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I could not get a good hold of it, I searched online quite a bit. This must have to do with safety but then non-polarized switches are used through out the world with no issues are risks, so my question why use polarized socket where one blade (neutral) is longer than the other (hot).

The best explanation I came across is, it has to do to with the lamps used in United States where you have to reach out to turn the lamp one and off and may accidentally touch the bulb part which in case of non-polarized socket could either be hot or neutral and can give a shock, but is that the only reason? Somebody could do the wrong wiring and that means you will still get a shock with polarized socket?

I could find other reasons like capacitor between ground and hot wires or making the chases connected to ground etc but I don't think they are the main reasons because again non-polarized switches are used through the world for appliances. So my question is why use polarized switch in the US? If electric lamp is the main reason, why not re-design the lamp and make it safer?

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    \$\begingroup\$ If you could rely on this being the case that would be a real security feature, however I know electricians who say that there is at least one wrongly wired outlet in every house, you just have to find it. \$\endgroup\$ – PlasmaHH Aug 27 '18 at 8:41
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    \$\begingroup\$ That's what my concern is, one wrong connection and ... \$\endgroup\$ – engineer Aug 27 '18 at 8:42
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    \$\begingroup\$ I always assumed that non-polarized sockets is a safety feature just because you must design all equipment to work even if someone made a wiring mistake. As an equipment designer you have absolutely no idea which one will be hot or neutral, and you can't tell the customer to plug it in one way only since you can't tell the difference on the connector. \$\endgroup\$ – pipe Aug 27 '18 at 9:11
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    \$\begingroup\$ It's the same with conductor insulation coloring. It seems like a good idea to make certain colors binding to certain functions but any seasoned electrician knows there's at least one lightswitch in any house which has live on green. You can't rely on it. It makes things less safe because people lose their caution. \$\endgroup\$ – Janka Aug 27 '18 at 13:30
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    \$\begingroup\$ I'm not entirely sure there is an authoritative answer to this question - if it ends up being mostly conjecture it may end up being closed as being primarily opinion-based. \$\endgroup\$ – Adam Lawrence Aug 27 '18 at 15:58
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It's more than that -- polarized plug make cheap single-pole switches much safer.

For example, look at the extension strip: cheap extension strip

  • With a polarized plug, it is perfectly safe to have a single-pole switch there-- one just makes sure it interrupts the "live" contact
  • With a non-polarized plug, one either has to put a more expensive double-pole switch, or to accept that half of the time, powered-off power strip would present a shock hazard. Neither of these options is very good.

It gets even worse with the fuses -- they are single-pole by definition. So if you have something protected by a fuse, and the fuse blows, it better be the fuse in the live wire! And in the non-polarized-plug land, a blown fuse might leave device non-working, but still energized -- which is pretty dangerous.

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  • \$\begingroup\$ Regarding shock hazards in non-polarized switch (with single pole)...it is never a good idea to touch expose wires weather live or neutral...and switches are always well insulated...so again the shock hazard is just more bookish and less realistic...almost every where single throw switches are used weather polarized or no polarized. \$\endgroup\$ – engineer Aug 27 '18 at 21:07
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    \$\begingroup\$ Unfortunately, the things are not perfect -- insulation fails, wires connect to metal enclosure, and this is when safety features become important. Given that non-polarized plugs do not have many benefits, why not get more safety almost for free? \$\endgroup\$ – theamk Aug 28 '18 at 2:46
  • \$\begingroup\$ At the cost of slightly higher cost of manufacturing switches and sockets (neutral and hot have to be different sizes) \$\endgroup\$ – engineer Aug 28 '18 at 5:58
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The reason is historical. WAAY back in the old days the neutral was used as a ground. My dad grew up in the 30s and it he said it was very common for washing machines and drills and light fixtures to have their metal cases connected to the neutral wire. Since there was always a few volts on the neutral due to wire resistance it was common to get a tingle if you grabbed a water pipe while touching a running appliance. I guess copper was expensive back then and an extra ground wire was a considered a luxury. So it was essential back then to polarize the plugs or people would die from full voltage appearing on the case.

You can imagine that the transition to requiring a separate ground wire couldn't happen overnight because you couldn't require people to throw out existing appliances. The fact that we still have polarized plugs means we are still in that transition.

