data center grounding

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dave_asdf

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I work for a data center and I am currently developing a grounding standard. The question I have is about connecting the grounding system to building steel. My design only connects to building steel at one point, the MGB. The MGB also connects to the utility power neutral at the multigrounded neutral. Power is fed to the equipment on the data center floor through a UPS and a few PDU's. Each PDU has a 4 wire 480/208-120 D/Y transformer. My question is, should the secondary of the transformers be grounded to building steel or to the MGB? We also have ground bars located around the center, which are usually used for rack grounds, that connect to the MGB would this be a better place to connect? Since we have a few transformers in the center i was concerned about unbalanced loads on each phase causing a grounding loop if all transformer secondaries were grounded to building steel and the input neutral. Please advise, Thanks.
 
Re: data center grounding

I think it was withdrawn July 29, 1997.

The IEEE Std 1100-1999 "Emerald Book" is good too.

[ December 12, 2005, 08:23 PM: Message edited by: ron ]
 
Re: data center grounding

Dave I have written data center grounding standards for MCI, and contributed to IEEE Emerald Book 2004 edition. I can send you a copy of the standard most Telcos use for data centers for free. It reflects what is written in FIP-94, Bellcore TRW-000295, ANSI T1.333-2001, etc.

To answer a couple of your questions of course you want to use building steel every chance you get. Every MGB should be connected directly to BS as close as physically possible.

Transformer, PDU's ect should be bonded to either BS, MGB, or what ever is closest per NEC 250. What that is depends on the design. For example if you are using a raised floor, you might use a SRG (signal reference grid) either made of #6 AWG or the floor stringers. The equipment racks would bond to the SRG, and the SRG is bonded to the MGB, all panels serving the area, building steel, and all under floor piping and race ways to form a mutli-point ground. DO NOT TRY TO MAKE A SPG SYSTEM USING A MGB. It will not work in a data centers. Nor should you use isolated receptacles. They will all be corrupted by the quipment and impossible to police and trace down violations.
 
Re: data center grounding

Thanks dereck, That's very close to what I was working on. You're right, we visited the possibility of an isolated ground (as some other data centers had tried it) and quickly decided that was not the way to go. We're going to setup the SRG which runs back to the MGB. we're also running all the transformers back to the MGB since it's a relatively small room. Thanks for your help.
 
Re: data center grounding

"Fips 94 is the only grounding book without mistakes" Mike Holt.
Its free on this website...
didn't FIPS 94 become the basis for an IEEE standard???
 
Re: data center grounding

Dave take this FWIW and consider this method. It should save some time, material, labor, and more effective.

Install a ground bar in the dead center of the room under the raised floor, call it a Raised Floor Ground Buss (RFB for short). Bond the RFB to the MGB, building steel, raceways, pipes, and any rebar under the floor if available. Run a bare #6 or 2/0 around the perimeter of the raised floor supported and bonded to the outside pedestals closing to form a loop. Run your grid (SRG) and use the perimeter to anchor and bond the ends. 2 x 2, 4 x 4 are the common SRG spacing (The smaller the loops are the lower the impedance at higher frequencies, or the higher the cut-off frequency). Then from the RFB run two #6 or 2/0 to the opposite corners of the perimeter cable. Any place where a conductor passes by building steel, raceway, pipes, rebar, etc bond to it.

Then take your transformer Xo leads to either building steel or the RFB. Your AHJ may require you to weld the RFB connection to building steel, and the transformer Xo to RFB connection. In my business most AHJ are satisfied with two-hole compression connectors and brass "DO NOT DISCONNECT" tags attached to the ground cables. However not all will accept this and require a weld.

Now for something you will not want to hear. It is not nessecary to use SRG's anymore in data centers using AC power. If you use PDU's and\or isolation tranformers, and run dedicated AC circuits with an EGC to the equipment racks using standard SG recepts (never use IGR) is more than enough. The SRG is a left over practice from the 70/80's when the communication cables used ground as part of the signal path like coaxes and RS-232 cables. Now days most I/O ports use eithernet, optical, or other balanced transmission methods. Ground is just for safety and has nothing to do with technical operations. :cool:

[ December 13, 2005, 06:11 PM: Message edited by: dereckbc ]
 
Re: data center grounding

Thanks, actually I'm happy to hear about the SRG. one of the guys i'm butting heads with on the grounding for our data centers insists on it. i'm guessing because he originally worked in data centers in the 80s.
 
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