From: dave@imax.imax.com (Dave Martindale)
Subject: Re: A question about 120VAC outlet wiring.
Organization: Imax Corporation, Mississauga Canada
Lines: 24

jeh@cmkrnl.com writes:
>
>Hmmm.  How are those orange "isolated ground" outlets (often used in
>computer rooms) wired?  

They require two separate grounds.  One ground goes to the ground pin
of the outlet, and the other ground wire is connected to the outlet's
mounting tabs (and thus grounds the box and faceplate screw and metal
faceplate, if any).

The box/faceplate ground goes to the normal distribution panel ground.
The outlet ground-pin wire is generally connected to an insulated
busbar in the distribution panel which is, in turn, connected to
the building-entrance main ground by its own wire.

So the two separate ground systems are connected together at the
building's service entrance.  But one is also connected to every
grounded piece of electrical equipment in the whole building,
while the isolated ground is shared only by the equipment plugged
into isolated-ground outlets of that panel.  If someone happens
to accidentally short one phase of a 600V feeder to ground,
the main building ground is likely to have a lot of noise on it
until the breaker trips or the fuse blows, while the isolated
ground will remain relatively noise-free.