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I have a GFCI receptacle (commonly found in North America) in my home. Recently I switched the power to the outlet off using the circuit breaker, then pressed the "Test" button on the outlet. To my surprise, the outlet tripped. The outlet does not reset unless I switch the power back on using the circuit breaker.

Why does the GFCI test functionality work when there is no power?

I watched a youtube video by Technology Connections regarding the GFCI receptacle. At 12:40 the video mentions that when the "Test" button is pressed, a resistor creates an intentional ground fault and causes the receptacle to trip.

I also read parent US5757598A. In Fig. 1, SOL1 receives power from the power source. So when there is no power, I expect GFCI Test to not work.

Is it a new design of the GFCI receptacle, or is there something wrong with my GFCI receptacle?

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  • Maybe it did not trip due to fault current (because there could not have been any if power was cut), but maybe due to it also acting due to mechanical force, to break the contacts? We don't know what kind of GFCI you have and how it works.
    – Justme
    Commented Feb 6 at 7:37
  • Some GFCI/receptacles are designed to use the TEST/RESET buttons as a switch. I wonder if maybe this is (a) one of those types and (b) if it is then a combination button that initially (soft press) does the usual actual test but when pressed far enough (hard press) actually interrupts the hot line.
    – manassehkatz-Moving 2 Codidact
    Commented Feb 6 at 19:10
  • It is wrong to say that the "test functionality worked". It's not defined what the buttons should do in the absence of power. It did something you didn't expect, but it wasn't test functionality. It's a product design question, and I'm sorry to get all "go ask your mother/father" but how to design the test circuit is probably an electrical engineering question, or some kind of engineering. ;)
    – jay613
    Commented Feb 6 at 22:09

3 Answers 3

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Some GFCI test buttons have a mechanical "off switch" functionality. This is why GFCIs should be tested and inspected using an outlet tester rather than just relying on the built-in test feature.

I can attest to the fact that failed GFCI receptacles may appear to "test" correctly using the built in button, while completely unresponsive to a ground fault tester.

Where this gets more complicated is with ungrounded GFCI receptacles. But I think that is off topic and may be a good separate question.

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A GFCI should not trip when the test button is pressud during a power outage.

The test button creates an intentional differential fault accross the sensing coil the gfci/rcd will detect and trip on, it does not generate an earth fault because they typically do not have an earth connection.

rcbo diagram

Without mains voltage, no current will go through this resistor.

Maybe the power wasn't isolated properly? Did you measure and verify it was isolated?
Always check before work!

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  • This all depends on the specific model. Some of the automatic self-test models just have an "off" switch, for example.
    – Robert Chapin
    Commented Feb 6 at 19:35
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It doesn’t take a lot to trip an RCD; even if power is ostensibly off you might find that there’s a big enough residual electric field to trip the device, although I’d be guessing where it would come from.

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