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Path: utzoo!attcan!uunet!fernwood!portal!cup.portal.com!thad
From: thad@cup.portal.com (Thad P Floryan)
Newsgroups: unix-pc.general,comp.sys.att
Subject: UPS/SPS testing re: AC protective ground
Message-ID: <35147@cup.portal.com>
Date: 22 Oct 90 10:35:31 GMT
Organization: The Portal System (TM)
Lines: 64

A few weeks ago a discussion concerning UPS and SPS (Uninterruptible and
Standby, respectively, Power Systems) appeared in this newsgroup.

I commented that one should NOT pull the wall plug (of the UPS) to test it,
and was asked "why?".  I commented "because you lose the protective ground."

Some people disagreed with that comment.  Please consider the following:

The purpose of a protective ground is to provide a safe circuit path as a
"crowbar" for any dangerous anomalies in electrical equipment.  The ground
is connected to the metal chassis of equipment (e.g. the UNIXPC) and it, not
you, is supposed to provide the "path of least resistance" for stray volts.

Sadly, many people ignore that, operating their systems with a 3-to-2 prong AC
adapter, and also ignore pin 1 ("FG (Frame Ground) Protection circuit.") on
their RS-232 connections.

If you have RS-232 device(s) connected to the system, or even if you have a
phone line connected to the RJ-11 jacks on the UNIXPC, you have "other" stuff
in the loop which, if faulty, could present hazardous voltages OR voltage
(potential) differences, especially if the devices are connected to separate
AC circuits (e.g. different breakers).

I remember 12 years ago in a new office building that everytime during the
first several days when I turned on my terminal, the computer in the other
office building would crash; problem was traced to NO ground in the AC outlet
in ONLY my office.  Thus, rather than the normal +/- 12V on the RS-232 lines,
there was some 70V on the line with respect to the computer on the other AC
power circuit; the RS-232 line length was some 500' measured with my TDR
equipment (and operated fine at 9600 baud after the AC outlet was grounded
(simply required the building maintenance guy to connect the conduit to the
in-the-wall box)).

For reference, the no-ground condition was discovered and located by my
purchasing a "SNAPIT Circuit Tester" Model 49662 at the local True Value HW
store.  This little gem (approx $5, and about the size of a 3-to-2 prong AC
adapter) has three lamps which indicate:

	R G G	(R=Red, G=Green)
	- - -
	. * .	open ground
	. . *	open neutral
	. . .	open hot
	* . *	hot/ground reversed
	* * .	hot/neutral reversed
	. * *	correct

Consider, too, the fact that SOCKETS have their contacts recessed for safety
because they present voltages, whereas PLUGS, with their exposed prongs, are
normally "dead" and receive voltage (~ server:client relationship :-)

HOWEVER, a UPS is generating AC independent of your local utility; I don't
want to be holding a plug thereof and, say, sneeze and drop the plug into my
lap or onto other equipment; can you fantasize "zzzzZAP! *P*O*W*" ?  :-)

To be fair, a UPS is supposed to isolate itself from the power line (and the
plug), but suppose the UPS is defective?  I simply don't want to take the
chance, and most mfrs of UPS/SPS systems DO state that one should NOT test
the unit by pulling its plug from the wall socket.

My systems are properly grounded at ALL TIMES, all my RS-232 connections have
pin 1 connected straight through, and my hands are NOT charred stumps.  :-)

Thad Floryan [ thad@cup.portal.com (OR) ..!sun!portal!cup.portal.com!thad ]