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    \$\begingroup\$ Note that, while polarization is better than nothing, it's still not a very good safety system. Half the outlets in my (late 1950s) house are wired backwards. \$\endgroup\$ – Adam Haun Aug 28 '18 at 17:19
  • \$\begingroup\$ This is dangerous conjecture. The addition of separate ground wire by no means eliminates the need for polarization! There always would be the devices that have no means of using ground wire (e.g. hairdryers don't have a chassis to connect it to) yet polarization is extremely important for them. The power switch on hot line minimizes the danger of electric shock when device is switched off. \$\endgroup\$ – Maple Aug 28 '18 at 17:21
  • \$\begingroup\$ Theoretically speaking, there's no issue with the case being at neutral on a single or split phase system. Neutral and ground are connected at the circuit breaker panel (in the US system, called TN-C-S). The safety issue is that if the neutral line were to open, then the case would become energized at line voltage. \$\endgroup\$ – user71659 Aug 28 '18 at 17:23
  • \$\begingroup\$ Explained more about the story in another answer. \$\endgroup\$ – Yuhong Bao Nov 25 '18 at 2:31
  • \$\begingroup\$ Since older plugs are often non-polarized (and, since modern plugs rarely are), the reason cannot be historical. \$\endgroup\$ – jpaugh May 21 at 21:54
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In the US, the current two-prong power outlet was invented by Hubbell back in the 1910s. Of course, Hubbell also invented the polarized outlet in the 1910s. Knapp at Hubbell also invented an grounded outlet that was incompatible in the 1910s (used in China and Australia today). At the time, both US and most of Europe used 110V to 125V with a neutral. When Europe switched to 220V, it was often off a 127/220V three phase feed, which means that the 220V had no neutral (still common until recently for example in Belgium and Norway). Fortunately most of Europe already had grounded outlets by the 1930s. In the US, it took until 1948 before NEMA 5-15 was standardized with the round ground prong we use today added to the original Hubbell outlet.

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If you search hard enough, you will probably find that "it seemed like a good idea at the time." It provides some increase in safety to have the outer part of a bulb socket connected to ground. The same is true of certain internal parts of some products. Some parts are more at risk of contacting external metal parts than others. Sometimes design detail decisions and electrical code rules are made for reasons that are rather weak. Even though we have double insulation and ground-fault interruption devices today, there is no particular reason to do away with polarized plugs and receptacles.

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Polarized plugs ensured AM radios worked on DC power, and that blenders rotated in the correct direction with DC power.

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    \$\begingroup\$ No. Just no. A rectifier (or any other device) can't, just by connecting to hot & neutral, "know" which is which. Plus I'd bet the blenders more often use an AC motor - why rectify if you can use AC instead? But if they did (as the AM radios) need to get DC, the circuit would not function any differently if hot & neutral were reversed. The only difference would be on a fault or relative to ground or safety if the case is connected to hot vs. neutral. \$\endgroup\$ – manassehkatz-Reinstate Monica Nov 25 '18 at 2:38
  • \$\begingroup\$ correct. Though DC systems were slowly on their way out by the time Hubbell invented the plug and socket, the way to use plug-in appliances was with an edison screw adapter, as described in the patent: patents.google.com/patent/US776326A/en \$\endgroup\$ – τεκ Nov 25 '18 at 2:59
  • \$\begingroup\$ It sounds like we are talking about two very different systems here. If indeed these polarized plugs are used for DC (what voltage? when was this actually done?) then that makes sense. But that would have been a very long time ago, and I seriously doubt that any currently used wiring in the US (beyond museum pieces) has not been totally replaced since the DC age. \$\endgroup\$ – manassehkatz-Reinstate Monica Nov 25 '18 at 3:29
  • \$\begingroup\$ Looks like the same outlet was used for Delco 32V DC systems. \$\endgroup\$ – Yuhong Bao Apr 25 at 4:29
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The polarization by itself has no meaning, of course. What you should be looking for is Grounding. When you understand what grounding is for, consider that polarization is a simple way to ensure correct ground connections.

To whoever down-voted this answer, here is what NEMA WD 1-1.08 has to say:

Polarization assures the correct positioning for proper mating of plugs and receptacles of the same rating.

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