I didn't do the wiring in this house, this has been happening since I moved in.
Basically, about 50% of the time that I turn off the bathroom fan, the GFCI outlet pops and needs to be reset. Once that happens, the fan no longer turns on until I reset the outlet. This only happens when shutting the fan off, not when turning it on or while it's running.
Does this likely indicate a problem with the wiring, the fan, or the outlet? I'm wondering what I should look at first.
I have seen this with fans several times, the fan motors create inductive kickback when spinning down this is enough to trip some GFCI's, if there are no outlets on this switched side of the circuit move the wiring to the line side of the GFCI. I have even seen motor circuits on a separate branch circuit cause this problem but they were usually larger 3+ hp motors in those cases both that I saw changing brands of GFCI did the trick, (the GFCI tested good I have a variable load to test so I know they were good just a bit more sensitive).
Here's what is happening. When you have a big magnetic coil, and power is flowing through it, it makes a magnetic field. When you suddenly interrupt that power, the magnetic field collapses, and this has a small force behind it. It is the nature of coils to resist changes in current flow. It does this by turning that small force into voltage, which increases until the desired current flows, or the force just runs out of energy. This is called an "inductive kick" and everything in the circuit must be insulated for it.
When the insulation fails, the high voltage will ignite an arc that the lower voltage will sustain. It can also leap across switches! In DC power over about 30 volts, this arc is Very Nasty Business because the arc will never stop.
However, with AC power, AC switches polarity 100-120 times a second (8-10 milliseconds). It is a sinewave, and part of the time it is near zero volts. The arc will self-extinguish at the next zero crossing. But in the meantime, the arc will flow enough current (potentially a lot of current) to trip the GFCI.
However... If you happen throw the switch at the right instant, you are near a zero-crossing for the voltage or magnetic field, so there's no magnetic field and no inductive kick. So no voltage spike.
So, no leakage and no GFCI trip.
Insulation is failing. This will get worse.
I would start by changing the switch because they are cheap. But prepare for a fan replacement, or at least take it down and clean it. Damp dust makes a great leakage path.
Note that switches have ratings, and their rating for a motor (inductive) load is much lower than a plain load. They may also have a similar "ballast" rating, because HID and older fluorescent lights have ballasts with a big inductive winding. Their tungsten rating has nothing to do with this.
I would try just replacing the GFCI switch first. Easy inexpensive way to see if that is all or something more complex. Do be sure to work safely and turn off breaker before changing out.
