[89241] in North American Network Operators' Group
Re: Presumed RF Interference
daemon@ATHENA.MIT.EDU (Steven M. Bellovin)
Sun Mar 5 23:44:59 2006
Date: Sun, 5 Mar 2006 23:44:29 -0500
From: "Steven M. Bellovin" <smb@cs.columbia.edu>
To: Robert Boyle <robert@tellurian.com>
Cc: wb8foz@nrk.com, nanog@merit.edu
In-Reply-To: <7.0.0.16.2.20060305223506.17a0fe00@tellurian.com>
Errors-To: owner-nanog@merit.edu
On Sun, 05 Mar 2006 23:30:13 -0500
Robert Boyle <robert@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