Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!ukma!xanth!mcnc!unccvax!dya
From: dya@unccvax.UUCP (York David Anthony @ WKTD, Wilmington, NC)
Newsgroups: sci.electronics
Subject: Re: Polarized Plugs
Message-ID: <1519@unccvax.UUCP>
Date: 8 Jun 89 19:28:56 GMT
References: <5558@clint.megatest.UUCP>
Distribution: usa
Organization: Univ. of NC at Charlotte, Charlotte, NC
Lines: 30

In article <5558@clint.megatest.UUCP>, jao@megatest.UUCP (John Oswalt) writes:
> What is the point of 2-pronged polarized electrical plugs?  (The kind
> where one prong is wider than the other.)  It seems to me that the only

	Back in the old days of radio and TV (and other appliances, no
doubt) there were scads of equipments which connected the chassis to one
side of the AC line.  The AC outlet is standardised so that the "wide"
blade is connected to the neutral (!= safety ground, although it is 
often bonded to earth ground at the service disconnect/breaker panel)
and the "narrow" blade is connected to 117 vac ("hot").

	The intent was to tie the chassis directly to the neutral (which
still could carry a lethal potential, though probably not nearly as
frequently as the hot) as an enhancement to safety.  Note that exposed
metal parts on such receivers still had to pass a leakage/hipot test
to earth (at least, later on).

	Line operated consumer electronics now have power transformers
or use full wave bridge rectification of the input; in the latter case,
the chassis floats 77 vac above ground.  Nevertheless, I see polarised
plugs on some line sets (why?). Exposed metal part isolation is much more
strict.

	Anyone making an extension cord, who wants product liability 
insurance coverage, makes one which maintains the polarization feature.
Any individual making such a device for trade had better use class I 
wiring, or preserve the polarisation feature.

York David Anthony
DataSPan, Inc.