blueman wrote:

Jeff Wisnia writes:


blueman wrote:

I was working last night on an electrical circuit that was shut off at
the breaker.
Nevertheless, the voltage from neutral to ground and from hot to
ground both measured about 30V. This was enough to cause my voltage
probe to buzz and to even cause a compact flourescent to light
dimly. The current across the circuit however measure just 0.7 mA.
Am I correct in assuming that this is probably just induced voltage
from neighboring wires that run alongside it or is there potentially
something more serious and sinister going on?
Thanks!


Measuring 30 volts from hot to ground with a high impedance voltmeter
can easily be caused by capacitive coupling to that that wire from
another wire in close proximity carrying full voltage. It's a common
question posed here. It could also occur if the breaker had popped
under overload many times and had a very high resistance leakage path
inside it through deposited vaporized contact material.

But, if you really measured 30 volts from neutral to ground (and
didn't intend to say that the measurement was from hot to ground.)
then there's something seriously wrong because the neutral should be
at ground potential, and there's no way that 700 microamps of
capacitive coupling or leakage is going to create 30 volts between two
wires which are connected together.

Better measure again to be sure, and if you still get 30 volts between
neutral and ground, my guess is that the ground lead ISN'T really
grounded and you're seeing an induced voltage on it too, relative to
neutral. THAT could be sinister.

Let us know what you find,



Good point! I did indeed find 30 volt both hot-to-ground and
neutral-to-ground.

However, I believe I can explain the neutral-to-ground voltage very
simply. The reason I was opening up the junction boxes in the basement
and checking voltages was that I was having some wacky behavior with
most of the circuit out but a few of the compact flourescents dimly
lit. It turns out that one of the neutrals had slipped out of the wire
nut. This could explain the induced current in both the neutral and
the hot since with the breaker off they were both effectively
"floating".

Let me know though if you think I am missing something.

Thanks.

Roger that, I think you're on track now. If you reconnected that neutral
wire nut and left the breaker open I'd bet dollars to donuts that you'd
still measure "something", but now less than 30 volts, with that meter
between hot and neutral or hot and ground, but nada between neutral and
ground. (Because the capacitance between the hot and neutral in that run
would form the lower leg of a capacitive voltage divider and shunt some
of that stray current to ground.)

Glad you got it fixed though!

Jeff

--
Jeffry Wisnia

(W1BSV + Brass Rat '57 EE)

"As long as there are final exams, there will be prayer in public
schools"