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I have an outdoor power outlet of which I'm not sure if it's GFCI or not (House was built in 1978). The outlet does not have Test/Reset buttons, but I don't know if outdoor outlets even have them. There's no labeling for GFCI anywhere,.

So I'm looking to purchase a GFCI Tester. (Those little three-pronged things with 2 yellow/1 red LED and a button)

But I'm not sure what should/would happen if I press the GFCI Button and the outlet is not actually GFCI. Should it trip the breaker in the breaker box, or should it do nothing and the LEDs on the tester just stay on?

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  • Nothing: testers just put a high resistor between Live and PE so you'll have for a small time a little leakage current (same as having fridge with bad contact that discharges little current over its metal casing so routed to earth by PE conductor).
    – DDS
    Commented Dec 20, 2018 at 11:57

3 Answers 3

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If the outlet, does not have test and reset buttons, it is clearly not a GFCI itself. However, it is possible that another outlet or circuit-breaker in the line before it has been wired to protect that outlet, as well. No harm will be done testing a non-GFCI outlet using the GFCI tester, but if it does trigger an interrupter elsewhere, you'll need to locate that to reset it.

That said, an outdoor outlet definitely should have GFCI protection, whether 'upstream' or intrinsic. If the GFCI tester does not trigger a protection device, a GFCI outlet should be installed ASAP!

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    Thanks, that's exactly what happened: Plugged in a tester into an outdoor outlet, and it tripped the fuse in my bathroom.
    – Michael Stum
    Commented Dec 31, 2018 at 15:04
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The "gfci" plug in testers put a resistor from the ground to hot in most cases to pull 6 ma of current this creates an imbalance that will trip every GFCI out there, if there is no ground the testers don't work. The no ground with the testers is more common on 2 wire upgrades that are protected by a gfci outlet in older homes to allow 3 wire outlets. In 78 there should be 3 wire outlets in most of the US. If you still have 2 wire electrical with no ground the tester won't work but the test button on the gfci will work.

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Those 3-lamp "magic 8-ball" testers are pretty lame, but in two ways.

  • the legends, like "hot-ground reverse" or "no hot". They are more useless than a magic 8-ball, because they are wildly speculating at the most likely (easy) cause in the context of new construction. They are wholly unprepared for the realities of maintaining old wiring, and their wrong guesses will waste hours of your time. The lights themselves can be quite useful, especially if you see them in the shade so you can see if some lights are half glowing. By the way, those should be neon, not LED.
  • we're seeing a new generation of those which actually have computers inside, and the computer gets between you and the lights. Good chance those lights are actually LED. Again, the computer is aiming to troubleshoot only the most obvious faults, particularly the ones that arise in new construction - if you have a real stumper, the computer will only mislead you.

Pushing the GFCI test button will cause a small (10ma) amount of leakage between hot and safety ground. Any common 8ma GFCI devices upstream of this point will trip.

If there is no GFCI upstream, the GFCI will not trip because it doesn't exist. The overcurrent protection device (the breaker) will not trip because it isn't a GFCI device and 10ma is not anywhere near an overcurrent.

If the receptacle is not grounded, the upstream GFCI will not trip because the intended 10ma of leakage is going from hot to nowhere. Since current flows in loops, it won't flow. In that case, you test the upstream device using its own button and make sure this outlet loses power.

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  • Thanks for the heads up. Wondering: Are those testers at least reliable for "Good" and "Not Good" differentiation?
    – Michael Stum
    Commented Dec 19, 2018 at 21:50
  • @MichaelStum they're intended for pass/fail testing of new construction where an installer has no reason to want to do it wrong. For instance a bootleg ground will fool them, but a new installer would never bootleg ground, it's more work than just using the real ground that is part of new wiring.
    – Harper - Reinstate Monica
    Commented Dec 19, 2018 at 22:21

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