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-   -   Fan causing GFCI to trip (http://www.diychatroom.com/f18/fan-causing-gfci-trip-64735/)

terryfitz 02-17-2010 07:14 PM

Fan causing GFCI to trip
 
I was called upon to try and fix a GFCI in a bathroom that occasionally trips. There is a 12/2 20 amp feeder coming into a three gang plastic box. In the box are the GFCI, a single pole switch to a fan, and a dimmer switch to a lighted medicine cabinet. Occasionally, the GFCI trips after you turn the overhead fan off (not on, but off.) All three units are pigtailed from the feed and the GFCI is wired to the "Line" side. I changed both switches out but not the GFCI (I didn't have one with me.) I also disconnected the hotwire to the dimmer. I've reached the point in the process of elimination where I think I will have to pull the fan unit but would prefer not to if someone has a better idea.

AllanJ 02-17-2010 10:48 PM

So the only time the GFCI trips is at the moment the fan is turned off?

Is there anything connected to the "load" terminals of the GFCI?

Is the GFCI test button unusually loose, for example if you slap the wall causing vibration, would the GFCI trip?

If the load connections are wire ends instead of screws, are the wire ends taped well and no single strands of wire poke through and could intermittently touch something of metal?

terryfitz 02-18-2010 05:33 AM

Hi Allen

Thanks for trying to help.

"So the only time the GFCI trips is at the moment the fan is turned off?"
Correct

"Is there anything connected to the "load" terminals of the GFCI?"
Yes, it goes to another receptacle.

The GFCI test button is fine, and I am using solid wire with no visible cuts.

secutanudu 02-18-2010 06:36 AM

Can you disconnect the everything connected to the load side of the GFCI, then try it? Just to officially rule out all the stuff after.

mikey48 02-18-2010 12:09 PM

The older GFI's trip with bathroom fans. I would replace the GFI if it is older.

AllanJ 02-18-2010 12:38 PM

When a motor, solenoid, or other inductive load is switched off, an inductive kickback aka back electromagnetic force is generated which can damage electronic switching. But I would not expect that to trip a GFCI unless the load was powered through the GFCI for example plugged into a receptacle on the load side.

HooKooDooKu 02-18-2010 01:05 PM

Quote:

Originally Posted by AllanJ (Post 402114)
When a motor, solenoid, or other inductive load is switched off, an inductive kickback aka back electromagnetic force is generated which can damage electronic switching. But I would not expect that to trip a GFCI unless the load was powered through the GFCI for example plugged into a receptacle on the load side.

This sort of describes the experience I have every once in a while similar to the OPs situation.

I wired up a workshop section of the garage with two circuits: 20 amp circuit for power tools, 15 amp circuit for lighting and low power loads.

The lights are shop lights plugged into a switched outlet. Every once in a while, when I turn the lights off, the 15 amp GFCI trips, and I know the switched outlet is on the load side of the GFCI.

HooKooDooKu 02-18-2010 02:11 PM

One of the physical laws of electricity is that current flowing through an inductive load can not change instantaniously.

I'm guessing that what might be happening is that when the switch is openned near the apex of the instantanious current flow of this AC circuit, the current flowing through the inductive load has to go somewhere. I think a part of it is finding its way through the GFCI, after all, the electronics in the GFCI create a (high resistance) path between the hot and neutral. This unexpected current flow through the GFCI is causing it to trip. The reason it doesn't always trip is because if the switch is openned when the instantanious current flow is near zero, there isn't any current flowing through the inductive load that needs to go anywhere.

I recall something called a "free wheeling diode" that would get connected to DC motors. The purpose of the diode was to provide a current flow path when the motor was still turning, but the power to the motor had been cut off.

If I'm right about what is causing the GFCI to trip, I wonder if we can't fix it with something similar to the free wheeling diode.

Try connecting a light in parallel with the fan, connected as physically close to the fan connections as possibile. The idea is that if there is an inductive load current flowing through the motor the moment the switch is openned, the current can continue to flow through the lightbulb rather than effecting the GFCI receptical.

terryfitz 02-20-2010 06:58 PM

Fixed
 
Well, it turned out the culprit was the GFCI although I don't really follow why since the fan was not on the load side. Regardless, it's fixed.

HooKooDooKu 02-20-2010 07:19 PM

Quote:

Originally Posted by terryfitz (Post 403360)
Well, it turned out the culprit was the GFCI although I don't really follow why since the fan was not on the load side. Regardless, it's fixed.

But you were not dealing with a ground fault. You were dealing with a faulty GFCI. A GFCI contains circuitry, and that circuitry is connected to both the line and load side. We've already talked about how a fan load can cause electrical signals even after the power is turned off... apparently that signal was finding its way back through the faulty circuitry and causing the GFCI to trip.

terryfitz 02-20-2010 07:27 PM

Right
 
You're right...I'm tired after a long day and had a brain circuit short.


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