# Finding a ground fault leak

#### MrSoftware

Joined Oct 29, 2013
2,027
Looking for ideas on how to follow a wire behind a wall. We just had about 80 foot of exterior wall replaced, that horrible T1-11 stuff replaced with Hardie board. Coincidentally, the GFCI breaker in my power panel, on the opposite side of that wall, will not reset, it trips instantly. I replaced the breaker, problem still exists. The power was off while they were working on the wall, so I have no idea what section of wall they were working on when the problem popped up. The hot and neutral are not shorted at all, infinite resistance according to my Fluke 87V. With only the neutral connected to the GFCI breaker, it still trips. So this leads me to believe the guys replacing the wall probably put a screw through the neutral somewhere behind the wall, and it's leaking enough current into the wall (S. FL everything is humid) to trip the GFCI. My question is, is there a tool I can connect to the neutral in the breaker panel, then go to the exterior wall and use it like sonar to follow that particular wire along the wall, potentially through conduit?

#### MisterBill2

Joined Jan 23, 2018
11,879

#### michael8

Joined Jan 11, 2015
287
# The GFCI trips when there is a difference in current between the two conductors through the sense coil . It is only that difference # that trips it. It does not matter which one or where the current flows to.

True, that's how the GFCI works internally
.

False, or not completely true. Look at the diagram at the bottom of page 1:

There are two transformers around the hot/neutral wires. One is driven by the GFCI circuit the other
is sensed. The two transformers are only coupled if one or both the hot or neutral are connected via
an outside connection between the source and load sides of the GFCI. At the source the normal
ground neutral bonding is a connection between ground and the neutral so a neutral ground fault
on the load side of the GFCI will couple the GFCI transformers triggering it. Thus a GFCI can detect
neutral to ground faults in addition to hot to ground faults.

#### MrSoftware

Joined Oct 29, 2013
2,027
Regarding where the current leak is happening: I disconnected all of the hots but left all the neutrals connected and it still trips. Whatever the problem is, it's right on the edge. With everything connected properly, sometimes the breaker will stay "on" for several minutes before tripping, then it will get in a mood where it trips instantly when I turn it on, over and over. The new breaker behaves the same as the old one so it's not a bad breaker. Regarding just swapping it with a regular breaker and putting the GFCI per-outlet: technically this might solve the problem of the breaker tripping, but would still leave me uneasy about having an actual short somewhere. I'm not sure what it's shorted to, maybe a neutral on a different circuit, but a short nonetheless.

#### crutschow

Joined Mar 14, 2008
29,785
False, or not completely true. Look at the diagram at the bottom of page 1:
Depends upon the design of the GFCI.
The older GFCIs don't look for ground-neutral faults.

#### crutschow

Joined Mar 14, 2008
29,785
Regarding where the current leak is happening: I disconnected all of the hots but left all the neutrals connected and it still trips
Have you opened the breaker box circuit neutral and checked the resistance between the neutral and ground wires?

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#### MisterBill2

Joined Jan 23, 2018
11,879
If you go farther into that data sheet then you learn that indeed, that second winding is for doing a a neutral to ground check, which makes it unclear as to what benefit that check delivers. If a portion of the load current is passing into the ground circuit instead of the neutral return, the result is that the trip threshold is reduced a bit. Or does the sensor controller somehow compensate for that??
It would provide a useful bit of information to temporarily disconnect the ground wire from that cable run, at least in the breaker panel. If the tripping stopped completely then you could think that it was a neutral to ground leakage problem. In that case, installing a GFCI device at each outlet and bypassing the one in the breaker panel would be the way to go. And a bit safer as well.