[sci.electronics] Polarized Plugs

wjm@mruxb.UUCP (mitchell) (11/10/88)

The reason 2-prong 120 volt plugs are polarized is for safety.  One side
of the common North American AC power system is connected to ground at
the service entrance panel.  This is in additon to the 3rd wire in
3-prong plugs, which is directly connected to ground.  Since some appliances,
like some TV sets, have the chassis directly connected one side of the line,
it is important that this side of the line be grounded!  Otherwise, someone
could get killed when touching both the chassis and something that is grounded
(like a water faucet or a properly grounded appliance).
This polarization is required by the National Electrical Code in the United
States as well as local electrical codes.
Regards,
Bill Mitchell

jao@megatest.UUCP (John Oswalt) (06/07/89)

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
difference in "signal" between the two prongs is a 1/120 second phase
shift.  It is a rare gadget indeed to which this matters.
-- 

John Oswalt (..!sun!megatest!jao)

dya@unccvax.UUCP (York David Anthony @ WKTD, Wilmington, NC) (06/09/89)

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.

howard@tp2.Waterloo.NCR.COM (Howard F. Steel) (06/09/89)

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
>difference in "signal" between the two prongs is a 1/120 second phase
>shift.

1 pin of the plug is connected to (in North America) the live side of the line
(120/117V); the other pin is connected to the earthing (grounding) conductor
of the mains transformer. This makes the one pin effectively 0 volts; the small
voltage measured on that pin (to ground) is a result of the line resistance
back to the actual ground.

>It is a rare gadget indeed to which this matters.

True, in the event that everthing is running normally, but if an insulation
fault were to occur and the plug was reversed, the chassis or other dead metal
parts of the unit could become live, presenting the owner/operator with a
potentially life threatening situation should they come into contact with a
real ground.

Most things which employ the 2 pin method rather than three prong are either
double insulated to reduce the liklihood of such a fault or the metal parts
are kept isolated from the AC line.

Net result is not too much apparent difference, but a lot of (excuse the pun)
potential difference.

-- 
Howard.Steel@Waterloo.NCR.COM  :-(  	I Think, Therefore I AM,
aka: Howard "The Duck" Steel   :-) 		I think

erk@americ.UUCP (Erick Parsons) (06/10/89)

><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
><difference in "signal" between the two prongs is a 1/120 second phase
><shift.  It is a rare gadget indeed to which this matters.
><John Oswalt (..!sun!megatest!jao)

In all likelihood (sp?) it is to keep the hot wire as completely isolated
from the frame of say.. a toaster, or any appliance that could be used
around a sink or tub or ? Remember that the neutral wire is the big prong
ALWAYS, and is common to the service ground at the service entrance panel.
If the wires were switched the frame of this toaster could be for some
reason further from ground potential and become a shock hazard should some
poor soul grab the toaster and the faucet at the same time.

--
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