The US common 110-125 VAC plug started out as a two blade symmetrical plug
and socket. At that time, it did not matter which was neutral ("earthy" in
UK parlance) and which was hot. The polarized two wire plug, which
accepted nonpolarized plugs in its jack but not the other way around, came
out I believe in the 1930s. It served two purposes: it kept hot chassis
equipment with its chassis at neutral rather than hot, at least
theoretically and it kept reverse polarity off DC appliances in those
situations where DC was provided for domestic use, in large apartment
buildings in major cities and in offices and even a few homes immediately
adjacent to DC rail services. There were apartments in New York City
supplied with DC only until quite modern times although it was
aggressively phased out once color TVs became de rigeur.
The third ground ("earthed", as distinct from "earthy" in UK
terminology)prong came in a little later.
In ANY polarized two pin or ANY three pin socket (I have never seen a 3
pin not polarized) it is a violation of code always and everywhere to have
any of the three hooked up wrong. Every US home (unless Amish or otherwise
without electricity) should have a standard common plug tester, available
everywhere, cheaply ($2-10) that plugs in and lets you know if the outlet
is wrong.
Of course, most homes built after WWII in the US have both "110" (which
has crept up to at least 117 and sometimes almost 130 volts most
everywhere) and "220" (double that) sockets and service. The 220 is
limited to the hardwired HVAC and to plugs for the range and dryer, and in
older homes or those with rooms off the air circulation pattern,
underwindow air conditioner outlets. It's interesting that there are at
least five different, incompatible 220 outlets in semi-common use. Some
are three (hot-hot-ground) and some are four (hot-hot-neutral-ground)
conductor, as some appliances use both internally. The three pin 220 ones
use only 220 internally, the ground not being allowed as a return.
http://www.arthistoryclub.com/art_history/Split_phase
http://www.stayonline.com/reference-nema-straight-blade.aspx
http://en.wikipedia.org/wiki/NEMA_connector
http://en.wikipedia.org/wiki/Domestic_AC_power_plugs_and_sockets
There are also 277 three pin outlets (window units in motels and such)
and various three phase connectors. Unlike in Europe, it is virtually
impossible to get three phase service into a residential swelling in most
areas in the US: utilities are convinced you are running a sweatshop or
growing dope hydroponically in the basement if you want it fitted.
Of course this is all different in most all other countries. My rule of
thumb is never buy the "foreign adaptors" sold to tourists but buy plugs
in country and make your own cables up as needed.
http://www.iec.ch/zone/plugsocket/ps_history.htm#universal
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