RCD tripping,- On lighting circuit???What!

Hi All,

I'm stumped on this one-maybe to much beer while the cup's on....

I've just wired in a new consumer unit and the lights keep tripping the RCD ,....It has been wired to it's own 6A MCB but it kept on tripping the RCD.
Further investigation showed that the the only 2 way lit up the tester(screwdriver) on the light switch screws ,....Checked further to find that the voltage to earth is 55v~ and when operating the switch it is 74v~

The wiring has confused me totally as the 1st light switch is one which controls a two way one side and a straightforward light. On the 2 way there is a twin & earth and a single red. The single red connected to the top terminal of the 2 way from here a jumper to the standard single light switch. On the bottom terminals the twin is connected 2 the terminals and the earth to the casing on the 2 way and a single cable to the single light switch.

The 2nd 2 way light switch is wired exactly as the first one,.... At the rose it appears to be a loop in system with the 75 & 50 showing up again when the 2 way is operated to the 2 "off" positions.

I did check and it was working fine (2 way)on the old board which was a rewireable wylex one. The wiring I have since been told has been done by the "electricity board" originally.

What have I missed!!!!- I know it's something that is right in front of me but I just can't see it!!!

Any help will be most appreciated- The client is now without lights tonite(said the telly would do!!!)-


Thanks in advance,.....

Is the light in question protected by the RCD? Is the CU a split load or does it have an RCD main switch?

My immediate thought (if it is a split load board) would be a borrowed neutral at the light on the 2 way switch. Also these voltages you are testing, what are you testing them with? Is it a multimeter? Also are you testing live to earth? If so it sounds like the earth for the circuit is missing.

Hi There,

i tested using a multi megger,....

The board is a split one but the lights are not on the rcd side,....

Also pls could you kindly elaborate on "borrowed neutral"....

Thanks

Covsparki

Sorry - The same voltages come up when I test at the rose,...
I.e The single red which I presume is the switch wire shows 50v to neutral & 75v to earth,.....

Once again using the multi megger,...

Thanks
Covspaki

I would say that it is almost certainly a borrowed neutral that is causing the RCD to trip.

This is where the live for the lighting circuit is fed from one circuit (probably on the non RCD side of the CU) and the neutral is 'borrowed' from a different circuit (probably on the RCD protected side of the CU). This will cuase an imbalance between the live and neutral that the RCD will notice, causing it to trip.

I was going to post you the diagram from for reference but seems to have dissapeared with darkernets demise so an explanation will have to suffice.

a "borrowed neutral" is where the live and neutral for a peice of equipment (nearly always a light) are taken from different circuits. Typically this happens in two situations.

1: when lights (usually wall lights) are added to an existing installation with the live taken from a lightswitch and the neutral taken from a conviniant socket

2: when there are two seperate lighting circuits and an upstairs/downstairs two way switching installation is done using only twin and earth cable.

borrowed neutrals present a risk to those working on an installation since they make it easy to accidently disconnect the neutral from an operating peice of equipment (which will then make the disconnected neutral live). Also if one of the circuits is on the RCD and the other isn't they will result in RCD trips.

P.S. as others have said it seems as well as whatever is tripping the RCD you also have some missing earthing on your lighting circuit. Do not ignore that issue.

P.P.S bas do you have copies of the images you posted in for reference using darkernet so you can upload them elsewhere?

Thanks - that makes sense,....

I take it no quick work arounds .... Needs a neutral from lighting circuit to eradicate the problem ???

Thanks for that - I knew the answer would be so simple that I would never think of it!!!!!!!!!

Once again thanks!

It could be that (on a s/l board) the neutral for the lights is on the wrong busbar.

Other possibilities for a non-RCD-protected circuit to trip the RCD include equipment like PC's and flu's (many, not just 1 or 2) are causing backfeeds onto the RCD.