I ran across an oddity today. HO asked me to fix dead convenience outlets on a big RV. It was parked in the driveway and plugged in to house power.
One circuit was fine, but the other showed 66V between hot and neutral, 119V between hot and ground on every outlet, including the GFCI that is first in line on that circuit. I opened the breaker panel and found no loose neutral wires; 119V between breaker and neutral bus.
Finally, I pulled out that GFCI outlet and measured 120V between hot and neutral LINE terminal screws, but only 66V between hot and neutral LOAD terminals, 66V between LINE hot and LOAD neutral.
Problem solved, but how could the GFCI do that?
Replies
I have no idea, except to pass on the time-honored solution to all questions of this ilk -- "phantom voltage." I.e., your digital meter was reading the effects of a small voltage on the neutral side, not the power side.
Mike Hennessy
Pittsburgh, PA
I thought of phantom voltage, but that usually entails considerable length of unterminated wire, yet I read 66V between "line" and "load" terminals on the device, not more than an inch apart. This suggests to me that the GFCI is "consuming" the voltage coming into it like a light wired in series.To test phantom voltage with my meter, I plugged one probe into the hot side of an outlet and touched the other to one prong of a 25 foot extension cord and read 4V; on a 50 foot extension cord, 13V, nowhere near the 66V of the GFCI. Curiously, when I looped a bare copper wire from one blade to the other of the extension cords, effectively doubling their length, the reading did not change, but plugging one cord into the other resulted in a 16V reading. Similarly, 60 feet of 12ga Romex read 4V but connecting white and black at the far end resulted in no change even though the length of wire was effectively doubled.
BruceT
>>Problem solved,.......<<
1) How did you solve it?
>>but how could the GFCI do that?<<
If you replaced the GFCI, and all worked properly then the answer is "the GFCI was broke!"
If you are asking for the mechanics of the GFCI - I have no clue.
Jim
Yes, I solved the problem by replacing the GFCI. I'm just curious as to how a GFCI can reduce voltage in a circuit and thought someone on this forum might know the answer.
BruceT
"I'm just curious as to how a GFCI can reduce voltage in a circuit "
Well, that's what a GFCI does when it trips. Sounds like a bad relay or bad connection internally.
"Well, that's what a GFCI does when it trips."When it trips, a GFCI shuts off the load side hot entirely. This thing had full voltage between both hot terminals and ground or line terminal, but only 66V between hot terminals and the load terminal.
BruceT