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From: levy@ttrdc.UUCP (Daniel R. Levy)
Newsgroups: sci.electronics
Subject: Re: Neutral-Ground Shorts
Message-ID: <1781@ttrdc.UUCP>
Date: Tue, 23-Jun-87 18:51:08 EDT
Article-I.D.: ttrdc.1781
Posted: Tue Jun 23 18:51:08 1987
Date-Received: Sat, 27-Jun-87 04:18:30 EDT
References: <820@sdcc12.ucsd.EDU> <583@inuxh.UUCP> <467@rlvd.UUCP> <258@uvicctr.UUCP>
Organization: AT&T, Skokie, IL
Lines: 34

In article <258@uvicctr.UUCP>, collinge@uvicctr.UUCP (Doug Collinge) writes:
> What I have wanted to know for years is,  why is residential service
> referred to ground at all?  I understand why they balance and ground 
> high-voltage transmission lines.  It seems to me that, if the transformer
> was used to isolate the residential service from ground everyone would
> be a lot safer.  How about it?

I think the idea is that, with the common household 240-volt service
(two 120-volt sides on the power transformer secondary with a tie point
in between that is grounded) the center point being deliberately grounded
prevents the worst case voltage to ground in the system from being any
higher than 120 volts.  Otherwise, if one side of the 240 became acci-
dentally grounded, the other side would be (unexpectedly) 240 volts with
respect to ground and would pose a (much) worse shock hazard than 120 
volts.  As it is, if either side of the 240 becomes accidentally grounded,
a fuse blows or a circuit breaker trips instead, alerting one to the problem
and causing no unexpected shock hazard.

(I saw this in a rather old elementary electricity handbook, so this may
or may not be on the level.  But it sort of makes sense, in that an insid-
ious shock hazard due to a short to ground could occur and never be detec-
ted until some unfortunate person found himself between the other side
of the 240 and ground.)

I suppose another problem would be that of controlling the maximum DC voltage
on the secondary side with respect to ground, to prevent insulation
breakdown in the transformer or for that matter in any equipment serviced
by the transformer which has a grounded case.  Even static electricity
could become a problem.
-- 
|------------dan levy------------|  Path: ..!{akgua,homxb,ihnp4,ltuxa,mvuxa,
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