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From an online source:

Your outlet tester may do double-duty as a GFCI outlet tester, simulating a ground-fault by the push of a button on it. There is a limitation here. Unlike a good GFI, which can test itself even without a good ground, the three-prong GFI outlet tester depends on a good ground to purposely trip the GFI; and this is also true when it is used to try tripping the GFI by pushing the tester's button at standard receptacles wired downstream from it.

If one doesn't trust the test button on the GFI, and one knows the outlet tester can give a false negative in this scenario, how does one verify the circuit protection?

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    I see no reason not to trust the test button. That's what it's there for. – Kris Apr 28 '17 at 1:10
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    @bishop -- how would the seller damage the test button during installation of the GFCI? – ThreePhaseEel Apr 28 '17 at 1:29
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    If the supply is 2 wires, testing a GFCI is harder than you think. – Harper Apr 28 '17 at 1:38
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    @ThreePhaseEel Because the seller wired it wrong, busted the solid state circuitry, then wired it right? According to a 1999 survey, 19% of GFI installs didn't provide protection, so it does happen... – bishop Apr 28 '17 at 1:46
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    I'm voting to close this question as off-topic because this has nothing to do with home improvements, it looks like a topic for a Blog. – Ed Beal Apr 28 '17 at 2:52
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If you were to read this: HOW GROUND FAULT CIRCUIT INTERRUPTERS WORK.

You can see that all you need to do is utilize a 15K Ohm Resistor across the Line and the Neutral to trip it. Pages 14 - 18 of that document should reflect this. You can make your own tester this way.

As noted by a commenter the resistor needs to be placed from neutral prior to the sensing circuit of the GFCI to the Hot After the GFCI sensing circuit. This will appropriately create a TEST circuit like that which is found in the documentation linked to..

While there are ways of getting around the need to remove the outlet from it's box it should not be attempted by anyone who does not know what they are doing. I will leave that to those who are knowledgeable enough to figure it out.

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    A resistor across line and neutral will do nothing as the current is still balanced. As the other answer noted, one must be on the "line" side and the other on the "load" side, so as to unbalance it. Unfortunately, this requires pulling the outlet out. – DoxyLover Apr 28 '17 at 20:11
  • @DoxyLover A resistor across the Line and Neutral is a load. And you are correct in that the neutral connection needs to be prior to the SENSE circuit. So no it is not balanced at that point.Reading the documentation - you can see this in the electrical diagram.It really depends on where his GFCI is located In an outlet or the CB. Personally I would open the box. Another option not recommended for a DIY'er (you need to know what you are doing) If there is another NON-GFCI outlet nearby he might be able to pull 1 wire from that neutral outlet through to his tester and then to his GFCI hot side. – Ken Apr 28 '17 at 22:48
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    My point is that your answer should state this and not require the OP to figure this out from the linked page. As your answer stands in isolation, there is no indication of needing to access the line terminals. – DoxyLover Apr 29 '17 at 0:18
  • @DoxyLover - point taken - specifically added it to the answer - so if he does not look at the documentation and diagram he can know what to do.. – Ken Apr 29 '17 at 6:34
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    I did read the linked documentation, which was an awesome reference. Why you've got a net negative bewilders me. – bishop May 10 '17 at 2:33
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You can put a load across the Line Live and the Load Neutral or the Load live and the Line neutral (this is what the test button will do) or between one of the Load conductors and a grounded conductor. This should trip the GFI.

However doing this while the GFI is properly installed is difficult and dangerous because the connections should be hidden and it requires you to poke around a live circuit.

GFIs without a ground are legal but the sockets should be marked with a "no equipment ground" label.

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