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I have a question about the three prong GFCI testers. I used one on a GFCI outlet with no ground and it first showed just the middle light (open ground), but when I press the test button on the tester (not on the outlet) it also shows the red light, meaning hot/neutral reversed. My research seemed to suggest that, with no ground, the tester cannot trigger the GFCI nor detect a reverse polarity. But I found this video which has confused me. Is the video just completely wrong or am I missing something obvious?

Video: https://www.youtube.com/watch?v=M5LkaMWOgJ4

The confusing part is at 1:30 where he plugs the tester into a (probably ungrounded outlet) and presses the test button and says it is mis-wired in response to showing hot/neutral reversal.

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    "I found this video which has confused me. Is the video just completely wrong or am I missing something obvious?" — Tell us what the video showed that confused you. Don't expect us to waste our time watching it, and definitely don't expect us to be able to figure out which part it was that confused you. Commented May 1 at 3:04
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    The tester shows what wiring it encounters by way of lights. When you press the button to cause a direct short to ground, the lights are indicating what the tester is sensing at that time! Don't put much stock in youtube videos, plenty are wrong or full of misinformation.
    – RMDman
    Commented May 1 at 14:22

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The test button connects live to ground, so now the neutral looks like a live because it has voltage to ground and to live.

I'm not going to watch a random you tube video.

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but when I press the test button on the tester (not on the outlet) it also shows the red light, meaning hot/neutral reversed

Disregard ALL lights when pushing the TEST button. If I were king, the Test button would have a shutter which automatically covered or darkened all the lights.

The GFCI testers are absolutely not designed to give a meaningful reading when the Test button is being held down. Don't even try to diagnose the wacky result you get.

Unfortunately, the sticker which says what the lights mean is wrong. That sticker is optimized for wiring mistakes in brand new construction. It never considered wiring defects in old work. So the label is so useless that I call the things "Magic 8-ball testers". I wish they had a more accurate label. For instance

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That's not entirely fair; the lights are perfectly useful if you know what they actually do.

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My research seemed to suggest that, with no ground, the tester cannot trigger the GFCI nor detect a reverse polarity.

That is correct. You identify lack of ground by 2 things:

  • The yellow light on the end does not light up
  • The GFCI test does not trip the GFCI.

Of course if the GFCI test doesn't work and the yellow light DOES light up, that suggest a bootleg ground (ground pin strapped to neutral at this receptacle). Remove that; it's unnecessary downline of a GFCI.

Also remember that NEC 110.3 and the GFCI instructions require "GFCI Protected" on any plain outlet protected by a GFCI elsewhere. Further, 406.4(D) requires "No Equipment Ground" as well where that is true. You don't need to use the supplied labels; any marking not handwritten will do. I like P-touch label maker, smallest font, + white cover.

The confusing part is at 1:30 where he plugs the tester into a (probably ungrounded outlet) and presses the test button and says it is mis-wired in response to showing hot/neutral reversal.

Because he doesn't know what the F he's doing. Unfortunately Youtube will let anybody make safety videos, and the algorithm does not care about accuracy and rewards style, connections, clever thumbnails, clickbait titles and playing the algorithm game. Ask any Youtuber with a silver plaque. They didn't get it by being accurate.

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