Doug Miller wrote:
In article , BobK207 wrote:
On Dec 26, 1:06 pm, M Q wrote:
HerHusband wrote:

I am skeptical that a properly working fan should trip a properly
working GFCI when the fan is turned off. If there is no ground fault
in the fan or switch, there can be no imbalance of currents (conservation
of electrons).

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.

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.


A double pole switch should not make a difference.

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.

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


I might suggest an intermittent ground fault in the
fan or switch when the fan is turned off.

Perhaps; but the explanation referenced above is IMO much more likely to be
correct.
Please let us know what the problem is/was when/if you figure it out.
Give MQ's theory a try. I would agree with his suggestion that you
might have an intermittent ground fault in the switch itself.

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.

Yea, seems remote.

--
bud--