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From: larry@kitty.UUCP (Larry Lippman)
Newsgroups: sci.electronics
Subject: Re: Neutral-Ground Shorts
Message-ID: <1816@kitty.UUCP>
Date: Thu, 25-Jun-87 23:51:55 EDT
Article-I.D.: kitty.1816
Posted: Thu Jun 25 23:51:55 1987
Date-Received: Sat, 27-Jun-87 04:39:35 EDT
References: <820@sdcc12.ucsd.EDU> <583@inuxh.UUCP> <467@rlvd.UUCP> <1781@ttrdc.UUCP>
Organization: Recognition Research Corp., Clarence, NY
Lines: 35
Summary: Limiting primary -> secondary fault voltages

In article <1781@ttrdc.UUCP>, levy@ttrdc.UUCP (Daniel R. Levy) 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 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 hit the nail on the head: the most fundamental reason for
grounding the secondary of an AC power distribution transformer is to ensure
that any primary (i.e., high-voltage side) to secondary transformer winding
fault will be of sufficient current magnitude to trip the primary side
overcurrent device (i.e., high-voltage fuse).
	Transformer faults involving primary to secondary conduction are not
uncommon, especially in transformers which are subject to overload and
thereby overheat, consequently stressing the dielectric oil and the winding
insulation (often paper).
	Needless to say, if the secondary were ungrounded, a primary to
secondary fault could result in not only lethal voltages present on all
interior building wiring, but would most likely cause a fire through
insulation breakdown and subsequent arcing.
	The trend today is for electrical utilities to switch to higher
distribution voltages feeding customer distribution transformers.  The
traditional 4160/2400 volt wye distribution is being replaced by 13200/7600
volt service.

<>  Larry Lippman @ Recognition Research Corp., Clarence, New York
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