I have just seen a domestic earth fault involving a cotton-braided Vulcanised Rubber Insulation (VRI) cable. A 60A branch fuse had blown and a friend asked me to look at the problem.
The live cable and its neutral had passed through a brass gland but to me the insulation appeared undamaged. I did not have a 500V insulation tester with me but I could find no low resistance to earth with an ohm-meter.
I replaced the fuse and when I closed the isolator the fuse again blew but this time the fault revealed itself as the live cable had shorted to earth on the inside of the brass gland.
The cable and fuse were was renewed and the problem was cleared.
My question is with 40 year old VRI cable what changes take place in the insulation? I know it gets brittle but assuming the cable is undisturbed and not under any vibration why would it fail like this? Could there have been a chemical change going on which gradually carbonised the insulation?
Beemer