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From: jeh@dcs.simpact.com
Newsgroups: sci.electronics
Subject: Re: polarity
Message-ID: <1991Apr24.111307.2296@dcs.simpact.com>
Date: 24 Apr 91 18:13:07 GMT
References: <11864@jarthur.Claremont.EDU> <1991Apr23.152315.22825@news.larc.nasa.gov>
Distribution: usa
Organization: Simpact Associates, San Diego CA
Lines: 55

In article <1991Apr23.152315.22825@news.larc.nasa.gov>,
 kludge@grissom.larc.nasa.gov ( Scott Dorsey) writes:
> 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.

fine so far.  

>    You can think of it as having a DC component of 84V and an AC component 
> of 84V.

you lost me here.  The hot side goes from +84 V to -84 V and back, referenced
to neutral, for a peak-to-peak swing of 168.  There is no "DC component"
anywhere.  Couldn't very well be, since your power is coming from a transformer
winding.  (Check it with a scope, set to DC coupling, making sure that you
don't overload the front end; the 10-meg resistor in the normal 10:1 probe will
suffice.) 

(Meanwhile, the other leg of your 240V residential service is going from 
-84 V to +84 V, so you can get 240V RMS for your electric dryer or air 
conditioner.  In such an outlet you typically have two hot leads and a ground,
not neutral.  Some of the 120V outlets and builtins in your house are connected
to one of the legs, and some to the other.  The neutral wire coming into your 
house is really just a center tap on a 240V secondary winding, which in turn
is usually excited by one side of a three-phase delta transformer on the power 
pole.) 

>    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).

total agreement here.  Also, light sockets (and other things with exposed 
contacts) are supposed to be wired so that the narrow blade of the plug (the hot
side) goes to the switch and then to the harder-to-reach of the contacts.  
Suppose you're removing a dead light bulb and forgot to turn the switch off 
first.  It's possible to touch the metal threads of the base of the bulb while 
they're still in contact with the socket, but since that's supposed to be the 
neutral side of the socket, it won't matter. 

	--- Jamie Hanrahan (x1116), Simpact Associates, San Diego CA
Chair, VMS Internals Working Group, U.S. DECUS VAX Systems SIG 
Internet:  jeh@dcs.simpact.com, or if that fails, jeh@crash.cts.com
Uucp:  ...{crash,scubed,decwrl}!simpact!jeh