ECN Forum Forums Technical Discussion Electrical Theory and Applications Trouble shooting mystery

I had a call involving a small guest house with one 20A circuit piped from the main house. The HO said every time it rains the 20A breaker trips. The piping leaves the exterior house panel in a 1-1/4" pipe with several other circuits to a 3R J Box on the flat roof of the carport. From there, the 20 amp circuit leaves in a 1/2" emt, down thru the roof and along the eave to the guest house.
The guest house is empty and the HO said the breaker still tripped on the last rain storm, even though everything was turned off and the fridge was unplugged. I did a check on the circuit with no loads and there was zero current flowing (dry day).
I am having trouble believing water alone could enter the conduit and cause a fault current large enough to trip the 20 amp breaker. And I can't even see where water could enter anyway. But suppose the conduit did fill with water, is it possible that it could cause a fault of enough magnitude to open a 20 amp breaker?
The only other possibility is an old underground post light circuit that is connected to the guest house, but the post light is long gone and, while the switch still energizes the underground wiring (in RMC), the HO says the switch, which is located on her patio, is always off. It was in fact off when I went there, and was behind a shelf unit which had to be moved to access it. I turned it on to verify that it was still energizing the underground circuit. While on, there is still no current flowing from the 20 amp breaker.
I am stumped and told her to call me as soon as it happens again, while it is still raining. If there is a possibility that water alone in the emt could cause this, I guess I will pull in some new THHN/THHW's.
Any ideas?

...BigB,..what type of wire is present inside the conduit now?? If there is so much as a tiny knick in the wire,and moisture gets in,it sets the stage for a low resistance short.The resulting arc can burn away for awhile,till it becomes a dead short or...clears itself by melting itself through.EMT rusts easily, and because of it's thinwall,can create jagged edges for conductors to be sliced up on.I would probably pull out the entire run,start from scratch,repair/replace any rotted or otherwise damaged pipe,and pull in new lines..but before that,with the circuit on...if the breaker holds,try to catch it on a day after or during a rainstorm,,BUT BE CAREFUL!!take a digital amprobe reading from the panel side, with EVERYTHING on the other end disconnected..including the switch to the defunct post light.if you open up the switch,and remove it,you may find a surprise "feed-out" circuit in the box going who knows where??...If you get a reading of 20 milliamps to 5 amps,with nothing connected,you've got yourself a low impedance short, or a leakage to ground..and that would be objectionable even at 1 amp... Good Luck!!
Russ
PS..I'm far from an authority in this scenario,but I've had my fair share of head-scratcher's and I usually find my culprit..

...Let us know how you make out,..e-mail me if you want!!



[This message has been edited by Attic Rat (edited 08-13-2004).]

Thanks A.R........I did an amprobe test but I will check it with my Fluke 27 inline ampmeter-much more sensitive.
The emt currently has older plastic insulated wires from I am guessing the sixties. The house itself is wired with cloth covered romex from the fifties.
I was only there for 40 min or so, I will probly need to spend more time when she calls back.
I usually find my culprit too, but it helps to go back armed with as much info as possible.
Thanks again....Brian in AZ.

P.S. Big rainstorm tonight!

[This message has been edited by BigB (edited 08-13-2004).]

No problemo,Brian,...Stranded or solid,...it makes a big difference when pullin' thru pipe...if they used solid,..chances are there's a knick somewhere in the old TW wire,..again, please keep me posted!! OK?...thanx,
Russ

BigB,
I had a similar problem. UF cable that over the years got deteriated. I hooked up a FLUKE 87 with the MIN/MAX mode on then i noticed that the MAX would register very high current draw then the breaker would trip. Then tried it again after about 5 minutes it registered some high current flow then the breaker tripped. I came to the conclusion that the UF cable was damaged somewhere between the house and the BBQ pit.

Thought this might help.

EDWARD

You might want to throw a megger on it. I have often checked wires with a Fluke and gotten the OL (over-range) on resistance. But a megger would show a dead short.

Just takes a little more umphf to get it to show up sometimes.