Couldn't this be possible with a shared neutral? The reason I ask is because
I was changing a fixture the other day for a woman, had the switch off (this
was just a single pole switch), unscrewed the fixture and checked the wires
with one of those pen like non contact voltage testers and there was still
juice. I figured it must just be a false reading so I took the wirenuts off
(the hot only had one wire with it in the wire nut and the neutral had two).
When I took the neutral wire nut off the wires came apart and arced a bit. I
thought it was strange, so I kept touching them together and pulling them
apart and noticed the clock on the microwave was turning on and off so I
shut off the microwave breaker too and the voltage went away. When I got
to the hallway fixture, same type of thing, tester read voltage pulled off
the wire nuts this time the neutral arced good with a nice sounding pop.
Turns out the hallway neutral was being shared with the computer room
neutral and being that the computer was drawing more current then the
microwave clock circuit I got a bigger arc until my disconnecting them
turned off the computer. This is just another thought because I don't know
if the OP posted whether he just test from neutral to hot or hot to ground
or neutral to ground. By the way this house was wired in 72 and I am not
sure if it was standard practice back then to share a neutral without a
double pole breaker, but I would imagine there are other circuits in there
that are wired this way.
The reason that you are getting a pop is because there is a load on
the neutral (microwave or computer) The electrons are being attracted
across the small gap. This is normal, and also why some people
incorrectly say you can't get shocked by the neutral. Even though the
light you are working on is off anything else downstream is still on.
I wired my sisters house. When she was in the kitchen she would get a
pop when she plugged in her fry daddy. She asked me if I had done
something wrong. I told her no. The reason the fry daddy pops is
because it is always on.
It was easy to explain to her in the kitchen why there was nothing
wrong. I took the plug for the can opener and plugged it in the
outlet. No pop. I then turned the can opener on and plugged it in.
Opening the neutral will cause the same type thing. It will also
shock you as it is 120V across the gap.
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