North American Network Operators Group Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical Re: Presumed RF Interference
On Sun, 05 Mar 2006 23:30:13 -0500 Robert Boyle <[email protected]> wrote: > > At 06:20 PM 3/5/2006, Steven M. Bellovin wrote: > >What might be useful -- ask an EE, not me -- is a circuit with an > >isolated ground. In that case, the ground wire from the power plug is > >routed all the way back to the breaker panel, and isn't connected to, > >say, the local electrical box that the cord is plugged into. I've seen > >computer equipment wired that way in the past. > > In the US, the NEC code states that the only place a neutral and a > ground should be bonded together is in the primary service entrance > facility or where the neutral is created. All subpanels will have > isolated grounds and neutrals. If you have three phase service and > use a delta (wye without the neutral) to wye transformer to create > the neutral, the neutral will be bonded to ground inside the > transformer cabinet. Eliminating the neutral is typically done to > save money when converting 277/480V to 120/208V (no neutral means a > reduced conductor count inside the conduit so smaller conduit can be > used since the extra copper for the neutral is eliminated on the > input side.) All grounds must be connected to the first metal box or > conduit they touch. If you are using plastic boxes with Romex, your > grounds will go all the back to your subpanel ground bar which will > not meet the neutral until the main breaker panel. More often in a > datacenter environment or a commercial facility, the wiring will be > BX under a raised floor or BX or EMT with THHN overhead. Either way, > the ground is connected inside the outlet box and wired directly back > to the breaker panel. The bonding in the box is to ensure there is no > voltage potential carried on any metal conduit. My NEC book is at the > office now and I'm home, but I'm pretty sure everything I have stated > from memory is accurate. > > Yes, I believe that that's correct, though I'm not going to dig out my copy of the NEC right now, either. I chose to leave out the part about separate panels. --Steven M. Bellovin, http://www.cs.columbia.edu/~smb
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