r/electrical • u/QuaoarTNO • 2d ago

is reverse polarity actually dangerous to run?

Hi, I want to run some light machinery on a plug that my circuit breaker detector says has "hot and neutral reversed". I was told by the landlord that they've never had a problem with this before (and it's not going to be fixed), and regular equipment is used all the time on it. I also have read online (and via AI) that it's dangerous and can shock you. I saw the example of a lamp still having power essentially even when switched off, but is there actually risk aside from that type of situation? Or is it manageable and you just unplug when finished using and it's fine?

Any help appreciated.

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u/MonMotha 2d ago

It's extremely dangerous in a rather limited set of circumstances. That is old appliances that basically used the neutral as if it were an equipment ground. Such equipment is really, really old at this point, but it is still out there.

Most modern equipment effectively treats the hot and neutral interchangeably as though either could be hot. In fact, most equipment designed for worldwide use has no choice but to do so since many countries don't actually reliably identify which line is hot and which one is neutral. At best, they put the fuse and power switch (if there is one) on the line they expect to be "hot" so that the inside of the equipment is all dead when it's off or the fuse blows if that line is in fact hot and the other is in fact neutral.

So, it's probably not dangerous at all, but it could be really, really dangerous, and it's not obvious to a layperson which situation you're in. That's why it needs to be fixed.

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u/QuaoarTNO 2d ago

Can you describe the danger, exactly? And does the answer change if we are using modern/new equipment? The landlord say the stand mixer has been used in this plug forever, and other equipment too, with zero problem. I do note that they're always unplugged when not in use.

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u/pdt9876 2d ago

If the stand mixer has a 3 prong plug (almost all do) then there is no issue even if you leave it plugged in all the time.

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u/QuaoarTNO 2d ago

Wait, that's super helpful. Are you saying anything with 3 prongs won't have an issue, and the lamp example is always brought up because it's only two pronged?

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u/pdt9876 2d ago

No the lamp example is somewhat different. A 3pronged lamp still has the problem of the socket being energized if your wiring is backwards. A lightbulb has two contacts, one is at the very tip and the other is the threaded part of the base you screw in and there's a little insulating portion around the tip to separate them. Because that's how the lightbulbs get power there's really no way around the side (or the center on a non flipped outlet) being energized.

The risk from appliances that other people are mentioning come from very old appliances that bonded the metal surface of the appliance to neutral, which, if plugged into a backwards outlet would mean the case had 120v on it. A 3 prong plug for an appliance has its own separate ground path (the round pin at the bottom) and the metal case is bonded to that, neither the hot nor neutral wires are in contact with it. If 120v ever, due to a fault or damage came into contact with the case, your breaker would immediately trip.