by Bill Brown, P.E.

The use of solid grounding for UPS-supplied power systems is currently a common practice in data centers. The use of conventional ground fault protection systems in this application often leads to circulating currents and nuisance tripping, resulting in the desire to defeat or exclude ground-fault protection altogether. This paper explores the nature of circulating currents in UPS-supplied power systems when solid system grounding is employed and the use of modified-differential ground fault protection to eliminate the effects of these circulating currents. In addition, the alternative of high-resistance grounding for this application is discussed, along with its advantages and disadvantages.