I have a similar problem. I noted that when the light bulb on the fan
fixture is illuminated the problem does not occur. Perhaps a
resistive load across the fan absorbs any power line spikes which
trip the GFCI. Just an idea.
Note: my return address contains no numeric characters.
In my case, the fan is just a fan. No light or heater in it.
Wiring is about as simple as it gets. A single incoming 12/2 cable coming
from the switch. Fan housing is grounded, hot and neutral are connected
according to the fan instructions (black to black, white to white. No
I suggested earlier the high voltage spike that can be produced in the
fan winding when the switch is turned off could produce capacitive
currents from the fan winding the motor poles (ground). The size of the
spike depends on where in the sine wave the fan is turned off - random
effect. Fans intended to be used on GFCI circuits could be built with
more winding isolation. I haven't seen aanswer yet that better fits what
In Mike's description above, if the light is connected across the fan
when the fan is turned off the light could absorb part of the spike from
the motor winding.
I don't think there is a fix other than "take the fan off the GFCI, or
replacing the fan".
If, as you suggest, there is a voltage spike when the fan is shut off,
and there is more capacitive coupling from one of the fan terminals to
ground than the other, then adding a capacitor across the fan leads
might reduce that.
Another possibility: I have often seen the armature of motors, when they
turn on or off, shift axially. If so, there could be a transient short.
Assume capacitance is equal from both hot and neutral to pole. Leakage
also depends on voltage. The neutral end is connected to pole (at the
service panel). The voltage from N-to-pole will be minimal. I suspect
the winding is constructed so the neutral end is most toward the pole
but don't know. Could try reversing the motor H-N leads.
Capacitor across H-N - maybe. The capacitor does not dissipate the
energy but may spread it over time and lower the peak voltage. MOV might
work but I wouldn't connect one L-N without protection. My guess is a
shunt resistance would probably have to be too low value so that it
dissipates significant energy.
I wouldn't guarantee trip is from capacitance-spike but it is the best
guess I can come up with.
If a supply wire was near an end of the armature - could be.
Note that GFCIs detect a fault from neutral to ground also. From the
schematics I have seen, I believe they do this by inducing a higher frequency
common voltage on the the hot and neutral. If there is neutral
to ground fault there will be a common mode current of this higher
frequency signal which will be detected by the same circuitry and trip.
The point is: don't ignore a ground fault on the neutral side.
What I remembered was inducing a current into just the neutral. A
manufacturer's datasheet shows common mode induction into both H & N
like you said. That would also give "neutral"-ground fault detection if
the hot and neutral wires were reversed. The data sheet shows a fullwave
rectified 60Hz source, which would be 120Hz with harmonics.
When there is load on a circuit with a GFCI, resistance will cause a
voltage from neutral to ground, and there will be current in a N-G fault
that will trip the GFCI. With the added circuitry a GFCI will trip on a
N-G fault with no load.
(datasheet is at
Detecting current difference between hot and neutral is a lot
simpler than that.
If the neutral is shorted to the ground, the ground wire acts
as a parallel conductor to the neutral which reduces the neutral's
current, hence, the hot and neutral won't match.
GFCI's work by running the neutral and ground around a magnetic core,
measuring the net magnetic flux, and triggering if it exceeds a certain
threshold. If the neutral and hot currents are the same, the magnetic
flux cancels out.
Age and Treachery will Triumph over Youth and Skill
I see that you didn't even read the link that you provided.
"To detect a Neutral to Ground fault there is a second transformer placed
upstream of the H-G sense transformer. A small drive signal is injected ...
which induces equal voltages on the H and N wires passing through its core."
A typo - neutral and hot instead of neutral and ground.
What is detected by adding the CT in MQ's post is a N-G fault with no
load. Without the added CT, the simple GFCI in Chris' post won't detect
a N-G fault until there is a load on the circuit. Other than that a GFCI
operates as Chris describes.
I was surprised to learn this function was built into GFCIs. But it only
adds 2 parts.
It seems rather strange even to do that. The slightest bit of load
on the circuit, either before or after the GFCI, will mean
that the neutral is at a different potential than the ground. If
the neutral shorts to the ground after the GFCI, then there will be
current flow thru the neutral at the GFCI to the ground without the hot
at the GFCI seeing anything.
Me too. I shoulda read a bit farther ;-)
Age and Treachery will Triumph over Youth and Skill
If the trip is caused by capacitance to ground and spike, and if the
capacitance to motor frame (ground)is lower from the neutral end of the
winding than the hot end, reversing the leads may help. I wouldn't bet
Probably not stated previously, a spike is not only high voltage, but
because it is short duration it is high frequency. High freq increases
Another possibility - capacitance from hot wire to ground wire in Romex
between fan and GFCI.
Sure. If you don't mind a light turned on with the fan, it works for
Mike. May work for the OP.
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.
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.