overheating cables

[dawsirn]

Was called to a property today where the night before they had smoke filling up a room and the smell of burning electrical cables. By the time I arrived later today (was only called an hour previous to my arrival) a builder who was working on a extension to the property had already cut out sections of the wall which had black charring to see if he could find the problem.

So when I arrived the customer explained the existing setup and I was surprised to see a previous extension the property with it's own submain was spurred of the ring. Now this spur wasn't even fused down. It was just a 45a DP switch supplying a 100a main switch consumer unit with various MCB's supplying various circuits ranging from sockets, lights and an outbuilding.

So this dodgy setup has been doing it's thing for a few years until it went up in smoke yesterday. After investigating a bit further where the builders had already started digging away I noticed one of the ring cables was installed behind the skirting board which is obviously bad enough but to make matters worse there's a wet under floor heating system throughout! all the cable I pulled back was showing signs of overheating. This was so bad it appears to have actually blackened the floor all along the bottom of the skirting board. All ceramic tiles. The builder had already pulled out celotex which was charred and the inside of the metal capping on the ring cable was charred also.

I think the straw the broke the camels back was the building work currently going on, as the tools etc are all plugged into sockets supplied from this spurred consumer unit.

In case anyone was wandering I decided to completely disconnect the circuit until further investigation and remedial work is carried out. I also found a Insulation resistance reading of 0.5Mohm between the Live and Earth on that circuit too. end to end readings were fine. r1 0.37 rn 0.37 r2 0.60


I've never seen it quite this bad before. I was wandering how hot the cable would have to be for it to actually cause this sort of damage?
Would the entire circuit be compromised now and full rewire of circuit recommended, or would I just have to try and replace local cables to where the heat damage was found. Not sure why the low insulation resistance reading exists but I've already recommend that is found too.

I've attached pictures but apologies for the crappy quality.

 

[Lucien Nunes]

It's a TN-C-S supply and we can infer from the loop impedance tests that it was not just a neutral but a CNE conductor that failed (15 ohms to both E & N). Depending on the non-electrical services and bonding, the entire installation load current could have been diverted at the supply terminals back through any CPCs that had an earth anywhere along their length, as presumably the transformer remained earthed at the pole. If the best earth was on something connected to this final, a 1.5 or even 1.0 CPC might have been carrying the full load including any showers etc. A short circuit on anything more than a 6A MCB would not have been cleared at all

I would be suspect thermal damage to any and all cables that have say <10 ohms to earth at any point beyind the CU and would test for this. Check whether the DNO will bear cost of repairs under the terms of supply in the ESQCR.

 

[Marvo]

As you say it's plausible with a poor CNE on a TN C S arrangement but there would need to be very few parallel earth paths within the property and lack of bonding or plastic services for water, gas etc to complete the perfect storm for a very dangerous situation. If the CPC was clearly the overheated conductor I'd also lodge a claim against the DNO.