Presumed RF Interference

Mon Mar 6 04:30:13 UTC 2006

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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