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Given old two-prong (ungrounded) wiring where a GFCI is installed nearest the panel and other two-prong outlets are attached to the load side of that GFCI, how does one confirm that all the outlets are properly protected?

As far as I understand things, a GFCI tester must leak some current to earth to confirm that it triggers a fault. With the earth pin unconnected that test can't be done with a conventional three-prong tester.

Is there a more sophisticated test? Or is there simply no such thing as downstream protection in ungrounded circuits? Must every outlet have its own GFCI?

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  • Don't waste your money installing GFCIs everywhere. One will suffice. Your notion is correct that a tester can't test a GFCI without a third path to leak current into.
    – Harper - Reinstate Monica
    Nov 26, 2017 at 20:42

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Testing the initial GFCI receptacle should be sufficient. No need to test the load side receptacles since they are protected by the GFCI. If the GFCI works, and the 2 prong receptacles are connected to the load side of the GFCI, then they are protected.

If however, you wish to pursue it, you would need a separate conductor run to a good ground. Insert the tester into a 3 prong to 2 prong adapter and attach the ground to the ground prong tab (or wire) of the adapter. Then plug in to each of the old 2 prong receptacles to test.

Good luck!

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    I don't have a lot of faith in what should work and I'm not comfortable putting the "GFCI protected" tag on an outlet I have not tested directly, no matter how much common sense tells me it must be protected. Anything could be going on within these 60-year-old walls. Looks like I'll have to build a rig for this.
    – sh1
    Nov 26, 2017 at 23:16
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I've done the cheater test before but to my mind it's not a valid test to simply trip the GFCI with the test button on the upstream, obviously anything south of it will die too provided it's connected to the load side of said GFCI. Like one person said, if the downstream 2 prongs are on the LOAD side of the GFCI, they are protected to the extent they are as such. But I want to know what actually trips the GFCI from the LOAD side 2 prong downstream outlets? Isn't a GFCI supposed to detect a fault between hot and neutral too? Like shouldn't it also detect an open or short on either hot or neutral? More specifically, what exactly trips the GFCI on the 2 prong side without a cheater? Also, they need to start mass producing 2 prong tamper proof outlets too! Hard to find them suckers if you can find them at all! Thanks!

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    What actually trips is current flowing through hot or neutral and not through the other. A short will trip anything/everything. But "open hot" or "open neutral" are "invisible" to the GFCI because they are not complete circuits. As far as 2-prong tamper-proof, not gonna' happen - anyone retrofitting is strongly encouraged to upgrade to 3-prong - either properly grounded or GFCI. Tamper-proof by itself only protects one small (important but small) problem mode - grounding (or GFCI) protects far more.
    – manassehkatz-Moving 2 Codidact
    Oct 19, 2020 at 1:36
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    Agreed on the opens. Actually, a GFCI on a 2 wire downstream setup will detect an unbalanced load between hot and neutral and trip! ;) That is, if there is more current coming in than is going out, it will trip the GFCI. Obviously no current will flow if either side is open. Still, I agree, there's nothing better than a proper 3 wire system!
    – Sparky
    Oct 19, 2020 at 1:48

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