AC Voltage between hot & neutral when circuit off

[svetz]

Wire on circuit 25 shows 119V
Turned off breaker 25 and retest; shows an initial high voltage (~300V) then quickly (<1s) fades to about 35V
Turn off breaker 19 and voltage between hot and neutral goes to zero.

C19 is L1 and C25 is L2.

C19 and C25 both power outdoor lights, C25 also powers some indoor stuff. Neither breaker trips by itself. Wires are all hidden in walls.
C25 has a set of 3 way switches on it for the outdoor lights (up and downstairs).

Any idea what the problem is or how to diagnose it?

 

[svetz]

My initial thought was a rusty screw/nail penetrating the insulation of the two circuits at some common point. In which case perhaps I could just swap C25 wth C23, same phase no foul? Not sure if the weird initial high voltage with a quick drop is indicative of capacitance in that situation.

But, if it's some sort of ground fault, then I probably need to locate what's going on. Not sure how I'd confirm that.

Thinking out loud here... let me know if any of this is bogus...

• If the hot were shorting to ground the circuit would trip.
• If neutral was shorting to ground you'd probably never know.
• For a ground fault, both C19 and C25 would have to be shorting to earth and then taking a highly resistive trip to ground in order to not trip the breakers. Older house, so the boxes are metal ... so I don't see how the hots could be shorting to earth.

 

[svetz]

Did I just fall victim to Phantom voltage? That is, am I measuring induced current because the wires are running parallel and the other circuit has a load running it I'm not aware of? Guess I need to check the current of that leak.

 

[gnubie]

Good old capacitive leakage, the bain of owners cheapie low power lighting that flashes and flashes.

 

[svetz]

I'll get back out there and do a low-impedance test and let you all know. But I bet that's it.

 

[svetz]

Yep... phantom voltage. Whew!

 

[Rider]

Yep... phantom voltage. Whew!

Well.... Halloween is coming...