[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.

   rgb

-- 
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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