The Dangers of Grounding Resistor Failure

Abstract

A resistance-grounded power system has a critical element that is often ignored-the neutral-grounding resistor (NGR). 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 NGR. The information presented here has shown that an open NGR is an undesirable situation. A system with an open NGR is subject to transient overvoltages, and so current sensing ground-fault protection will not indicate the presence of a ground fault. A ground fault then remains on the system and might escalate to a phase-to-phase fault. NGRs are subject to failures related to thermal overload, lightning, storms, earthquakes, wildlife, extended service life, manufacturing defects, vibration, corrosion, and improper specification or installation. A well-designed NGR monitor provides continuous protection against failures that previously rendered ground-fault protection, coordination, and annunciation systems inoperative, as well as leaving the system exposed to damaging transient overvoltages. An NGR monitor provides confidence that current-sensing ground-fault protection will operate as designed on the next ground fault.