New RCD box, live connected to earth on one ring main Ask questions and find answers to many subjects relating to electrics and electrical work

[andyb1979]

Hi everyone,

This is my first post here, so apologies if this has already been asked. Im doing some electrical work in my house, I do have the 16th edition regulations certificate so I'm legal (Im not a full-time electrician nor do I have part P though) and I have a question about fixing an existing ring main.

I've just replaced an ancient fuse box with a shiny new one with RCD, however one of the existing circuits was causing the RCD to trip and on inspection, neutral is connected to earth in this circuit. This is the sockets for the first floor. There's also a boiler connected to this ring main and about 9 sockets in a few different rooms.

I've powered down the ring main, taken all the sockets off the walls and inspected them, no problems there. So my fear is there is an incorrect connection between neutral/earth in a junction box somewhere under the floor! :(

My question is this - how can I identify and fix this problem? I had the idea to take all the sockets off the walls and using a long lead, find which was the first/last to the fuse box and try to identify which sockets the problem occured between, then replace just that bit of wire - Im trying to avoid rewiring the entire ring main as its just too much work.

If anyone has any techniques / advice for this sort of problem, please let me know as Im sortof feeling around in the dark on this one (no pun intended!) ...

Cheers,

AndyB1979

[ericmark]

Unless you have some clues as to where the half method is best way forward. Pick a point approx half way in ring main. Split it and find which half is bad. The take bad half and split it in half and find bad half. And so on. Remember to start with also split at consumer unit.
Having said that borrowed neutrals are more likely than earth neutral. Everything in consumer unit switched off test both all RCD lines to Non RCD neutral and all Non RCD lines to RCD neutral and ensure all are clear.
Normal is where someone wants extra lights from light switch which has no neutral so run to nearest socket. Once you have found fault I would try removing bulbs and see if it goes.

[andyb1979]

Thanks for your advice,

That's a good idea, splitting the main half way, that way I narrow down to the faulty area quickly.

Good point about the borrowed neutral. That is scary. If i re-wire a socket or two then half the lighting circuit stops working its not going to be pretty.

I'll let you know how I get on!

[andyb1979]

Hey I fixed it!

Using the split the circuit and narrow down the problem technique, I isolated two sockets that had the earth/neutral connection problem. These were next to each other so I reckoned there's probably a fault under the floor there.

Took the floorboards up, a couple of junction boxes there, but no fault, then I noticed the wire for one socket (which was a spur) went up behind a piece of architrave. I popped the architrave off and there it was, a screw straight through a wire!

Not my doing I stress, this was here when we bought the building. Im surprised no-one noticed it before now!

Thanks for your help :)

[ericmark]

I am glad you found it. Well done.