GFCI Tripping Repeatedly?

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We recently remodeled my in-laws bathroom and, as per code, the entire 20 amp bathroom circuit is protected by a 20A GFCI outlet. Everything was inspected and functions as it should.
Unfortunately, the GFCI is tripping occasionally when the bathroom vent fan is turned "off". It never trips when turning on the fan, only when turning it off. And, it may go a few weeks before it trips, and then suddenly trip three times in a row. Then it'll work fine again for a few weeks.
I suspected a bad GFCI, so I replaced it with a new one, and also replaced the switch that controls the fan. It doesn't seem to trip as often now, but it tripped for me again last night when we were visiting.
The only remaining item seems to be the fan itself, but I'm curious if there's some other possible cause I might be overlooking? The fan has been installed for a few months now, so I probably can't return it at this point.
I thought maybe condensation from moisture draining back through the vent pipe (it had to be routed up over a beam to get out through the side wall). But, the tripping doesn't really seem to be related to when the shower was used recently.
Thoughts?
Anthony
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HerHusband wrote:

Probably motor body and fan enclosure does not have or have poor ground?
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I have had trouble with motor loads tripping GFI's.
The fact that it trips when the fan is turned "off" leads me to believe what you're seeing is not a true ground fault (current leakage to ground) but "merely" a difference between the instantaneous currents on the hot & neutral wires.
I'm an ME but I did study some motor & circuit stuff in school.......here's my best attempt at an explanation, when the fan is turned off, the magnetic field of the windings of the motor collapses & induces a current back thru the neutral (since the hot lead is switched & therefore open).
The GFI senses this slight current as mismatch between hot & neutral currents & trios.
cheers Bob
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Hi Bob,
If this is the story, what is the proper way to deal with this? Is there a type of motor that can be specified and which would minimize this effect? Would it be acceptable practice to switch both the neutral and hot to the motor? If so do double pole switches break both connections sufficiently close to simultaneously to avoid the induced current imbalance?
Thanks, Wayne
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Wayne-
Sorry for the short post (the potential cause & no solution)...... I was off to the airport to pickup one of my NorCal sons
Your questions are all good ones (I had / have them as well but don't really have any answers)
If the fan location requires GFI maybe one just has to deal with the nuisance trips?
Per John's post (& I agree), fans may or may not need GFI protection depending on their location relative to the bath or shower.
I just re-did my bath & didn't GFI protect the fan or the light I was going to but it wasn't required; even though I wanted to for curiosity & extra safety.
I also was going to suggest a double pole to switch the hot & the neutral...... but in all my electrical experience.....switching the neutral is a major no-no.
cheers Bob
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Incorrect. Switching *only* the neutral is a major no-no. There's nothing wrong with switching both the neutral and the hot.
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Doug Miller (alphageek at milmac dot com)
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On Mon, 24 Dec 2007 16:23:28 GMT, Wayne Whitney

An easy "try this" will be to swap out the washroom fan with a new one or one from another bathroom. The puzzling thing is the exhaust fan motor draws so little power that even stopping the motor by hand should not cause any circuit hiccups (and will not cause the motor to overheat.)
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BobK207 wrote:

The field collapses 60 times a second when running...
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yeah but BOTH the hot & the neutral are still connected when the fan is running.....
I believe it is the "collapse" with the hot open & the neutral still connected that causes the problem for the GFI current sensing circuit.
My comments were meant to provide a plausible reason for the behavior "GFI trip on fan shutdown"
& your theory is?
Sounds like Doug's comment that its ok to switch both should solve the issue.
OP-
Is the fan / switch wiring such that you can easily install a DPST switch and switch the hot & the neutral to the fan?
cheers Bob
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I was afraid of something like that. At this point, there's not much I can do to rewire the fan to be off the GFCI.

Yes, I could install a DPST, but I can't say I've seen them in stores? Then again, I've never really been looking for them.
Anthony
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Anthony-
If you have a Home Depot near by, they probably have the DPST
here's a link to listing of the Leviton double pole products (white), HD (at least in my area) stocks Leviton products
http://www.leviton.com/OA_HTML/xxlcfbuibeCSrdSrchResults.jsp?cg=&kw=double-pole++white&ds=0&dr &st=kw&cpg=0
the 1202-2W switch looks to be the one you'd want.
The DPST solution will only work IF the neutral to the fan comes from the switch box & can be safely broken there.
cheers Bob
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Bob,
Thanks for the info!

We have a couple of HD's and a Lowes nearby. I'll have a look next time I'm there. I've never had a need for them before, so I've probably looked over them and didn't know it.

Yep, the power comes into the 3-gang switch box, then separate cables run to the fans and lights. It'll be no problem switching to the DPST switch.
I'll have to wait and see if changing the switch will prevent the GFCI from tripping when the fan is turned off, but it's a relatively simple and inexpensive thing to try. Thanks for the tip.
Anthony
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If the GFCI is in the same three gang box as the switch for the fan and lights you should be able to reconfigure the wiring to eliminate the light and fan from the GFCI protection.
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No, the GFCI is in a different box on a different wall. Power comes from the GFCI over to the 3-gang box, then three cables exit the switch box to go to the fixtures (ceiling lights, vanity lights, bath fan).
There wouldn't be an easy way to take the fan off the GFCI.
I wired both bathrooms in our own house the same way, with the fan on the GFCI, and we've never had a GFCI trip. So it has been odd that my in-laws GFCI trips so often with the same wiring scheme.
Anthony
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Sure there would. Just disconnect the cable coming from the load side of the GFCI outlet, and connect it to the line side . There is no need to connect any of the loads you have through a gfci

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RBM wrote:

Some exhaust fans are specifically listed to be installed with a GFCI protected supply. The concern is that the vibration of the fan is more likely to cause a fault and a person who is standing in a shower is particularly vulnerable to electric shock. Removing the GFCI protection from the shower fan may be a violation of the electric code requirement for installing all devices in conformity with the instructions included in the listing or labeling.
You can purchase a combination device that has the switch and a GFCI on the same yoke and use that to control the fan. It would make a lot more sense however to open the neutral splice in the three gang box and test for a ground fault on each of the cables that are not the supply. The one that shows a measurable resistance to ground from the load side of the switch with the switch open and the neutral splices open is the one that is ground faulted. If you cannot get the fault to read a measurable resistance with the circuits thus disconnected from the supply you can either open and inspect all three load outlets for a pinched wire or an over tightened cable clamp or you can rent an insulation tester and subject the three cable's insulated conductors to three hundred volts. Insulation testers give the resistance of the conductors insulation under a specific test voltage. If you use any higher setting than three hundred volts you are likely to cause faults rather than locate the existing fault. -- Tom Horne
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In his case, the fan is not located over the shower or tub

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HerHusband wrote:

I think you'll be shocked at what they charge for a DPST.
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120 times per second
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Whether it collapses 60, 120, or a thousand times a second when running is not relevant, because when the fan is running, both wires are connected to the GFCI. With power to the fan routed through a single-pole switch so that only the hot conductor is opened by the switch, only *one* wire is connected to the GFCI, thus making it possible for the collapsing field to cause imbalanced hot and neutral currents at the GFCI.
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Regards,
Doug Miller (alphageek at milmac dot com)
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