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.

 

[Midwest]

You could cut a 400mm circular hole above, use Wago Box to joint cable above then run new cable to what is below damage. What is below damage?

 

[Sparra]

You could cut a 400mm circular hole above, use Wago Box to joint cable above then run new cable to what is below damage. What is below damage?

The cable in question runs from the damage point up the wall to the first of 2 wall lights. So in this case there is a light above and good cable below.
So I assumed the advise would be to cut out the damaged area and replace the section of cable up to the light.
Sounds like you're advising a wago mf lighting box? I didn't much fancy chopping out the wall in this block of flats, so wondered how you all feel about splice and heat shrink, or even wago spliceline connector and heat shrink which I've seen posted as a solution elsewhere but wanted to ask your view as to whether that's possible/sensible?

 

[timhoward]

You need to end up with two layers of insulation and reasonable physical protection.

Crimps are fine for a joint, but they need containing or surrounding. I’d think self amalgamating tape would be better than heat shrink.
What’s the wall construction, is it a stud and plasterboard wall? That would determine my approach.

(You don’t always have enough give on the original cables to join them up again and get anything around them and a fair bit of wall needs removing even to expose enough to crimp them.)

 

[Sparra]

(You don’t always have enough give on the original cables to join them up again and get anything around them and a fair bit of wall needs removing even to expose enough to crimp them.)

That's why I don't want to dig much our really, I fear it's concrete. Surely not, but maybe. Plasterboard on top. But judging by how much of the pin was in the wall, there's not a lot of space at all.

 

[Sparra]

You need to end up with two layers of insulation and reasonable physical protection.

Crimps are fine for a joint, but they need containing or surrounding. I’d think self amalgamating tape would be better than heat shrink.

Hmm. Hadn't thought of that. The conductor itself has a reading which, incredibly, suggests it's not damaged. Whereas a crimp connection must be good. It's definitely don't want to crimp and leave a connection with worse continuity to what's there already. What's your feeling on using self amalgamating tape over the existing cable and damage? It must just be a pinprick. It provides insulation but not mechanical protection, right? It's hard to imagine a pinprick in a sheath that's been reinsulated with self amalgamating tape being a problem...

 

[timhoward]

As I'm sure you know the trouble with these situations is having to 2nd guess the actual damage.
The IR is good so the conductors are not likely to touch or arc.
The RN-R2 suggests the conductors haven't been partially broken leading to a higher resistance.

If you had a thermal camera you could stick a big load on and see if there's a hot spot showing.

It would be very tempting to let sleeping dogs lie, but unfortunately it needs examining just in case e.g. next week a vibration causes the earth to break.

I'd very carefully cut out a rectangle 1 inch wide, 2 inches tall (without cutting through the cable!), carefully cut away the outer insulation, have a look, and if all is ok wrap the neutral with a couple of turns of insulating tape, and then the whole lot with AM tape.
A bit of MakeGood from screwfix, carefully sanded, and it will be good as new unless it's wallpapered of course, in which case the first job is trying to cut out the wallpaper to reinstate afterwards.

 

[Midwest]

The cable in question runs from the damage point up the wall to the first of 2 wall lights. So in this case there is a light above and good cable below.
So I assumed the advise would be to cut out the damaged area and replace the section of cable up to the light.
Sounds like you're advising a wago mf lighting box? I didn't much fancy chopping out the wall in this block of flats, so wondered how you all feel about splice and heat shrink, or even wago spliceline connector and heat shrink which I've seen posted as a solution elsewhere but wanted to ask your view as to whether that's possible/sensible?

That would be your call. Any joint must be available for inspection, i.e if it fails you know where it is. Wago connectors are only maintenance free, because of the box they are installed it. Putting spliced/soldered joints in a wall is a bit dodgy.

 

[Spoon]

Any joint must be available for inspection,

I can't see that being practical all the time and while it would be great to do that, does the regs state it?

 

[Midwest]

I can't see that being practical all the time and while it would be great to do that, does the regs state it?

All screw terminal joints should be accessible, not sure I’d be comfortable with crimped or soldered joints not, being reliant on the installers skills therein.

 

[Spoon]

All screw terminal joints should be accessible, not sure I’d be comfortable with crimped or soldered joints not, being reliant on the installers skills therein.

We can all agree with screw terminals being accessible but personally I'd go with reg 526.3 for soldered or crimped connections.

 

[Rockingit]

Just thinking outside the box a bit - assuming that the cable is a feed to the light, how far apart are they? As in, would it be an option to reposition the light down to the damage?

 

[Midwest]

We can all agree with screw terminals being accessible but personally I'd go with reg 526.3 for soldered or crimped connections.

I personally wouldn't, especially 6242y cable etc. Though that might not be the cable in this instant.

 

[Sparra]

You're all superstars, thanks for your messages.
You've def helped me clarify my thoughts on the matter. Namely 1) don't just leave it. I've already had one sleepless night about it and I don't fancy more, let alone the worst happening. You've just confirmed it. 2) am going to start with timhoward's advice to carefully cut out and inspect. Wrap if appropriate. I rather like the idea of dropping the lights, though it might look rather odd. I'll present that as an option. Worst case, a light box goes in.
But #midwest, it's 1.5mm t&e. I've not tried soldering or crimping it, does it just not work? The fact that you've said you wouldn't leads me to think this isn't the place to try it for the first time!

 

[pirate]

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.

 

[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.