Ring Main that isn't

[robaxeman]

I'm replacing a single socket in the Dining Room with a double socket - sounds easy? When I unscrewed the existing socket (having isolated the ring) I noticed that one of the Neutral wires was loose - but thought nothing of it. I disconnected the socket, removed the single box, chased out the wall for a double box, fixed the box, wired up a new double socket, reset the circuit breaker - pop! The circuit breaker would not reset - just kept tripping. I disconnected the new socket (leaving wires safe) and reset the circuit breaker - no problems. Tried wiring up the socket again - kept tripping. I disconnected the socket and simply wired the cables together using terminal block, just to connect the ring - circuit breaker kept tripping.

So now I'm stuck. All the sockets work fine on this circuit as long as I don't connect the offending socket.

Any thoughts?

Cheers,

R.

 

[Lectrician]

What breaker pops? Is it an MCB or an RCD?

If you leave the neutrals seperate from each other, does this stop the tripping?

Can you test for ring continuity at the socket? Test for continuity between L and L, N and N, E and E.

 

[securespark]

This seems almost certainly to be caused by something you have done.

When you removed the old socket and chased out, did you damage any of the cables?

I cannot understand why connecting the conductors together trips the supply UNLESS:

1. You are connecting live to neutral.

OR

2. One pair of conductors in the socket you have altered are RP. IE, connected to the wrong terminals in the preceding one (the black (or blue) conductor is actually live & the red (or brown) is actually the neutral).

 

[robaxeman]

Hi Lectrician,

Its an MCB that trips - the one for the socket circuits. When I test for continuity at the socket, I have continuity between all wires - L-L, L-N, L-E. Can this be right?

Hi Securespark,
The cables don't look damaged, but I guess that's still possible. I'm definitely connecting L-L and N-N - but then as you say maybe they're reversed at another socket. I'll check.

Thanks for your help.


Cheers,

Robin

 

[JohnD]

I spy a nail through a cable

or maybe no grommett and a metal box has chafed through the insulation.

 

[Space cat]

In the original socket, only one neutral wire was loose. Correct? Also, I assume the socket worked because you don't say otherwise.

The only thing you have done differently is to connect that loose neutral wire to the other neutral wire which, we know, is a good neutral. This means two things: that neutral wire is shorted to live somewhere down the line AND its connection back to the CU is broken.

As a test (not a solution) isolate the offending neutral wire in choc-block and test your sockets. Are there any that don't work? Also test anything that might be attached to the ring through an FCU.

Unfortunately, this will not be an easy fault to track down unless you know where the cable runs are.

When I test for continuity at the socket, I have continuity between all wires - L-L, L-N, L-E. Can this be right?

L-L continuity is expected in a ring. You should be worried if you don't have it.

A meter may show some L-N continuity if you have stuff plugged in or connected through FCUs. A mains transformer primary has a low DC resistance. Filament bulbs also have surprisingly low resistance when cold.

L-E continuity should NOT happen. In your case, I can only suggest that you have a neutral-earth short somewhere (probably in the same place as the live-neutral short) and the 'continuity' is from live through a low resistance load to neutral and thence to earth.

 

[ColJack]

just out of curiosity, you don't live in tamworth do you?

we did a hospital - flat conversion there and the show flat had a screw through a cable from a towel rail, and no time to fix it before it opened, so we split the ring and took the fault section out of it.. making it a radial on a 20A breaker..

it was a temp fix to get the showflat open for selling..

someone from the main electricians that I was subbied to was supposed to go back and fix it once all the flats were sold ( it obviously needed holes cut in the ceiling etc to run the new cable )

 

[robaxeman]

Spot on JohnD and Spacecat. The problem was exactly as you say - Neutral-Earth short caused by chaffed insulation presumably due to no grommet on the original box. Fortunately, there was a very short cable run to the next socket and I was able to replace the cable - everything is now working fine.

Thanks all for your help - it is much appreciated.

Cheers,

R.