[89240] in North American Network Operators' Group
Re: Presumed RF Interference
daemon@ATHENA.MIT.EDU (Robert Boyle)
Sun Mar 5 23:31:17 2006
Date: Sun, 05 Mar 2006 23:30:13 -0500
To: "Steven M. Bellovin" <smb@cs.columbia.edu>, wb8foz@nrk.com
From: Robert Boyle <robert@tellurian.com>
Cc: nanog@merit.edu
In-Reply-To: <20060305182059.90ab46ff.smb@cs.columbia.edu>
Errors-To: owner-nanog@merit.edu
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.
-Robert
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