I have a fan in the barn for animals, and the fan has been running for over a week non-stop. I noticed the grill was covered with filth and was not blowing real well. Since it's mounted high on a wall, I knew it would be hard to clean by hand. I unplugged it and used a garden hose to completely blast it clean.
After the cleaning, I waited 20 minutes for it to dry, then I plugged it back into the GFI outlet it's always plugged into. It spun up for a few seconds, then the GFI tripped. This did not surprise me, water probably got into the motor. I let it sit another half hour, then plugged it back in, and the GFI tripped again. I again waited awhile, then tried once more, and again the GFI tripped.
Knowing in this humid weather things are not drying well, I opted to run an extension cord to another non-GFI outlet about 12 feet away, and plugged the fan in that outlet. The fan ran fine, but the moment I plugged the fan in to that outlet the GFI blew again.
Ok, here's the oddity. The GFI is on a completely different breaker than the non-gfi outlet I used for the fan. Nothing else was plugged into that GFI, although the ceiling light in that part of the barn is connected to that GFI and it was turned on. But none of that should have anything to do with the GFI, since the fan was not only plugged into a completely different circuit, but is on the opposite side of the 220-0-220 center tap. (next breaker down).
How in the hell could plugging in that fan to a totally different circuit cause that GFI to trip? This makes no sense.....
I should mention that I worked as an electrician for years, and have electronic experience too. Yet, this is entirely beyond common sense.
Note: After running the fan for 10 minutes, I plugged it back in the GFI with no problem. It just needed to dry.