Nicked live core - heat shrink OK?

Had a sparky doing some electrics, including a fused spur running off a ring main. He left the fuse out of the spur plate (ring main behind it) while I finished off work.

When I'd finished and came to to push the fuse in, I got a big BANG and spark/smoke and it tripped the consumer unit RCD. First time I've seen this, pretty scary.

On investigation, it looks like he'd nicked the insulation through to a live core on the ring main and when I pushed the fuse in it was enough to push that live core against the metal box at the back - the red core is slightly blackened either side on the nick.

I'd prefer to cut out the damage but I've tried pulling on the twin and earth and there's no spare, so it would leave the cable quite short/tricky to reinstate.

Am I OK to bind up the nicked core with insulation tape and/or cover with heat shrink tubing? If it's much more than that I'll have to get him out again, but given the complete pigs ear that company have done of some LV downlighters (three melted connector blocks, said they'd checked the rest, I didn't believe they had and indeed - two more gone brown, one of which was split with the metal connector inside badly split in 2 places...) - I'd rather not let them set foot anywhere near where I'm working ever again.

So, any wise person know the approved way of fixing a mains nicked core in a back box (single, space is tight)?

Many thanks.

The damage to the cable from the arcing and burning at the fault will have affected several inches of cable due to heat softening the copper and combustion products ( conductive carbon ) contaminating the insulation.

Your choice, do it properly or bodge it and have another fault some time in the future.

Oh. That makes sense - unfortunately! So you mean digging out and replacing that whole run of cable to the next box? That would definitely be something they'd have to come back and do. Drat.

You might also consider digging out the one backbox so you can fit a deeper one. That will give you room to work with the cable ends after you have cut back to undamaged. A momentary flash from a short that blows the fuse will not IMO have caused very extensive heat damage, but you can trim it back and see.

Although not very elegant, it is possible, using say a 45mm backbox, to crimp on core extensions or to use a 32A connector block inside the box, and still have room to put your FCU on the front.

If the cable between sockets is run in oval conduit, then it is very much easier to do repairs and alterations. The extra cost is tiny.

I expect you will also be asking around friends and neighbours to see if they can recommend a more reliable local firm.

Or you could add another socket to the system and use it an accessible junction box with which to join the replacement cable. An extra socket is always welcome. It could be on either the ring circuit or the spur depending on which cable needs to be replaced.

very good point

Done it. Trimmed the damaged wire back about half an inch and its insulation an extra half inch. No blacked insulation on that side of the original damage. had to get a new spur plate as the screw head on the supply live was totally gone.

What a hassle!

And yes, we are looking for a new electrician now. Having recently used a local plumbing firm (highly praised on Checkatrade) who charged £300 for two guys to be here for 2 hours - and then having to re-do some of their work (I tried to leave bad feedback on Checkatrade but wasn't able to, a subject I now see has been well covered elsewhere on DIYNOT, suggesting Checkatrade are basically in the business of patting everyone on the back so the fees keep rolling in which is also the strong impression I got), I'm obviously pretty terrible at selecting tradesmen... I can do a great many jobs myself so when I call in an expert I expect them to be better than me, not for me to have to spend hours correcting their slapdash mess afterwards. Bring back proper apprenticeships?

Driver23 said:

(highly praised on Checkatrade)

Click to expand...

They are a total waste of space.

Get recommendations from friends, neighbours, relatives work colleagues.

I once used a builder who was recommended by a friend and kicked him off the job half way through, he was such a bodger. My friend's house is now suffering from badly cracked walls! Many friends and neighbours are not well qualified to give informed opinions beyond things like how clean the site was kept.

And when trades are working I tend to leave them alone as I know it's pretty off-putting having someone stare at you while you work. When I come along at the end I sometimes mention concerns/actual faults I can see and usually get an excuse/disagreement/shrug. A Brazilian I worked with on a renovation said the builders out there are scared to keep the customers happy as they simply won't get paid if not and its the opposite in the UK. Might be some truth in that. I'm just too British!

Why oh why can't building control give honest opinions? They are best placed of anyone.