> The primary purpose of the equipment grounding conductor
> (green wire) is to
> provide a low-impedance path for fault current on a branch
> circuit. The
> lowest-possible impedance is realized only when the EGC is
> routed alongside
> the phase conductor. Article 250.24(C)(1) states: "This
> conductor shall be
> routed with the phase conductor..." I didn't quote the entire
> sentence,
> because it is very long.
I understand that. What I was saying is that isn't the *only* ground path.
You're almost guaranteed to have multiple ground fault paths, therefore
EGC's from both systems are already tied together by virtue of those other
paths. Even if you didn't intentionally jumper the EGC's together, if you
had one device plugged into one outlet and another plugged into another
outlet and both devices mounted in the same rack (or, even just sitting on
top of each other), the EGC's will be electrically joined through the units'
metal cabinet exteriors.
> The reason for keeping the equipment grounding conductors
> separate is very
> simple; we are talking about a hospital, not some ordinary
> radio shack.
> The critical (red) buss likely feeds catheter labs, dialysis machines,
> mechanical respirators, and similar equipment that is
> connected to human
> bodies. Even a few milliamperes of stray current flowing through a
> grounding conductor may take a side trip through a patient's
> heart and kill
> them.
Well, now we're getting into a completely different topic. Line powered
medical equipment is wholly isolated from the patient. While I'm no expert
on the subject, there's something called the isolation barrier which wholly
isolates the patient from the electrical system.
> One of the previous posters suggested putting the computer on
> the white
> buss, with the repeater on the red buss. That's a good idea,
> but be careful
> to use a fiber-optic link to pass data between them, else the
> two devices
> will have a common ground connection through the data cable's shield.
Depending on what the interface is between the two, that may or may not be
necessary. Twisted pair Ethernet is transformer-coupled, so no additional
isolation is necessary (just don't use shielded cable). For simple things
like control logic (PTT, COR, whatever), just use an optocoupler. For
audio, transformers.
--- Jeff WN3A
