1978 The Reality of High-Resistance Grounding

In recognition of the successful applications of high-resistance grounding of industrial power systems primarily in the continuous process industries, an attempt is made to identify the broad application areas as well as to offer qualifications regarding the application of the high-resistance grounding concept. High-resistance grounding (arranged to alarm only) has proven to be a viable grounding mode for 600-V and 5-kV systems with an inherent total system charging current to ground (3 Ico) of about 5.5 A or less. Beyond this current limit and in applications at 13.8 kV, high-resistance gounding appears to require direct tripping to prevent fault escalation prior to fault locating and removal. The original objective??avoidance of unscheduled shutdowns??could not be realized. The subsequent solution of direct tripping, rather than alarming only, is shown to result in a grounding scheme which is considered to be subordinate to the low-resistance (about 400 A) grounding scheme. In discussing the probable failure mode of motor windings, it is suggested that a ground fault in a motor winding, operating on a high- resistance grounded system, may be caused by an escalated tum-to-turn insulation failure. An indefinite prolongation of the resultant ground fault is likely to result in a further escalation to a phase-to-phase fault which increases the probability of unscheduled shutdowns and excessive motor damage. A precautionary note is included regarding the effect of static conversion equipment on high-resistance grounding.