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Re: Presumed RF Interference

  • From: Robert Boyle
  • Date: Sun Mar 05 23:32:14 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.


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