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From: kludge@grissom.larc.nasa.gov ( Scott Dorsey)
Newsgroups: sci.electronics
Subject: Re: polarity
Keywords: polarity, stupid, babooze
Message-ID: <1991Apr23.152315.22825@news.larc.nasa.gov>
Date: 23 Apr 91 15:23:15 GMT
References: <11864@jarthur.Claremont.EDU>
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Reply-To: kludge@grissom.larc.nasa.gov ( Scott Dorsey)
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In article <11864@jarthur.Claremont.EDU> cliff@jarthur.Claremont.EDU (Clifford Stein) writes:
>What's the purpose of polarized wall outlets with an AC
>signal?  I don't understand.   Is it really bad to force
>something to plug in the wrong way?

   You are thinking that the reference ground in your house is at zero volts
"the Ground they tell you about in Physics class" and that the AC power 
alternates at + and - 84 volts around that ground (so that your peak voltage
would be 168 volts from one side of the plug to the other, or 120VRMS).  Just
like they show you in class.
   If this were the case, then you'd be able to get 60 volts (rms) between
each side of the plug and ground.  But this isn't the case.  Try it with a 
meter.  You get 120V on one side, and zero on the other (hopefully) if you
measure from the two prongs to the building ground.
   You can think of it as having a DC component of 84V and an AC component 
of 84V.
   This means that if you stick a fork into one prong, you won't feel anything
and if you stick a fork into the other you are likely to be killed.  The
polarized plug is usually a safety thing to make sure that the switch turns
off the hot side of the line instead of the cold side.  In Europe, they often
turn both off (which is the real solution).
   Yeah, it's a stupid question.  But nobody ever told me any of this stuff
when I was an EE student either, so don't feel bad.
--scott