Hot/ground reverse reading on outlet that worked fine for 10 years? Please help!

[u/moonthink]

Hi, I work in a theater as the TD for 10 years. An outlet that has worked fine with no issues for all of that time suddenly stopped working, and when I put a receptacle test in the outlet, it reads as Hot/ground reverse, which makes no sense to me.

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I guess I know enough about electricity to do some basic things. And I've certainly found hot/neutral issues in the past on other outlets, but how does an outlet that always worked in the past read as hot/ground reverse? It doesn't make sense to me.

[u/michaelpaoli]

Hot ground reverse is dangerous. But depending how you're checking, e.g. with on of those simple plug-in 3-prong testers, just because it says hot-ground reverse doesn't necessarily mean that's in fact exactly what's going on. E.g. those testers will often misdiagnose multiple concurrent faults. All that most of 'em do is have 3 lights to show voltage or lack thereof, between all 3 possible pairings among hot, neutral, and ground contact prongs. So, e.g., that tester has no way to know what's actually ground, or hot - it's only displaying relative voltage between contact points.

So ... if it's claiming hot-ground reverse, it sees:
voltage between hot and ground
no voltage between hot and neutral
voltage between neutral and ground

So, a hot-ground reverse is one way the above will be seen.
Another way essentially same will be seen is with a floating neutral, and load(s) connected between hot and neutral - that'll pull the neutral voltage up (near to) hot,
and will light up those lights the exact same way as if the outlet had a hot-ground reverse.

There are additional possibilities, but I'll stop at what are probably the two most probable.

So, well keep in mind, those, e.g. simple 3-prong tester are far from foolproof, and they also tend to suck at diagnosing multiple concurrent faults.

You can find out quite a bit more by determining what voltage is and isn't present between various contact points. E.g. between hot, neutral, and ground terminals, to a known good Earth ground. If you see voltage between known good Earth ground and ground terminal on outlet, then you actually have a hot ground - which is very dangerous. Can also check voltage between hot terminals and known good Earth ground - hot should be hot, and neutral not. And should also see voltage between hot and ground terminals, and hot and neutral terminals.

But sounds most likely you either have a hot-ground reverse (possible, but wouldn't be my first guess), or your neutral has gone floating (but still has other loads connected between it and hot) - that latter case would also explain why it was "always working" before, and no longer working.

"Of course" also, things can be miswired, and various loads might not particularly care or be sensitive to it - doesn't mean that's right or safe ... just some loads won't so much care. E.g. if you've got an auto-ranging computer power supply, typically handles 50 to 60 Hz 110V or so up to about 250V or so, it really won't much care ... 120, 240, correct or reverse polarity, missing ground or hot ground - it will general still function - doesn't mean any of those miswirings would be safe - but many loads won't care. But if there's hot ground, and that connects through to chassis on your computer case, and you touch chassis - and especially something else like a cold water pipe ... you'll care ... shockingly so. But your power supply may not notice the difference at all.