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I've lived in a variety of different places where polarities have been different on a per-outlet basis, ie the neutral is where the hot should be. I've never noticed any ill effects from this.

Are there any side-effects of having the polarities reversed? My understanding is that almost any AC device out there will just simply flow the other way if the polarity is backwards.

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It entirely depends on the country and its code. There are countries where it is perfectly safe and the side effects are minimal and can be countered by just turning the plug around – PlasmaHH Feb 15 at 11:02
It is not safe as some equipment (incorrectly) assumes the neutral and earths are the same, and therefore connects neutral to the metal case. – Walker Feb 15 at 11:36
By swapping polarities, do you mean exchanging the LINE and NEUTRAL connections? Or, do you mean flipping the polarity of the LINE, such that when it should be positive, it's negative, and when its negative, it's positive? – Pigrew Feb 15 at 14:08
@Walker: Such things really exist? Glad I never seen some around here... – PlasmaHH Feb 15 at 14:49
@Pigrew Polarity is kind of a misnomer with AC power in the negative/positive sense. DC current flows one-way, from anode to cathode. But AC current constantly changes direction. Forget positive and negative with AC. "Polarity" is really just a reference to the line conductor being connected to one of the poles of the inner coil in the transformer outside. In the U.S., there are two line conductors, one from each pole of the smaller transformer coil, and a grounded "neutral" coming out of the center. AC appliances don't care about polarity, it's a human safety issue. – Craig Feb 15 at 17:19
up vote 16 down vote accepted

There's no danger to the attached load. AC current reverses direction many times per second (100 times if you have 50Hz AC, 120 times if you have 60Hz AC).

It literally makes no difference at all for the equipment.

Human safety is another matter. The hot conductor is generally dangerous if you touch it, while the grounded ("neutral") is generally (not always) safe. However, if equipment has a single-pole switch, it is only breaking the conductor it assumes is the hot conductor. If you have the hot and neutral reversed, then the appliance is still energized even if the switch is off.

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For instance, I had a lamp with this problem. It means that the screw portion of the bulb socket is the hot portion, and if your hand touches the base of the bulb as it is screwed in, you receive a shock. The shock indirectly resulted in the immediate destruction of the lamp. – Eric Hauenstein Feb 15 at 15:57
@EricHauenstein haha--I'll bet it did. ;-) – Craig Jun 27 at 0:25

You're correct that in an AC system, the electricity flows both directions and therefore will allow items to function.

However, these items can't be regarded as safe.

When two wires come into a device, and it has a switch, the switch is designed to interrupt the hot wire. If the polarity were reversed and the switch on the device were turned off - it would be interrupting the grounded (neutral) wire. This means if you turned off a light and stuck your finger in the bulb receptacle (Safety note: I don't recommend it.) You could be providing a ground-fault current path and complete the circuit!

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The question isn't discussing whether I replaced the ground with the hot or the ground with the neutral, but the neutral with the hot. If the hot and neutral were swapped in a switch, the switch would continue to work. – Naftuli Tzvi Kay Feb 15 at 5:01
@NaftuliTzviKay, not if you consider that the switch is there to make it safe to work on the light! – Walker Feb 15 at 11:33

Just to add something. Technically reversing the polarity of an outlet can kill a device if the device was improperly implemented. IE if the device uses a half wave rectifier instead of a full wave bridge rectifier you will be providing an AC source through the entire ground plane of the DC components thus resulting in a visit from the blue smoke monster.

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I'm pretty sure this isn't true, unless the device was seriously poorly designed. – Daniel Griscom Feb 15 at 12:39
@DanielGriscom as I said improperly implemented. I don't think you can even get UL certified without a bridge rectifier (that use that type of power supply). I was just stating the problem with reversing polarity from an engineering point of view. – Austi01101110 Feb 15 at 19:49

The relative voltage between hot and neutral is constantly reversing but that doesn't mean that the two conductors are the same. Relative to earth the hot is varying (both positive and negative) while the neutral stays at roughly zero voltage relative to earth.

In particular if a single pole switching or protection device ends up in the neutral due to reverse polarity it can leave the appliance in an "off but live" state which is undesirable. A single pole protective device that ends up in the neutral also can't provide any protection against faults to earth.

It is possible to design applicances that meet safety requirements even if live and neutral are reversed and most modern appliances are likely to be built this way since in many parts of the world unpolarised plugs are the norm.

However if you live in a country where correct polarity is expected (such as the US and the UK) you should respect that expectation to ensure that all equipment is safe, even older equipment or equipment that was built in your country and never intended to be exported.

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