Nails through ring main cable

[NickB_99]

Hi,

Just moved in to a new place fairly recently (built ~1970s) and been removing shelving from one of the downstairs rooms. There is a ring main socket outlet near the floor, with the cabling running down behind the plaster from above. Unfortunately, when the socket was put in, the wiring did not line up and is about 10" to the side. When the previous owner put the shelves in, they nailed the batons either side, which has had the effect of putting two nails through the cable. The plaster is weak so have now removed and inspected the cable below the plastic shield over the two cables to the socket. Both nails have gone through the same cable.

A short length of one of the cables (about 4 foot) will need repairing. My two questions are:

1) Can I replace this length of cable with some new cable and join with a maintenance free joint to the original cable just above the top nail hole? And if so, what type of joint is acceptable? From what I've read, a trad junction box can't be used as it is to be buried behind plaster. The wiring is the old colour coding of red, black & unshielded earth with multi-cores for each. I'm aware that the colours have changed now.

2) We have recently had the electrics upgraded to remove the old fashioned fuse wire consumer unit, in favour of a more modern one with circuit breakers etc. This sockets circuit is now protected by a 32A MCB. Assuming I can do this repair to the protected circuit, is 2.5mm2 Twin and Core acceptable for this repair? I picked some up at the DIY superstore and said it was ok for ring main & sockets up to 23A.

Thanks in advance for any advice.

 

[securespark]

Yes, you can make a permanent repair to cable. You need some good quality kit to do it & some heat shrink sleeving. Think someone posted some pix once showing how best to terminate and dress the joint.

Was it you, RF????

Use the same size cable & don't forget to put a two-colours sticker near the DB if it does not already have one.

 

[TicklyT]

First problem is that the cables shouldn't be there in the first place!

They should be horizontal or vertical within the width of an accessory (switch, socket etc) or within 15mm of a corner. There is information about safe zones elsewhere on here.

First choice would be to replace the whole length of the damaged cable (and move it to a safe zone).

Second would be crimps, applied with a proper ratchet crimper, and heatshrink sleeving.

Another option may be to chop a back box into the wall to accommodate the repair. This would then create it's own safe zone for the existing cable run in line with the box. You could use choc blocs as the joint would be accessible behind a blank plate, or possibly even fit a socket if the position isn't too out of place

 

[GaryMo]

They should be horizontal or vertical within the width of an accessory (switch, socket etc) or within 15mm of a corner. There is information about safe zones elsewhere on here.

Just correcting your typo - 15cm, 150mm.

 

[NickB_99]

Use the same size cable & don't forget to put a two-colours sticker near the DB if it does not already have one.

Sorry for the naivety securespark...do you mean I should use 4mm2 T&E cable for the same size as the 32A MCB ? Also, the electrician who fitted the new consumer unit also added 3 new double sockets for me into 3 of the rooms. Shouldn't they have applied this sticker when the work was done? (I'm guessing DB = Distribution Board). I read some of the posts which say it depends if it is Radial or not. TBH not sure what I have here! but I guess a higher rated cable would cover both types.

First problem is that the cables shouldn't be there in the first place!

Completely agree! Though feel a bit hard done by that the job wasn't done right at the time. Although, have just re-measured and actually looks about 3" outside the left edge of the single socket. Can the single be removed and extra space chipped out to the left and a double socket put in instead? Then it would fall within the edges of the socket - compliant?

 

[securespark]

Apologies, guys...missed the bit about the cable not being in safe zones.



As for the cable, if the circuit is socket outlets fed by a 32A device, then it's either a ring on 2 x 2.5mm² or a radial on 4mm².

Your vintage of cable may well be imperial in which case it will be tricky to id. But it shouldn't be difficult to look at a cut ends of 2.5, 4 and your cable to visually determine which is the suitable cable to go for.

 

[pdcelec]

if it is imperial, it will be tinned and either have 3 or 7 strands. 3/036 or 7/029. The details of these cables are available if required

 

[ban-all-sheds]

When the previous owner put the shelves in, they nailed the batons either side, which has had the effect of putting two nails through the cable.

Through which parts of the cable?

Was the socket working OK?

We have recently had the electrics upgraded to remove the old fashioned fuse wire consumer unit, in favour of a more modern one with circuit breakers etc.

Did the electrician test all the circuits before and after?

What doe the IEC for the CU replacement show for continuity on that ring?

There is information about safe zones elsewhere on here.

• http://www.voltimum.co.uk/files/gb/attachments/niu/l/attachments/c5-43.pdf
• http://web.archive.org/web/20061110111450/http://www.niceic.org.uk/downloads/concealedcables.pdf
• IEE article
• IEE Article on 17th Edition requirements for concealed cables.

Can the single be removed and extra space chipped out to the left and a double socket put in instead? Then it would fall within the edges of the socket - compliant?

That would be fine.

 

[NickB_99]

if it is imperial, it will be tinned and either have 3 or 7 strands. 3/036 or 7/029. The details of these cables are available if required

Have had a closer look at the cable. It has 7 strands in both the live & neutral. The earth has 3.
I did some comparisons...2.5mm2 is a bit smaller; 6mm2 is much bigger. Went to the DIY store today and showed a section of cable. Recommendation was 2.5mm2 would be adequate and I've since replaced cable up to the socket. All seems ok, no tripping circuits.

Through which parts of the cable?

Was the socket working OK?

Both holes had caught the left hand cable quite squarely. My concern was between the lower hole and the socket (about 2 foot distance). The inside of the cable was completely blackened (but insulation looked intact).
The plug has never been used since we moved in so can't confirm if working ok. Or indeed what was tested when the CU was added.

Did the electrician test all the circuits before and after?

I would need to check with the sparky as to what circuits were tested. They added 1 new double on the downstairs cicuit and 2 upstairs. There are two MCBs in the CU labelled 'sockets' and checked they are down/up today.

What doe the IEC for the CU replacement show for continuity on that ring?

Sorry! not sure what you mean by this
The work was planned in two phases. Phase 1 - CU and extra sockets. Phase 2 - Kitchen is being done later and waiting for wall to be taken out. I was expecting there would be certification for all the work done, but was expecting it to be provided after phase 2 complete? & possibly test results if that's what the IEC is? The company is NICEIC and ECA registered, so hopefully reputable.

However, since the info from ban-all-sheds I've noted that the safe zone extends below the sockets also (thanks for the URLs!). I've checked the 3 new sockets and on one upstairs, the cables come in at about 45 degrees below from the left. It definitely steps outside the safe zone . I'm thinking that I should follow up with them. Given the invoice has now arrived for payment of phase 1, prob worth talking to them to verify this socket & testing to date? Also clarify what paperwork I should get...

Any other advice welcome!

And especially thanks for the feedback to date