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In North America sockets and plugs design makes sure you'll be able to plug only in a certain way: enter image description here(source)

Why is this important for AC? Moreover, many plugs, such as cellphone chargers, are in fact symmetric, i.e. not "sensitive" to the way they are plugged.

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    \$\begingroup\$ In fact in a lot of countries the receptables are as symmetric as the plugs \$\endgroup\$ – PlasmaHH Jul 21 '15 at 19:23
  • \$\begingroup\$ This subject has also been discussed here: electronics.stackexchange.com/questions/124215/… \$\endgroup\$ – Charles Cowie Jul 21 '15 at 21:49
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    \$\begingroup\$ That type of socket (NEMA1) has been prohibited since 1974 (for new constructions). The plug (polarized or not) is still kosher though. \$\endgroup\$ – Fizz Oct 1 '15 at 22:26
  • \$\begingroup\$ More answers here. \$\endgroup\$ – feetwet Nov 20 '18 at 23:00
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The wide slot is supposed to be the neutral, the narrow slot the hot.

Neutral is nominally supposed to be near ground potential. However, there's no guarantee the receptacle was wired up correctly.

If it is wired correctly, and if a correctly wired polarized plug is used, then the threads on something like an Edison-base light bulb will be near ground potential and there is less chance of an electrical shock than if the screw is at 120VAC with respect to ground. So, it's 'safer'.

enter image description here

It's also backward-compatible with older non-polarized plugs that have two narrow blades, however newer plugs that are polarized are not compatible with older receptacles (barring the improper use of tin snips).

Edit: As kabZX points out, when one side of the line is switched or (most importantly) when one is fused, it should be the hot side only.

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    \$\begingroup\$ Also when the equipment has a spst power switch, it should end up opening the live/hot wire and not the neutral one. \$\endgroup\$ – kabZX Jul 21 '15 at 19:12
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    \$\begingroup\$ @kabZX Good point. It used to be verboten to open up the neutral however they seem to have relaxed that rule (at least when an earth is present) to accommodate international approvals- many computer power switches open both sides, for example. Maybe someone knows the UL/CSA history. \$\endgroup\$ – Spehro Pefhany Jul 21 '15 at 19:16
  • \$\begingroup\$ Or, as is my experience with some "common folk" in stead of tin snips/pliers/dremel just idiotically huge brute force. :-) Further I can only tell you that in zhe Eurrops, the assumption is always made that everything will always be wired badly (except for ground?) and thus no polarity tricks and always Dual Pole switches, unless there's reinforced insulation. \$\endgroup\$ – Asmyldof Jul 21 '15 at 19:18
  • \$\begingroup\$ I still don't understand: if the "neutral" is close to ground, then the user still has 50% for an electric shock if they "happen" to touch the hot... So why bother in the first place? \$\endgroup\$ – Sparkler Jul 21 '15 at 22:30
  • \$\begingroup\$ @Sparkler Yeah, I guess there's a historical factor at work. They were not originally polarized but were 'always' hot and neutral. In the UK in construction sites I understand they use 110V centre-tapped so either line voltage wrt ground is 55VAC. Much safer. \$\endgroup\$ – Spehro Pefhany Jul 21 '15 at 23:29
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The "hot" supply wire is more dangerous to a person than the "neutral" return wire: Neutral is closer to ground potential.

For safety: Equipment that has potential for end user to come into contact with one of the wires needs to ensure it is the neutral return wire, not the hot supply wire.

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  • \$\begingroup\$ "Equipment that has potential for end user to come into contact with one of the wires ..." Can you give an example of such a device and country where this is acceptable? I can't. \$\endgroup\$ – Transistor Nov 21 '18 at 7:05
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Think that you have two wired communication devices whose ground is not isolated from the mains ground (i.e. supposed to be connected to the wall ground). And the communication line is requiring the ground wire to run between the two parties. Now assume you have reversed the polarity on one of the devices. Boom! Ground is shorted to the phase line. Well, anyway it is not a good practice to make such a devices, but it may happen.

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  • \$\begingroup\$ Safety regulations have long forbidden mains-powered devices from having exposed metal connected to either power lead. The only common exception is Edison-base light sockets, which are grandfathered for compatibility. Unscrewing a light bulb will often expose the metal base before disconnecting it from (what should be) the neutral. \$\endgroup\$ – supercat Jul 21 '15 at 19:44

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