Electrical - How could this happen?

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.
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*Are those outlets being fed by a three wire circuit with a shared neutral?
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On 7/20/2011 3:04 AM, snipped-for-privacy@myplace.com wrote:

Since the light is off the load of the GFCI device, it may be possible that the GFCI load neutral got co-mingled with the neutral from the other circuit
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I agree, I'd suspect the wiring is not what you think it is as my first guess too.
But I wonder if both are on a sub panel. A barn was mentioned so I'm guessing the wiring is a tlittle more complicated than typical residential. If you have a long run to a sub panel and then have one gfci circuit on that sub panel plus one non-gfci circuit would it be possible for events on the non-gfci circuit to affect the gfci? They would share common neutral and ground back to the main box.
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sometimes the surge or spark can confuse a GFI.
Mark
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wrote:

As the OP in this topic, I think your reply makes more sense than any others, because I surely see no other explanation.
To answer the other replies: There is no shared neutral.
The GFI outlet is located in a small side room off the barn. This room contains a water hydrant (spigot). The reason for the GFI is mainly because of the heat tape that is used on that hydrant in winter. (for safety reasons). There are two outlets powered by that GFI in that same room. (neither were in use during the incident). The other thing on that GFI is the ceiling light, which I have wanted to rewire to remove the light from that GFI since it's annoying when the GFI trips the light goes out. One of these days I'll find time to change that.
The non-gfi outlet I used to run the "wet" fan, is directly above the breaker box. It's a separate breaker and is only 12 inches from the breaker at most. All outlets in the barn located lower on the walls are GFI protected to protect the animals. This outlet was originally installed high on the wall for an electric fencer. Too high for animals to contact. However the fencer has since been moved to a different location. Thus, this outlet and it's breaker are normally not used at all, except to maybe power an electric tool when I am fixing something.
Therefore, there is no shared neutral involved.
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On Jul 20, 3:00pm, snipped-for-privacy@myplace.com wrote:

Yes, but is this all on a sub panel at the barn with separated neutral and ground?
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On Wed, 20 Jul 2011 12:18:00 -0700 (PDT), jamesgangnc

NO, it's a completely separate panel with it's own main breaker, and ground rods. A line comes to that barn from the meter pole. There are 3 separate main panels on this farm. One in the house, one in this barn, and one in the garage. All are fed by overhead triplex cables that originate from the meter pole. Each panel has it's own main breaker and grounding system (rods). It's NOT a sub panel.
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On Jul 20, 3:40pm, snipped-for-privacy@myplace.com wrote:

Ground and neutral are connected on the same bus bar at that panel then?
Having a main breaker and a ground rod does not distinguish a sub panel from a main panel. A sub panel will have separate ground and neutral bus bars. A main panel will not. I have a sub panel in my garage. It has a main breaker and ot's own ground rod. It's still a sub panel.
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On Wed, 20 Jul 2011 12:47:01 -0700 (PDT), jamesgangnc

There is a disconnect on my meter pole, but whether it's called a sub panel or not dont matter to the original question. We're just tossing around words. But none of this matters in the original question. If there was a shared neutral on those circuits, I could understand where that may affect the GFI operation, but whether the panel is called a sub panel or not does not matter.
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On 7/20/2011 2:04 AM, snipped-for-privacy@myplace.com wrote:

reason #496 not to have gfci's.
--
Steve Barker
remove the "not" from my address to email
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On Fri, 22 Jul 2011 06:31:56 -0700, "Bill"

Yes, I did do all the wiring in there. Actually I built the addition on the barn where the water hydrant is, so it would be inside the barn rather than outdoors. Then I put in the GFI circuits in there because of the hydrant and the heat tape that I put on the pipes in winter. So I know exactly what is wired and how. As I said before I have the ceiling light connected to the GFI, which I want to change. At the time it was convenient and I had other priorities to do. Yet everything is wired correctly using grounded cable.
There is no possible way that the ground from the circuits in that room would affect that other outlet since that other outlet is on it's own breaker and is mounted right above the breaker box. That's what puzzles me. I even ran into and old friend yesterday, another retired electrician. He could not make sense of this either, except to say that the GFI might be defective. I dont think that's the case either because it works just fine now that the fan is dry.
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On Jul 23, 2:44am, snipped-for-privacy@myplace.com wrote:

maybe the GFCI got a little wet when you sprayed the fan?
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