Old BX 2 wire cable rubber insulation/cloth. changed a light fixture. after completing installation with the switch off I started putting in the light bulbs-- lights flikered! voltage test showed 85 volts in the switch loop when switch is off. I know there is a fault in the switch loop. Did a continuity test in the switch box but no continuity. when breaker is off- no voltage so there is no other source. Just want to know how a lower voltage such as the 85v read I have slips thru "evidently poor" insulators without the conductors actually "completely touching". If there was good contact in this case the lights would have stayed on. any master electrician or journeyman who knows please reply. Thx. (PS I took out the switch loop- naturally)
Rubber insulation can biodegrade into brittle chips or powder. The wires themselves become superficially oxidized over time. Rubber, which contains sulphur, actually hastens "oxidation" of the wire surface. So wires can touch making electrical contact but not a good contact.
If you have a fault in a cable of any vintage, you are going to have to replace the cable. Now if you know where the fault is, for example you put a nail through the cable, you can put two junction boxes there (one if the cable has enough slack).
Don't forget, when measuring voltages, measure between a switch terminal and neutral (with some intuition, you can make sense out of measuring from the switch terminal to ground). Don't measure between the two switch terminals.
Do you get coverage you can count on? From the bag of fertilizer? From your lawn sprinklers?