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Saw the good answer to this question: Is there a difference between the two conductors in 120VAC wiring?

But I'm wondering as a followup, since the two primary prongs in an AC mains plug are different, why (in many plug configurations) are you allowed to insert the plug in either direction? Is there ever a case where the appliance cares which line is "hot" and which is "neutral"? Is there ever a case where it's important enough not to screw up?

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3 Answers

up vote 8 down vote accepted

In switched appliances, the appliance's on/off switch needs to be wired to the hot side of the connection. Otherwise, you'd have current running through the device when it's powered off, just looking for an opportunity to cause a short. For non-switched devices, I don't believe it makes a difference.

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Thanks. How do we explain the many appliances with "switches", especially older ones (just as an example, a 1980s stereo receiver/amp that I use) that use 2-prong unpolarized plugs? In Europe, the simpler two-round-pole plugs are essentially unpolarized as well. Are these just luck of the draw? –  quixoto Jun 6 '11 at 2:50
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They were made in a time when the outlets themselves didn't have the different polarities, just two small slots. It's possible that the switch turned off both wires. But I suspect that safety was just less of a concern than it is today. –  BMitch Jun 6 '11 at 2:55
    
@quixoto - two prong (ungrounded) appliances must be double insulated and be designed that you can't open them up without unplugging them (although that's a bit dubious sometimes). A lot of equipment today doesn't have a switch on the input line at all. –  mgb Jun 6 '11 at 15:44
    
If it's powered off there should be no current running through the device, just the voltage. And it's not necessary, just maybe nice to have, or there wouldn't be unpolarized plugs. –  starblue Jun 10 '11 at 7:37
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It's a quirk in the history of consumer electrical devices.

The earliest consumer electrical devices were, in fact, lightbulbs. And they don't care which contact is active, although you probably do if you use an Edison Screw socket. When sockets started to appear, using two identical parallel prongs was simple and obvious, which was probably why it was chosen. A similar thought and markeing process would have led to the two round pins of the European socket.

It was only after consumer electricity became ubiquitous that safety started being considered. That led to ground pins, and then polarised plugs and then switches, all applied to the entrenched standards retroactively. It is illuminating to look at later common electrical plugs, such as the Australian one. It was designed from the beginning to include an earth, be polarised and have switches.

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could you provide a link to the Australian design? I'd be curious to see it. –  JYelton Jun 9 '11 at 22:39
    
@JYelton Australian plugs are called "Type I". en.wikipedia.org/wiki/Power_plugs#Type_I –  staticsan Jun 10 '11 at 2:49
    
I believe that polarized plugs came out before ground pins. I have seen many ancient outlets, most of them are polarized, none of them have a ground pin. ( on 15 amp outlets ) –  Brad Gilbert Jun 25 '11 at 4:47
    
@Brad Gilbert Yes, I wasn't trying to claim that grounds came first, just that they one one in a list of things added for safety's sake. –  staticsan Jun 28 '11 at 2:31
    
Then perhaps you should change "and then" in your answer. –  Brad Gilbert Jul 25 '11 at 17:02
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In some countries, at least in Europe, the outlets are polarized, so that appliances can 'know' which one is the hot wire.

One Example are the monstrous plugs in the UK. BS 1363

The system I like best is the one they have in Switzerland. Stecker-Typ J It is designed in such a way, that unpolarized plugs (e.g. "europlugs") can be inserted either way, but polarized plugs, that also carry the third prong for protection, can only be inserted the right way. An at the same time, these plugs need significantly less space compared to the bigger round plugs used in most of europe.

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The US is similar: en.wikipedia.org/wiki/Mains_electricity_by_country . The question was likely related to quixoto's last question to replacing the plug itself, which risks reversing the hot/neutral wires going into the device. –  BMitch Jun 6 '11 at 11:17
    
German plugs aren't polarized. Also, the euro plugs for small devices without protective earth aren't polarized and fit most european sockets. –  starblue Jun 10 '11 at 7:40
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