Thanks

Richard S. Anderson, RCDD CSI CDT
http://www.servamatic.net

Also someone made the point that fax machines and copiers don't need to be boded to ground even though they are consdidered IT equipment. That really doesn't hold water. Most copiers / faxes I have used have a plastic cabinet that isolates the internal metal from human contact.  The internal metal is bonded to ground through the power cord and there is no potential for the cabinet to become energized from a faulted circuit.  This is different than a metal server cabinet in a DC that may become energized from a faulted circuit via cable tray floor grid or other metal conductor.

But let's go to something basic like a equipment lineups. Assuming there is AC powered in the equipment rack, means an AC circuit is ran to it. Per code there is an EGC ran with the branch ciruit. Many of the equipment racks have more than one AC circuit in them. Most use at least use 2 from a UPS for redundancy. Ok the next rak is installed next to the rack we just talked about. It is bolted to the adjacent frame, and has its 1 to 4 AC branch ciruits ran to it with each circuit having an EGC. Ok now we a a line up with several equipment racks, and a lot of AC branch circuits with lots of EGC to form one massive ground plane. Adding a supplemental ground would add no value at all except to the contractors bank account. This is what the PE is telling you.

So you ask what happens if a 208 or 480 volt line came into contact with the line-up.  Well first thing the voltage is not 208 or 480. Those are phase to phase voltages. Phase to ground is 120 and 277 volts. Anyway the fuse or circuit breaker would operate immediately like it is suppose too. It has dozens of EGC's providing the plannned fault path.

Moving onto cable racks, well that is classified as a raceway, and NEC Article 392 covers cable trays. Guess what? They have to be bonded or th emis used term grounded. 392.7 states cable trays must be grounded per NEC 250. NEC 250 tells you that any enclosure shall be grounded and sized to to clear any fault likely imposed on it. That leads you to NEC 260.122 for the size of the bonding conduct. The size is determined by the size of the circuit breaker size of the largest circuit likely to be encountered. If you are looking for a 1-size fits all, then a # 6 AWG should do the trick as it will clear a 200 amp breaker safely and quickly. 

Hope that helps

DereckC MSEE, PE

Moderator Mike Holt Code Forum

Thanks for your comments. I understand that technically 208 and 480 may not be the exact correct voltages.  It has been a long time since I took three-phase power classe and it is irrelevant anyway in the discussion.  I will be sure to let the PE, electricians, our DC manager and multiple manufacturers of UPS and computer room PDUs that they are incorrectly referring to the voltages as 208 and 480 Vac and see how well that is received.  Also I think you are misinterpeting my use of the word "rack".  I am not referroing to ladder rack used to support cabling.  I was referring to telecom equipment racks, you know the aluminum or steel, 7' x 19" or 23" equipment racks used sometimes in telecom rooms and data centers that may be attached to metal cable tray or ladder rack.  Do these need to be bonded ground?

All I was trying to point out is if a 208 line were to fault to ground, the voltage is 120 to ground, phase to phase voltage is 208. That is why you will see in the specs 208/120 or 480/277. That tells you the circuit is 3-phase wye connected.

Yes, it does not matter if it is cable rack or equipment racks, they have to be bonded.

DereckC MSEE, PE

Moderator Mike Holt Code Forum

Nightlake Ltd U.K.