Repairing damaged cable in wall

[Sparra]

Hi guys, so I attended a property today where the lights were tripping with a neutral to earth fault. It turned out that a week previously the tenant had tapped a little pin into the wall to hang a picture, incredibly he says it's been fine for a week and only started tripping over this weekend. On removing the pin the IR neutral to earth is back to out of range. And rn-r2 is 0.6, only 0.1 higher than the next nearest fitting. But it still needs repairing I hear you say. The cable runs down, where some rather nice solid flooring would be ruined by being lifted. If I replace just that section of cable from the light down to the damage point, how do you all feel about splicing cables and burying them? Does that count as maintenance free? I don't fancy burying a whole mf box.

 

[Sparra]

Joining/mending the cable is one aspect, and hiding it neatly is another. Client put up a shelf unit, and one of the fixing screws punctured a cable to an adjacent switch which controlled the overhead light. She didn't know about zones, and the screw was directly vertically above the switch. It was a dot n dab plasterboard wall, so not much room to work with. I dug out the plasterboard and sank a 16mm back-box, cut and jointed the cable, put a blank faceplate on and moved the shelf a couple of inches so the faceplate was obscured by items on the shelf, and stuck a label on that said "electrical joint behind here". A picture could have been hung over the faceplate as it was at a suitable height. The joint is safe, it's accessible, but virtually invisible.

Hmm, yes I agree the ideal solution is to always hang a picture over it. Afterall, the damage was caused by a nail for picture so it's a good place for one. I feel somewhat persuaded to argue that it's necessary to do this.

 

[GBDamo]

Hmm, yes I agree the ideal solution is to always hang a picture over it. Afterall, the damage was caused by a nail for picture so it's a good place for one. I feel somewhat persuaded to argue that it's necessary to do this.

I've fixed this in the past by using a miltitool to take out the plaster (75mm x 75mm) around the damage, cut the cable, fit a 16mm galv back box to house the damage repair and then fit a blank plate.

This may not sit well with some from a aesthetic point of view but when weighed against the cost of redecoration they soon come round.

It also acts as a reminder not to be such an idiot in future.

 

[Lucien Nunes]

I would expose and examine. If there was clearly negligible damage to the conductor, I would prefer to seal the individual conductors and then the whole cable, without cutting and joining. Whether you do that with self-amalgamating tape or whatever is more of a question. IMO there is a strong argument for not creating a joint if one is not necessary.

The IR is good so the conductors are not likely to touch or arc.

I'm not sure we can infer this. There is probably no carbonised insulation or existing arc damage, but if the wall is dry the damage could be extensive without showing up as low IR. If it should become damp in the future, then leakage into the wall fabric could occur if a large amount of conductor is exposed.

The RN-R2 suggests the conductors haven't been partially broken leading to a higher resistance.

This definitely doesn't follow. If for example a 5mm section was damaged so that only 10% of the cross-section remained, within a run of 5m of cable, the increase in resistance would only be 0.9% and not readily detectable.

With all the mention of crimps, remember that some types are not suitable for solid conductors. They might make a connection but not a reliable and durable one.