Presumed RF Interference

Mon Mar 6 04:44:29 UTC 2006

On Sun, 05 Mar 2006 23:30:13 -0500
Robert Boyle <robert at tellurian.com> 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