McGraw-Hill Construction’s Engineering News-Record took a ahot at figuring out what’s wrong (ELECTRICALLY) at the big new NSA Data Center in Utah. The author is from a consulting and technology design firm. The resulting article provides stabs at answers to 9 questions, including #7
Q 7: Is the grounding system appropriate to a facility of this magnitude and sophistication?
The code maximum of 25 Ohms ground resistance [NEC 250.53(A)(2)] is way too high for the ground system of a mission critical data center. The ANSI/J-STD-607-A, which should be used for data centers as well as telecom installations, calls for a maximum of 5 Ohms.
And hopefully, no one included Isolated Grounds or “IG” circuits in the design, although they would not likely have anything to do with the present problem.
In a data center IG circuits are a waste of copper and a potential source of multiple ground paths and eddy current problems. But proper installation of a technology ground reference system is not easy to achieve.
Electricians like to make nice neat bends in wires, but the high-frequency spikes that grounds are supposed to drain away don’t go around sharp corners. Getting electricians to properly radius every bend can be challenging, particularly when a large number of craftsmen are working on a high time pressure project.
Since a robust ground reference is necessary for breakers to clear fault conditions before they can propagate upstream, might we question whether sufficient attention was given to this part of the design and installation?
There is also #8
Has breaker coordination been done in conjunction with the arc flash analysis?
and #9
Have Surge Protection Devices been included at all necessary points?
We didn’t excerpt the answers here to not wreak havoc on ENR’s copyright. As of 12/5/13, the article (dated 11/12/13) was freely available (in front, not behind, the magazine’s firewall) – here.
Source: http://electricalcontractor.com/?p=12060
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