copper earth wires blackened

Moisture

Probably its corroded due to cantact with air over a long period of time, it could be damp, but , as I have not seen the premises thats a guess, if the cause of this was overheating them the pvc would be in a sorry conditiion in all the sockets

Ben

the blackening was like a carbon coat on the wire as when i touched it my finger was black. The Zs readings were quite high but within spec. Any Ideas ?

This sounds like a plasticiser agent that was used in old style pvc/pvc cables similar to the green goo from late 60's T&E. Usually this leaches out of the cable once the it has been subjected to long periods of heavy loading and heating. You need to check your IR values as usually this condition will adversely affect the readings meaning a fair chance of rewiring.

green goo/slime is usually quite noticable. Did you see any traces? See my pics...

Jdsparky said:

Moisture

Click to expand...

Second this.

no just a very fine black carbon like substance. This only appears on bare earth cable and also face plate screws seam discoloured. Any ideas

Sounds like a damp issue as indicated by other posts then if you have no sticky green goo.

Come on boys learn your chemistry if you mix copper and damp conditions with oxygen you get copper oxide which is green wire instead of metallic copper shine although not the same as the green goo suggested earlier on, the blackening can be caused by contact with gases produced from cleaning product, sulpher will aslo turn copper black its very common to find blackened copper in florida where everyone has a swimming pool and the treatment and cleaning fluids over time can turn the copper black from the gasses they produce.....

Having said this ive found this on alot of occasions and not just the earth but sometimes the lines, even when pvc coated the blackening is present throughout so i would possibly put it down to a coating in manufacturing to stop the pvc sticking to much to the copper been vapourised in a fault as temp' can rocket for a split second as thousands of amps flow.

Its a shot in the dark but happy to hear other guesses...

Ive disregarded Chines Drywall epidemic from about 2004 to 08 as this didn't really effect uk as far as im aware ....WIKI it if your lost on that one.