Proper grounding is much more than a simple go/no-go application. The National Electrical Code (NEC) is the basis for the well-known 25-ohm standard. This should, however, not be taken as the be-all and end-all of grounding as it (unfortunately) is often regarded. The NEC requirement is reasonably forgiving, with the practical consideration that a contractor cannot be held responsible for putting in an extensive and expensive ground grid just because he is unlucky enough to be working for a customer resting on high resistivity soil. Meeting the Code’s requirement assures that there is at least a working ground electrode in place, the structure is not purely “floating,” and a practical degree of lightning protection is in force. The Code’s principle concern is electrical safety. It is not intended to guarantee performance. For commercial enterprises, however, performance should rank a close second to safety in terms of the operation of electrical equipment. Simply “meeting Code” can still leave a large gap between nominal and optimal grounding, and this contingency should be investigated to be reduced or eliminated.
With the influx of data center construction, special considerations have to be accounted for when designing its grounding system. This business model relies heavily on redundancy and cannot afford a second of down-time if any at all. This is where exceeding the NEC requirements becomes the standard and obtaining a near zero resistance becomes the focal point of the design, not cost. Insurance agencies and similar authorities typically recommend five ohms or less for a commercial ground, with specialized applications often more rigorous. Ideally, ground resistance would be zero, but this “standard” is only of theoretical interest. In the practical sense, one cannot expect to achieve zero resistance, but there are notable benefits to getting as close as possible.
Chemical laden grounding electrodes, multi-oriented rods comprised of various metals, ufer grounds installed just shy of the water table, all options and more to be considered when designing a grounding scheme. With no guarantee however engineers mostly rely on soil resistivity tables, historical data, and NEC standards to develop these schemes. And as long term studies are published and new technologies arise, we will see a new emphasis on the changing on the standards to keep up with the demand.