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From: pierson@cimcad.enet.dec.com (Dave Pierson)
Newsgroups: sci.electronics
Subject: Re: polarity
Keywords: ac, power, ground, neutral
Message-ID: <1477@rust.zso.dec.com>
Date: 26 Apr 91 01:26:26 GMT
Sender: guest@rust.zso.dec.com
Distribution: usa
Organization: Digital Equipment Corporation
Lines: 57

In article <1991Apr24.111307.2296@dcs.simpact.com>, jeh@dcs.simpact.com
writes...
>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.
	He said think of it as.  He didnt say it was...

>  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.
	urmm, well, sorta...  The neutral is REQUIRED by the National Electric
	Code to be tied to ground (as defined).  This forces the neutral to
	"stay put".  this can be considered DC, if one likes.

	The hot will swing plus 168/minus 168 around the neutral.
	[exception: some "220/240" circuits are actually two phases of a
	grounded neutral three phase circuit.  The voltages will decrease
	moderately, and phasing will show up....]

	I can think of two advantages of "forcing" (via wide blade) the
connection "polarity": reduced capacitive leakage and reduced 60 Hz "noise",
where this is relavant.
=======================================================
Someone [Shailendra?] whose posting i lost referred to the breaker/fuse being in
the neutral, for safety.  I suspect that was a typo, as the explanation was
correct.  The fuse/breaker MUST be in the HOT line, else the load is
hot-to-ground even with the fuse/breaker open.  [this commentary applies to
grounded neutral circuits only....  Marine, RR and others are sometimes full
floating...]

thanks
dave pierson			|the facts, as accurately as i can manage,
Digital Equipment Corporation	|the opinions, my own.
600 Nickerson Rd
Marlboro, Mass
01752				pierson@cimnet.enet.dec.com
"He has read everything, and, to his credit, written nothing."  A J Raffles