slightly exposed live wire behind plug socket

[ravsaidha]

There was a slight nick and clearly something exposed which is why it short circuited. But he had checked the wire itself and was happy with it all before placing a sleeve. He actually didn’t seem that fussed with it and just felt it was touching metal which is why it just needed to be covered...

 

[ban-all-sheds]

You would not be the first person to encounter an electrician who is not very good.

IF the wire when live made contact with an earthed back box then it WILL have been damaged, and that's all there is to it.

 

[mattylad]

The Op stated that the power was off and has not said it has shorted at all. So the scenario where making a live contact is not applicable here.

That's all there is to it too.

 

[ravsaidha]

Well when I tried turning the mains on the fuse kept tripping. Not sure if that makes a difference?

Obviously bit worried if the electrician has just done a bodge job

 

[Jackrae]

The black material evident in the photograph comes from burning and the burning comes from arcing which also causes depletion of copper, so in simple terms the copper is damaged. Add to that the fact that "the fuse kept tripping" suggests either excessive current (fuse or MCB) or an earth-fault if RCD protected. But an earth fault resulting in an RCD trip probably would not result in any burning.

 

[John D v2.0]

Agreed with the above, someone did that in our place at some point and the screw was obliterated at the end and there was a lot of black. But it was just sitting there exposed for a long time without I'll effect.
In reality the thin bit of the wire may get too warm and further damage the insulation if there's a lot of current in that part of the ring. But there are probably a lot in service like that.

 

[ban-all-sheds]

There probably are. But there should not be any which have been ministered to by a professional electrician.

 

[JohnW2]

.... "the fuse kept tripping" suggests either excessive current (fuse or MCB) or an earth-fault if RCD protected. But an earth fault resulting in an RCD trip probably would not result in any burning.

I'm not sure why you are trying to make that distinction.

We effectively know that the problem arose because of an L-E fault (earthed screw/box touching live conductor), and exactly the same current would have flowed through that fault (until a protective device stopped it) whether or not the circuit were RCD protected. I therefore wonder why you are saying that burning would not be likely if there was RCD protection (but likely without RCD protection) - are you thinking that an RCD would limit the fault current to a shorter duration than would an MCB, or what?

Kind Regards, John

 

[Jackrae]

I stand corrected but from a semantic stand-point "probably" isn't the same as "not be likely".
What I was endeavouring to suggest was that a "minor" L-E fault (ie a 'leakage') might trip an RCD specifically if the current wasn't high enough to trip an MCB or blow a fuse.

 

[JohnW2]

I stand corrected but from a semantic stand-point "probably" isn't the same as "not be likely". What I was endeavouring to suggest was that a "minor" L-E fault (ie a 'leakage') might trip an RCD specifically if the current wasn't high enough to trip an MCB or blow a fuse.

Fair enough. It wasn't actually the semantics of 'how likely' that I was questioning.

Given that the evidence (both circumstantial and visible) is that the problem was a full-blown L-E 'short' (not a 'minor leakage'), I was concerned that what you wrote might perpetuate the common misconception in a good few people's minds that an RCD can somehow limit the magnitude of L-E current (to 30mA or whatever) - whereas, as you will know, all it can limit is the duration of whatever fault current is flowing.

Kind Regards, John

 

[John D v2.0]

I don't agree there is guaranteed to be damage at the site of the short. If the fault was created when the power was off, and was creating a fault of negligible impedance, then there would be little voltage across the fault and therefore little heat generated.
Otherwise we would be suggesting to replace the whole circuit every time the MCB tripped.
If in the other hand, as in my case, the fault presumably had a relatively high impedance, there would have been a lot of heat and damage until the wire fuse or wire or screw parted.
Don't forget that the let through of an MCB at 5xIn wouldn't actually be that great.

 

[JohnW2]

I don't agree there is guaranteed to be damage at the site of the short. If the fault was created when the power was off, and was creating a fault of negligible impedance, then there would be little voltage across the fault and therefore little heat generated. .... Otherwise we would be suggesting to replace the whole circuit every time the MCB tripped.

Indeed. However, I don't think it's being suggested that heating of the cable, in general, is a problem - after all, the requirements of ADS are deemed to leave the cable unscathed. I think it is being suggested that the local damage can result from localised heating ('in the fault') due to a fault of non-negligible impedance and/or arcing at the point of the fault.

Kind Regards, John