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From: eshop@saturn.ucsc.edu (Jim Warner)
Newsgroups: comp.dcom.lans,comp.dcom.modems
Subject: Re: Fluorescent lights and data cables
Message-ID: <1769@saturn.ucsc.edu>
Date: 29 Jan 88 02:37:45 GMT
References: <5253@columbia.edu> <865@cfa.cfa.harvard.EDU>
Reply-To: eshop@saturn.ucsc.edu (Jim Warner)
Organization: University of California, Santa Cruz; CIS/CE
Lines: 44

In article <865@cfa.cfa.harvard.EDU> wyatt@cfa.harvard.EDU (Bill Wyatt) writes:
>
>I can't actually comment on the above question, but I have a warning
>that may not have occurred to you: Make SURE you have separate
>circuits for your computers and other quiet power units (including
>separate GROUND at the outside water main or whatever) that isolate
>from the lights and other noisy equipment. 

I disagree that computers are electrically quiet.  But more important,
DO NOT ASK FOR SEPARATE GROUNDS.  The purpose of grounding electrical
equipment is primarily for safety.  Separate "isolated" ground systems
are a violation of the NEC.  If you are responsible for spec'ing the
power for a new installation, be sure to get a copy of "Guideline
On Electrical Power for ADP Installations", FIPS Publication 94,
U.S. Dept of Commerce, National Bureau of Standards.

There are no clean grounds and dirty grounds.  Ground is ground. 
But rather than go on and on...just get the book.
>
>In our building, built in 1972-3, the fluourescent lights leak a
>voltage into the neutral wire, so that there's 1.5 volts between the
>neutral and ground. This is usually OK, but can apparently affect the
>life of our computers. Since DEC specifies a limit of 0.7 volts, we've
>sometimes had hassles with field service over the issue.
>Unfortunately, rewiring is completely out of the question. 
>
I'm not sure I believe this.  The resistance of 12 gauge copper wire
is 1.6 ohms/1000 ft.  At its rated load (20 Amps), DEC's 0.7 volt
rule would require your circuits to be no more than 21 ft from the
panel.  

One thing you should ask for is dedicated outlets for the custodians
to use for their floor waxers and vacuum cleaners.  This is especially
important as workstations start to show up in offices.  We've seen
the following: a custodian plugs his waxer into a circuit being used
by a computer.  The waxer, along with the computer, exceeds the 
rating of the breaker and it opens.  At the instant the breaker
opens there is lots of energy stored in the magnetic field in
the motor.  The surge from the motor can't go back to the public
utility because the breaker is open.  Guess where it goes....

jim warner
Sr Development Engineer
University of Ca, Santa Cruz