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We have multiple GFCI cords, such as this one (the GFCI is not an outlet). When plugged into a certain outlet, they all consistently trip with any load higher than a few watts. When plugged into any other outlet, they work fine.

Shouldn't this be physically impossible? If a GFCI just measures the current difference between the two halves of the circuit, no configuration on the supply-side should ever cause it to trip, right?

(For example, a ground-fault inside the outlet would cause the voltage/current to the GFCI to drop, but the current coming in and going out should still be the same, by Kirchoff's current law)

I assume this is caused by some nuance of GFCI implementations I don't know about, which is why I'm asking here.

Notes:

  • An outlet tester shows the outlet is wired ok, so the hot/neutral aren't swapped.
  • When using the outlet tester with the GFCI, the GFCI does not trip. But various loads ranging from 250 W to 1500 W all cause it to trip. Those same loads with the same GFCI don't trip in other outlets.
  • I've tried multiple GFCIs with multiple devices, all with the same results.
  • The outlet measures 120V

[Edit] One of the GFCIs seems to know when it's plugged into the bad outlet, even with no devices plugged in. When plugged into the bad outlet, the 'test' function on the GFCI does not work. When plugged into any other outlet, it does. The GFCI still trips with any device plugged in, which again should be impossible...

[Edit 2] This should not have been migrated. It is an electronics question, not a DIY question. Sigh.

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  • I would meter the outlet with a proper meter and physically check the wiring. This is definitely unusual behavior. I'd also check if a 12A load will run off the outlet without a GFCI. Putting the whole circuit on a GFCI breaker would be interesting, as would rewiring the receptacle with a GFCI plug. All of that said, this should probably be on Home Improvement Stack Exchange.
    – K H
    Commented Feb 13, 2021 at 2:59
  • @BlueRaja-DannyPflughoeft -- do devices work in the outlet in question without the GFCI in the circuit? I think I know what's doing this, but want to be sure
    – ThreePhaseEel
    Commented Feb 18, 2021 at 1:57
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    I do agree with you on the migration - EE tends to throw anything here if it mentions "house" or "appliance" or "consumer device of any sort". If you'd included a schematic of a GFCI and asked about that, they might have kept it there... :)
    – FreeMan
    Commented Feb 23 at 18:17

2 Answers 2

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Shouldn't this be physically impossible? Maybe in theory, but let's come up with some real-world hypotheses.

There is a phenomenon of GFCI "nuisance tripping" when the current flow is noisy, or contains higher frequency components. I believe it's because the differential transformer that senses imbalance in the line and neutral currents, is not so perfect at cancelling our those two currents at frequencies above 60 Hz.

So, first hypothesis: loose or corroded connection in the line or neutral wire feeding this outlet. The simple outlet tester or basic voltmeter, would not detect this, so you may have to just repeat your experiment, remembering that the fault could be in this outlet, or anywhere upstream toward the distribution box. Power lines are often "daisy chained" through several outlet boxes. Identify all outlets on this circuit and repeat your experiment on each. That might help determine where the poor connection lies.

The other hypothesis, less likely, is that there is another appliance on this circuit, like a motor with sparking brushes, causing noise on the line. Again, knowing what this branch feeds, you should be able to disconnect any possible culprits and repeat the experiment.

I'm interested to hear what you find.

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  • Hmm. It's an outdoor outlet, and the only receptacle on the circuit. It is branched off a subpanel, with a well-pump connected to the same panel; however the issue happens even when the pump is not running.
    – BlueRaja - Danny Pflughoeft
    Commented Feb 13, 2021 at 4:42
  • Aha, an outdoor outlet. Maybe corrosion in the wire connections, or inside the outlet contacts. I’d replace that outlet as a worthwhile test.
    – Mark Leavitt
    Commented Feb 13, 2021 at 4:54
  • Hrm, I replaced the outlet, all of the wiring between the outlet and subpanel, and the breaker, but the issue is still happening.
    – BlueRaja - Danny Pflughoeft
    Commented Feb 13, 2021 at 23:42
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An outlet tester shows the outlet is wired ok, so the hot/neutral aren't swapped.

That is not important. You should check if the neutral/protective earth are swapped, there is a very big possibility, that they are.

The other possibility is that a load is wrongly connected, and the only correctly wired outlet is the one which trips, all the others don't have the PE wire, so they never trip.

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  • How could swapping neutral and ground at the outlet trip the GFCI? The current through the hot/neutral will still be the same - the GFCI can't know that ground gets swapped later in the circuit! If they were swapped at the load that would make sense, but that's not what's happening here (as mentioned, I tried multiple devices, all of which trip when plugged into the bad outlet but not a good one)
    – BlueRaja - Danny Pflughoeft
    Commented Feb 13, 2021 at 23:41

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