High-resistance grounding of low-voltage systems: a standard for the petroleum and chemical industry

A debate has existed in the petroleum and chemical industry for years concerning low-voltage (480-600 V) power systems grounding. Since reliability and continuity of service are very important, some engineers in the past preferred using an ungrounded system. The practicality of such ungrounded systems becomes questionable as the extent of coverage increases. Few ungrounded low-voltage systems are presently being designed due to the possible destructive nature of transient overvoltages resulting from an arcing ground fault. Most systems now utilize either a solidly grounded or high-resistance grounded source. This paper begins with a brief discussion on ungrounded, solidly grounded and high-resistance grounded systems. Benefits and limitations of each system are also discussed. It is shown that the use of high-resistance grounded low-voltage systems makes good sense in the petrochemical industry. Design, construction, operation and maintenance factors for such systems are discussed and analyzed together with systems when three-phase four-wire loads are present. Finally, operational problems and some appropriate solutions are discussed where significant variable-speed drive loads are utilized. It is suggested that this should become a standard of the industry and the solidly grounded system should be used only in applications where the high-resistance grounded system becomes impractical.