[Beowulf] Re: HVAC and room cooling... wires

Robert G. Brown rgb at phy.duke.edu
Fri Feb 6 09:30:10 EST 2004

On Fri, 6 Feb 2004, Nathan Littlepage wrote:

> > That's not crazy, that's actually rather sane.  What would be crazy
> > would be grounding the neutrals and/or ground wire in 
> > different places.
> > Can you say "ground loop"?
> > 
> Grounding loops.. truly a bane. I remember one instance where someone
> wired a telecommunications switch to two different grounds. The -48v DC
> power had it's own ground, and someone had grounded the chassis to a
> different feed. I little lesser know fact was the lightning rod on the
> tower next to the building was linked to the same ground as the power.
> When lightning did strike, nothing but smoke as the charge rolled from
> one ground to the other on each bay.

There is also a memorable instance of powered racks with incoming two
phase power split into two circuits having a polarity reversal so its
neutral wire on one circuit was 120V above chassic ground and the
neutral on the other circuit.  When somebody plugged a single unit with
components on both lines -- I think it was more like "meltdown and
fire".  Not really a ground loop, of course...

...but plenty of people have been electrocuted or fires started
because there was a lot more resistance on the neutral line to a remote
"ground" than there was to a nice, local, piece of metal.  Basically,
AFAICT there is really nothing in the NEC or CEC that is "stupid".  In
fact, I think that most of the code has undergone a near-Darwinian
selection process, as in electricians who fail to wire to code (and
often their clients) not infrequently fail to reproduce.

I don't think code is conservative ENOUGH, if anything, and like to
overwire for any given situation.  12-2 is just as easy and cheap to
work with as 14-2, for example.  10-2 unfortunately is not, but it
gives me comfort to use it whereever I can.  And I kinda wish that all
circuit breakers were GFCI by code as well, not just ones servicing
lines near water and pipes.  However, these are still available as user
choices -- code permits you to go over, just not under.

Anybody curious about wiring should definitely google for the electrical
wiring FAQ site.  It explains wiring in relatively simple terms.


Robert G. Brown	                       http://www.phy.duke.edu/~rgb/
Duke University Dept. of Physics, Box 90305
Durham, N.C. 27708-0305
Phone: 1-919-660-2567  Fax: 919-660-2525     email:rgb at phy.duke.edu

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