Update. The GFCI can also trip when the power supply was put on the load side instead of the input. Here the input is open. So it's not current inbalance in the sense coil, please explain how can inductive kick can trip it when ac supply is put in the load side.

Here is a GFCI youtube video that continuously trips when you use water shaded pole motor. It doesn't trip on other appliance, The motor was surrounded by plastic, glass surface and glove hands so there is no way current can escape. Can the tripping be caused by inductive kick from motor inducing capacitive coupling inside the circuit? How do you refute this theory?


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    Might be good question for electronics SE.
    – BowlOfRed
    Jan 18, 2019 at 22:45

1 Answer 1


GFCI devices basically sense a current imbalance - more current going out on the hot than is returning on the neutral, or vice versa.

In a simple circuit, say just a resistive load like a light bulb, the two are always balanced, and if they are not, there is probably current travelling through a ground path. This is generally unsafe, and why GFCIs are made to interrupt power when they sense this imbalance.

However many motors have capacitors in them, and can upset that simple balance without any ground fault, without any current on any ground path. GFCI devices may see that imbalance and trip, even though there is no ground fault present.

Motors also store energy in the electrical field, it's kind of like electrical inertia, and when the circuit is opened, the motor generates a high voltage pulse - inductive kickback - which again could disrupt the balance of current seen at the GFCI.

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    @jtl - if you are familiar with the water flow analogy for electrical current - a capacitor is like a storage tank in the works. If it's empty, water flows in the intake pipe but not out the output pipe until the tank fills. That's the imbalance. Jan 18, 2019 at 23:14
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    Shaded pole motors don't usually use caps, but inductance will cause the exact opposite effect from a cap and can cause a trip with no path to ground. Motor circuits can produce hundreds of volts of "kickback" this can be seen on a oscope the spike is two short for most meters to display.
    – Ed Beal
    Jan 18, 2019 at 23:36
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    Ok now I think it is a complete answer.+
    – Ed Beal
    Jan 19, 2019 at 19:39
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    @batsplatsterson You are talking about the displacement current. I have seen gfci trip in my house due to voltage spikes in line due to large inductive load switching in next house. Jan 23, 2019 at 10:25
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    Jtl, I have seen motors on a separate branch circuit cause GFCI trips, especially on dual winding start motors when the clutch changes the windings there is a big spike created. On DC motors the spikes are even worse. Relays on control circuits are enough to cause tripping in some cases if there are not suppression devices such as diodes or MOV's, this may be the case for your power supply. Electronics in power supplies do consume power even with no output device there are losses in transformers if a step down type, and switching supplies create all kinds of harmonics that AFCI breakers trip
    – Ed Beal
    Jan 23, 2019 at 18:00

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