[VAC] Re: GFI Trips when rains--Help!

["Dr. Gerald N. Johnson, electrical engineer" <geraldj@xxxxxxxxxx>]

30 amp receptacles generally don't have GFI. My SOB trips the GFI in my
barn when it rains. I've not determined why. Probably its dirt in the
outlet boxes and main panel in the trailer. The GFI trips when current
leaks to the ground wire of the three instead of returning on the
neutral wire. The usual trip point is just under 5 milliamps because
that's below the threshold of feeling that current. Grounding the
trailer won't affect the GFI, the trailer is supposed to already be
grounded through the third wire of the cord and wiring and the trips of
the GFI hint that the existing ground is effective, at least a the low
current. (a 100 watt lamp takes 833 milliamps at 120 volts).

Grounding the trailer with an 8' driven ground rod connected to the
ground (but not to the neutral) in the trailer main panel can prevent
tickles if the ground wire isn't always connecting well and you still
have that leakage but won't make the GFI any happier.

As Scott says, rotten wire insulation can leak that much when wet, but I
expect there's a mouse nest or a build up of dust and grunge (even dirty
spider webs) in the main panel and the outlet boxes in the trailer. You
may be able to separate them by tripping the branch circuit breakers in
the trailer (A/C filled with dirt to contact wiring terminals or relay
contacts can be a problem too) but that test may not be conclusive
because of it being a general leakage problem and disconnecting any
fraction of them may be enough to keep the GFI from tripping. I expect
it to be dirt because dirt's conductivity increases with high humidity.
Dirty outlet boxes have been a perpetual problem in dairy barns, always
blamed by electric utilities as being the local causes of shock problems
that were more often caused by ground currents from the high voltage
distribution line returning to local earth through the farmer's
secondary grounds. When a cow experiences a volt or two between milk and
nose or wet feet she can become uncooperative or ill. Reading between
the lines of such experiment reports I figure the cow that became
"uncooperative" kicked the experimenters through the side of the barn.
As she should have.

Gerald J.