bet Kev the kitchen fitter was behind that

 

[Wilko]

I’ll bet Kev the kitchen fitter was behind that

Quote from Kev of Kev's Kitchens "Only 3 wires, how hard could it be?"
Well found @teacherspark!