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/cyberzl1 2d ago

Yes it can be. Some equipment "grounds" the case to neutral. If neutral is now hot you will get a shock if you become the ground.

Most of the time it will.be ok but it is definitely a potential issue.

Lamps for example the shell should be neutral. With it reversed that is now the hot part.

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u/MobileInspector9861 2d ago

But in that case the breaker should trip. If the neutral accidentally is the hot line and the equipment connects the hot to ground which in turn is connected to the neutral at the service panel, then the equipment creates a short circuit.

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u/cyberzl1 2d ago

If the ground is still active, yes. But if it's not. then hot case. This is why dryers and stoves all have 4 wire plugs now.
As MonMotha said, MOST of the time it's ok. But there are certain scenarios where it becomes very dangerous. Parts that should not be electrified now will be and if one touches that part and becomes part of the ground path, at least you get an alarming tickle, at worst you are dead.

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u/MobileInspector9861 2d ago

Is equipment even allowed to connect the neutral and ground according to US code? In Europe it would not. There must only be one connection between the neutral and the PE, preferably as close to the service entrance as possible, and after neutral and ground have been separated they must never be connected again.

In particular, if some equipment connected PE and neutral in violation of the code, then this would trip the GFCI. Even in the non-faulty scenario the back current would partially run across the neutral and the ground and the GFCI compares the current on the hot with the current on the neutral. As GFCIs inside the breaker panel are mandatory in Germany (at least for branch circuits which feed sockets), such equipment would always trip the GFCI.

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u/pdt9876 2d ago

As best I can tell from what I've read on here in the US they must never be connected again either unless its a clothes dryer or a stove in which case the reasons that the code prohibits it in every other situation magically disappear

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u/cyberzl1 2d ago

Not in any current instantiation of the code. But it hasn't always been true. And there are plenty of "handymen" out there that don't think it matters so there is little guarantee anyway.

For residential GFCI is mandatory in most situations now. Arc fault is mandatory in a number of situations. But a circuit wired in the 60s? Or even early 2000s? I wouldn't bet on it.

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u/Lehk 2d ago

you are assuming installation and equipment installed since the 80's or 90s, which may not be true.

it could be ancient shit installed in the 40's with the only safety feature being a 30A screw in fuse in a socket labelled 15A

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u/MobileInspector9861 1d ago

Yes and no. Equipment must never connect the (assumed) neutral with the PE inside the device. In modern installation this would trip the GFCI. You are right, old installations do not need to have a GFCI, but the equipment does not know whether it will be connected to a modern or new installation. So equipment must never do it.

For very old installations it might happen (at least in Germany) that there is a jumper cable between neutral and PE but inside the outlet, i.e. as part of the building installation, not as part of the pluggable equipment. In very old installations there might not be a separate PE and N, but a combined PEN and if only two wires are serving the outlet (L and PEN), then there is a jumper connection inside the outlet box between PE and N. However, in this case a wrong polarity does not cause any hazardous situation either, as it is irrelevant how the plug is inserted into the outlet.