In article , bud-- wrote:
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.

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.

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.

Yes, it does -- and with current reaching the motor through an SPST switch,
*one* of those two wires is disconnected from the GFCI when the switch is
opened, but the other is not. And that's why the GFCI sees an imbalance -- and
why I think a DPST switch will eliminate the problem.

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.

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


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.

Pretty good assumption, I'd say.

--
Regards,
Doug Miller (alphageek at milmac dot com)

It's time to throw all their damned tea in the harbor again.