I have a receptacle tester with a button to test GFCI. It works properly in tripping the GFCI if the outlet itself is a GFCI outlet. However when I test it on circuits which have a GFCI breaker nothing happens. is my tester broken is my breaker functioning improperly what's the issue here?
asked Mar 14, 2021 at 15:47
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2 Answers
If the receptacle is grounded: Yes it should trip it.
If the receptacle has some semi-grounding that is insufficient to clear a dead-short bolted fault (i.e. incapable of flowing 200A without setting the house on fire): Yes it should trip it. And that is fine. A bolted fault is also a ground fault, which should trip the GFCI, ending the event.
If the receptacle is not grounded: No, it should not trip it. That would be disturbing if it did.
If the receptacle is not grounded, but a tester indicates grounded: Then a) no, it should not trip it, and b) that is a bootleg ground that defeats the entire purpose of even having a GFCI /facepalm /Darwin_Award and needs to be corrected ASAP.
If you stick the GFCI tester in a 2-prong cheater and come off the ground tab with a separate wire run across the house to a part of the electrical grounding system: then yes, it should trip it.
answered Mar 14, 2021 at 16:30
Harper - Reinstate MonicaHarper - Reinstate Monica
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1It would seem like safety might be improved if someone made a GFCI outlet specifically for applications with no equipment ground, which would connect the grounding pin weakly to neutral (e.g. via 180K resistor and capacitor that together would allow less than 2mA to flow), but trip if significant voltage appeared on that pin. Otherwise, a failure in plugged-in equipment failure that shorts hot to ground wouldn't be able to trip a GFCI unless someone touches it and provides an alternative path to ground.– supercatCommented Mar 15, 2021 at 18:16
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@supercat But where would you use it? Can you still legally run receptacle circuits without a ground wire? I'm afraid you're not allowed to stick such a fancy receptacle into an old ungrounded circuit and then claim that the whole thing is still grandfathered in. (But I know next to nothing about the NEC. It certainly wouldn't be allowed this side of the pond.)– TooTeaCommented Mar 15, 2021 at 19:39
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@TooTea: Given a choice between installing an "ordinary" GFCI outlet to a location without a useful ground wire (which is allowed in the USA, provided the outlet is marked 'No equipment ground'), or installing one with additional safety features, I would think the latter should be seen as preferable. Alternatively, one could construct a device that would attach to four of the five wires on an existing GFCI (line hot/neutral, load hot, and grounding) would add such safety functionality to it. Just a few resistors, a couple diodes, and a transistor.– supercatCommented Mar 15, 2021 at 20:10
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@supercat OK, this just shows my level of ignorance of the NEC. You're right, it would be better. Question is whether the potential market for such devices is big enough to pay for the design and certification. I assume these "No equipment ground" receptacles aren't super common.– TooTeaCommented Mar 15, 2021 at 20:19
Harper's answer is, as usual, totally correct. But to explain why:
GFCI (whether breaker or receptacle) detects a difference between current on hot and current on neutral (or two hots if 240V, but we can ignore that for simplicity).
The internal test of a GFCI does whatever is necessary to test.
An external test - e.g., built in to a 3-light "magic 8-ball" tester - has to cheat a little bit. It takes a bit of the current that would normally be going from hot to neutral and sends it to ground instead. If the ground pin is properly connected then this works and the GFCI trips. If the ground pin is not connected (which can be legitimate it properly labeled when using a GFCI) then the external test tries to send current to ground, but it has no place to go and so that circuit is not completed and the GFCI does not trip, but this does not have any effect on proper operation of the GFCI.
The one exception, as noted by Harper, is a bootleg ground. This is ground connected to neutral in the receptacle. If you have that, a 3-light tester will show everything "OK" (because it can't tell the difference between neutral and ground) but the GFCI test function of that tester will not work. That is the one combination which is an indication of a serious problem. "Open ground" + "GFCI test fail" is OK.
answered Mar 14, 2021 at 16:39
manassehkatz-Moving 2 Codidactmanassehkatz-Moving 2 Codidact
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1It should be possible to put a big enough inductor between hot and neutral to trip a GFCI.– JoshuaCommented Mar 15, 2021 at 2:19
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1Something that is capacitively coupled to ground could trip a GFCI, but inductance in and of itself could not. When motors trip GFCIs, it is often because of protection devices which clamp voltage spikes to ground.– supercatCommented Mar 15, 2021 at 18:17
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The internal test of a GFCI is just a switch that connects hot to ground (line-side neutral) via a suitable resistor to create the rated ground fault current.– TooTeaCommented Mar 15, 2021 at 19:42
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@TooTea Really? That is the way an external tester works. But if that's the case then the Test button on a GFCI will not work if there is no ground. Which goes against "GFCI in lieu of ground". So I wonder if that's the case or not... Maybe Harper knows... Commented Mar 15, 2021 at 19:51
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I mean a GFCI breaker. We don't have receptacle GFCIs around here. But I assume it works the same way, because for breakers the idea is to make the test resemble a real fault as closely as possible, so that you don't get false positive tests (test trips while a real fault does not).– TooTeaCommented Mar 15, 2021 at 19:54