Jeff, 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.
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. It the cath lab, there are usually grounding monitors that will cause an alarm if any stray current is detected. The notion that someone is jury-rigging an electrical connection that will make common connections between a white and a red system is frightening. Although such a connection will likely be passive most of the time, if a fault occurs in the equipment, that fault may perturb critical medical equipment- with tragic results. In the hospital where I have a repeater on the red circuit, that circuit is separately-derived by definition, since it has a separate transformer upstream of the automatic transfer switch. Even though I am a certified electrical inspector, I asked for an independent inspection and a signed certificate of compliance before I plugged anything in. 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. 73, Eric Lemmon WB6FLY -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of Jeff DePolo Sent: Monday, July 14, 2008 8:20 AM To: [email protected] Subject: RE: [Repeater-Builder] Switching a Repeater Betwen AC Sources > It is important to switch all three power leads, not just the > hot lead, > because neutrals from separate sources cannot be made common- > especially > when a separately-derived power source is involved- and the equipment > grounds must guide any fault current back to the source via > the most direct > route, which is alongside the respective hot conductor. Eric, you probably have more experience with industrial electrical than I do, but I can't recall anywhere in NEC where switching EGC is permitted, even with separately-derived systems. My copy of NEC is back at the shop, maybe you have yours handy and can provide a reference for my own edjumication. I would tend to think that in a large industrial complex like a hospital or high-rise building that the distributed, and unavoidable, ground paths, (i.e. the entire building including conduits, building steel, concrete, water pipes, etc.) is almost guaranteed to pose a much lower impedance fault current path than can a single equipment grounding conductor run alongside the hot. As a sidebar, many, if not most, emergency generator installations at communication sites are not seperately-derived systems because the neutrals *are* tied together at the transfer switch. However, in the instant case, where it is unknown whether or not the "red" and "white" are on separate systems, I agree that switching the neutral and hot is the only safe way to go. --- Jeff WN3A
