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.