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 conce rning low-voltage (480-600 V) power systems grounding. Since reliability an d continuity of service are very important, some engineers in the past pref erred using an ungrounded system. The practicality of such ungrounded syste m becomes questionable as the extent of coverage increases. Few ungrounded low-voltage systems are presently being designed due to the possible destru ctive nature of transient overvoltages resulting from an arcing ground faul t, Most systems now utilize either a solidly grounded or high-resistance gr ounded source. This paper begins with a brief discussion on ungrounded, sol idly grounded, and high-resistance grounded systems. Benefits and limitatio ns of each system are also discussed. It is shown that the use of high-resi stance grounded low-voltage systems makes good sense in the petrochemical i ndustry. Design, construction, operation, and maintenance factors for such systems are discussed and analyzed together with systems when three-phase f our-wire loads are present. Finally, operational problems and some appropri ate solutions are discussed where significant variable-speed drive loads ar e utilized. It is suggested that this should become a standard of the indus try and the solidly grounded system should be used only in applications whe re the high-resistance grounded system becomes impractical.