GFCI Tripping Repeatedly?

This is incorrect; see an earlier post regarding the collapse of the magnetic field in the motor windings when power is switched off. The consequent backcurrent generated does indeed produce a brief, slight current imbalance. This is evidently of sufficient magnitude and duration to exceed the GFCI's trip threshold.

Of course it should. By opening both the neutral and the hot, a DPST switch prevents the backcurrent from reaching the GFCI, thus preventing the GFCI from ever seeing the imbalance.

Perhaps; but the explanation referenced above is IMO much more likely to be correct.

Very doubtful. Given that it's a single-pole switch, this would require an intermittent, high-resistance short to ground that doesn't draw enough current to make an audible arc. Hard to imagine how that could happen.

Easy enough to test -- but the results are predictable: it won't make any difference.

It's possible that this particular GFCI is a little more sensitive than it should be, and swapping it with a different one may cure the problem.

Switching both the hot and the neutral through a double-pole switch is almost certain to fix it.

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

As I mentioned in my original post, I already replaced the switch and GFCI with new ones, thinking the switch was bad or the GFCI was faulty. It seemed to reduce the tripping slightly, but did not eliminate it.

Replacing the fan is kind of my last resort.

Anthony

Only if there is leakage to ground. The leakage could be capacitive currents from winding to motor poles. Else, as M Q wrote, how could the current be different in the 2 wires? Field collapse can create a high voltage spike but it creates a current in the circuit through both wires.

Could be high voltage produces current from high side (hot) to grounded motor poles by capacitive current?. Spike is short duration (high frequency) increasing the capacitive current. Size of voltage spike depends on where in the current sine wave the switch opens.

Assuming timing of both poles opening is close enough together that the GFCI doesn't trip.

Yea, seems remote.

NOT correct. A GFCI trips any time there is an imbalance between the hot and neutral currents, that exceeds the thresholds for duration and current. It is NOT necessary for there to be ANY leakage to ground.

Yes, it does -- and with current reaching the motor through an SPST switch,

Possible, I suppose... but I'm not convinced.

Pretty good assumption, I'd say.

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

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

Of course. But how do you get a different current in the wires of a bath fan without ground leakage.

Still not explained - how is the current not the same in both wires unless there is some form of leakage to ground. The current is the same in both wires for a light bulb. Why not the fan.

Wall switches generally operate slowly - apparently intentionally. (You can typically move the handle to a position that causes the contacts to arc - not that arcing is relevant.) I have read the intent is that the contact break be slow enough to reduce inductive kick from magnetic field collapse. GFCIs may operate in 1/2 cycle. It is not obvious to me that slow operation and random variation would not result in timing difference between poles of 1/2 cycle in some switches.

In his case, the fan is not located over the shower or tub

As noted above, see an earlier post in this thread (not mine) regarding magnetic field collapse and back-EMF.

As noted above, see an earlier post in this thread, etc.

Because a light bulb is a resistive load, and a fan is an inductive load, at least in part.

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.

With only "one" wire connected to the GFI there is no difference of potential either. It kind of makes induced currents impossible too, but run with it. :)

Want to bet on the double pole switch?

Maybe you ought to spend a little more time thinking about that...

Sure.

I have to admit, I'm a little skeptical about the double-pole switch. Whether resistive or inductive, how can you have current flow unless the circuit is completed? Open the hot wire and current shouldn't flow through the neutral either, unless there really was a fault in the wiring somewhere? I was an electronic tech MANY years ago, so maybe I've just been away from this stuff too long. :)

In any case, my own attempts at solving the problem have failed, so I'm willing to give it a try. I bought a double-pole switch yesterday ($9) and plan to install it in the next day or two. Unfortunately, due to the erratic nature of the problem, I may not know for a month or more whether it solved the tripping problem. But, if it does trip again, at least I can rule out the switch once and for all.

Thanks for the feedback!

Anthony

check and see if the positve(black) and neg(white) wires are connected to the coresponding black and white wires in the fan, if they are reversed u will trip a gfci

So test it.

Back-EMF can flow through the neutral wire because the neutral is connected to ground at the breaker box.

Back-EMF from the collapsing magnetic field in the motor windings when it's switched off.

Yep. Let us know what happens.

You want to explain why you think that's the case?

You basically have a 2 wire inductor. You are saying that when the circuit is broken, magnetic field collapse produces a current flow in one wire that is not equal to the current flow in the other wire. You may be in line for a Nobel Prize. Or maybe not.