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I replaced a non-protected receptacle with a GFCI type and now my circuit tester trips the GFCI as soon as it is plugged in. The receptacle tested fine before replacement, ground, neutral, hot, etc. I've double checked the hot/neutral/ground connections. I've also tried it with the downstream load disconnected.

The GFCI trips the moment I plug in my circuit tester (before pushing the test button). Otherwise things check out OK. I can plug in a power tool without it tripping. Also, the integrated test/reset button work correctly.

I've read that the integrated test button is the only one that truly matters. Is this correct or should I be concerned? I'm sure a home inspector will red flag this if their tester has the same result.

Update

My tester is by "Commercial Electric". The tester is one of the palm sized three prong type with three leds to indicate the condition of the circuit and a button on top to test GFCI. It's new from the local box store.

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  • What's the make and model of the tester? Is the tester new, old, in good condition, etc.?
    – Tester101
    Aug 1, 2016 at 10:55
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    It depends on how your particular tester is built. If it tests against the ground pin, then it creates the exact live-to-ground current that's supposed to trip GFCI. Powertool is not a good test device, as they're usually designed as insulated - they don't have a ground pin in their plugs. Try something with grounded plug, a desktop computer or 3-pin laptop charger.
    – Agent_L
    Aug 1, 2016 at 10:55
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    I would check your tester on another GFCI outlet. It is possible the trip function of the tester is stuck. If your outlet runs any tools or devices I believe the tester is bad. If the Test & reset buttons on the outlet work I would bet the tester is bad.
    – Ed Beal
    Aug 1, 2016 at 12:58

2 Answers 2

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As well it should.

Normally, loads are connected between hot and neutral. Appliances are not supposed to connect hot or neutral to ground; ground is only a shield.

The GFCI compares current on the hot and neutral wires. They should be the same. If they are not, current has found another route, possibly through the grounding system (which isn't supposed to happen) and potentially through some poor human.

Circuit testers are trying to test whether ground is connected... cheaply. They mis-use "hot" as a power source, by connecting a light bulb between hot and ground. If ground is connected correctly, this will light.

In other words, it intentionally creates a hot-ground fault (by sticking a light bulb there). This is exactly the condition GFCIs are designed to detect.

I'm not talking about any ground-fault-test the tester may also have.

So why do testers often work? With a perfect GFCI, they wouldn't work. I suspect it is because GFCI's have detection thresholds above zero, and that is often enough for these testers to "get away with it". I even think there may be a tacit agreement among manufacturers for this, but obviously, lower sensitivity impinges safety. Remember a shock which only stuns you can kill you with secondary effects like falling or drowning.

So either your GFCI is pretty good, or your tester is pretty bad.

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    Note that the GFCI trips before I push the test button on the tester, but operates normally with the test button on the plug.
    – tharen
    Aug 1, 2016 at 14:57
  • I've never taken one of these apart, but I'm not sure that's how they work. When plugged in, the unit should light up properly (assuming the wiring is proper). When the test button on the unit is pressed, it simulates a ground-fault to trip the GFCI. In normal use, the GFCI should only trip when the test button on the tester is pressed.
    – Tester101
    Aug 1, 2016 at 15:12
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    I'm speaking of its regular function, not any GFCI test it may also have. Admittedly I don't know which circuit tester he's talking about. But unless the detector strings a long cable back to a reference ground, it can only test ground by testing its relationship with one of the other two pins (hot or neutral). Any such test must, inherently, be a ground fault. A theoretically perfect GFCI would always detect this; you may have gotten away with it due to detection thresholds. I am saying, don't bet on that.
    – Harper - Reinstate Monica
    Aug 1, 2016 at 15:32
  • @Harper, In the back of my mind that's kinda what I wondered. Could my new GFCI receptacle be sensitive enough to detect the circuit tester and trigger a fault. It seemed unlikely that the manufacturer of the tester would not have caught that, but what do I know?
    – tharen
    Aug 1, 2016 at 16:09
  • Reducing the trip threshold below a certain point will increase the frequency of false-trip events caused by things like capacitive coupling; false trips don't increase safety.
    – supercat
    Dec 9, 2016 at 23:33
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The short answer is yes, some testers can give inconclusive results. The GFCI receptacle is able to detect the GFCI and an incorrectly wired load can trip the receptacle breaker.

I puchased a new GFCI/circuit tester and it properly identified a 'down stream' load with the hot and neutral reversed. I swapped the hot and neutral on the second outlet and all was good.

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