Data Room Grounding

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jburgess

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We are currently working with grounding requirements for Data Rooms. These rooms have a copper ground bus installed in each room and we usually have five or six rooms. Technology folks
install a #6 ground that bonds these together along with bonding data cable trays etc. with this ground. We are supposed to ground these bars to the building ground per NEC requirements. We usually run a 4/0 ground from each to the building service ground point. I have ran across a few University technical specifications that allow the bus bars to be connected to building steel. Our take on this is this ground system is really there to eliminate noise on the data circuits in lieu of people protection that this might be an acceptable method. The NEC as we read it isn't very clear in it's requirements for these type grounds. QUESTION-is building steel acceptable for these grounds and would it be acceptable to ground only one of these ground bars since they are interconnected with #6?
Another point that we need some advise on is building steel itself. Large buildings have expansion isolators at various places. How do these isolators affect the continuity of the steel system? Seems like there might be several
building sections that might be at different potential or does the foundation rebar connect it all together.

Thanks for any info you may have.
JB
 
Re: Data Room Grounding

Short Answer:
Building steel is bonded and required to be used.

Long Answer:
I design a lot of protective and signal grounding systems for data centers and telephone offices, and my first piece of advice to obtain a copy of the IEEE Emerald Book and Tellcordia TRW-000-295. Unless you thoroughly understand the concepts and the objective you are more than likely to cause more problems than you will solve. The grounding is broken down in various sub-system and are kept separate from each other and cannot be allowed to mingled with each other. The various sub-systems are ground electrodes, lightning/discharge, AC Equipment Grounds (ACEG or EGC?s), DC equipment ground, Integrated grounds for miscellaneous equipment, and signal ground.

You mentioned a ground bar in each room to bond the cable trays. That is part of the integrated ground system, it?s a dedicated system. It has nothing to do with reducing noise. In fact it creates noise. Typically in a equipment room you would install a Raised Floor Bus (RFB) under the floor. Then the RFB would have a dedicated 2/0-to-750 run back to the ground electrode system on a bus bar called an Office Principle Ground Point (OPGP). The RFB would also be bonded to the nearest building steel with a 3/0. Then a Signal Reference Grid (SRG) is constructed of # 6-to-2/0 on 4-feet centers directly under the equipment line-ups. The purpose of the RFB is the termination point to connect the X0?s of all the PDU?s and/or isolation transformer to form a new single point ground that is free of common mode noise. The SRG is then used to bond all the equipment frames together to form a high frequency, low impedance path between equipment frames and to the source (PDU).

I also install a ground bar at the 8?-2? above finished floor in each room called a Area Floor Buss (AFB). It is terminated to the OPGP and building steel in the same manner as the RFB. This is a miscellaneous ground bar to bond cable racks, auxiliary framing, pipes, conduit, doorframes, etc. This is not for noise purposes, it is to provide a planned low frequency, low impedance, back to any source that might accidentally come in contact with. It is useless for noise reduction.

Another bar that is installed is where DC plants are installed is called a Master Ground Bar (MGB). The MGB is used to establish an Isolated single point ground to reference the return buss of the DC plant, DC signal ground, DC equipment ground, and cable racks transporting DC power. Again it is terminated to the OPGP and building steel.
 
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