WHY NEUTRAL-GROUNDING RESISTORS NEED CONTINUOUS MONITORING

A resistance-grounded power system has a critical element that is often ignored-the neutral-g rounding resistor. A resistance-grounded power system should have it's neutral-grounding resistor continuously monitored. During a single-phase-to-ground fault, current flows from the transformer or generator winding through the faulted-phase conductor to the fault and to ground, returning to the source winding through the ground-return path and the neutral-grounding resistor. When a neutral-grounding resistor fails, the failure mode is usually open circuit leaving the ground-return path open. Current-sensing ground-fault protection, which is the type most commonly employed in a resistance-grounded system, will not operate with an open resistor, and the advantages of resistance grounding are unknowingly lost. Inadvertent operation with an ungrounded system and inoperative ground-fault protection can be avoided by using a continuous neutral-grounding-resistor monitor. This paper reviews the benefits of resistance grounding, compares continuous resistor monitoring with planned-maintenance testing and inspection, and defines the characteristics required of a neutral-grounding-resistor-monitor. Three case studies are presented that show the need for continuous monitoring of the neutral-grounding resistor.