I have a house that was built in 1923 which has the original BX armored cable throughout. At the switches, fixtures, and outlets, some of the cloth-over-rubber sheathing has become brittle, cracked, and is falling off. I have repaired some of this with heat shrink wrap and it seems to work well. My question is, if some of the frayed cloth strands come in contact with the copper conductor under the heat shrink, is it a fire risk? In other words, when I wrap the wire in heat shrink, some of the cloth strands under the heat shrink will be touching the copper wire. Should I be concerned about it catching fire? Thanks.
As I understand it, normal cloth is not electrically conductive, so it shouldn't be a significant problem if some of it touches the conductor. My bigger worry would be a failure of insulation leading to a short-circuit in the cable. Applying the heatshrink will have inevitablly involved moving the wires around increasing the chances of cracking.
I would be very wary of touching the installation any more than you absoloutely have to before you are able to re-wire it.