The purpose of a neutral grounding resistor is to limitthe ground fault current to a safe level so that all the electrical equipmentin the power system is protected. The resistor should be the only current pathbetween the neutral of power transformers or power generators and ground.

When the neutral of a system is not grounded it ispossible for destructive transient overvoltages to appear from line to groundduring normal switching of a circuit having a line-to-ground fault. Experiencehas proved that these overvoltages cause aging and failure of insulation atlocations on the system other than at the point of fault. In this way, arelatively unimportant line-to-ground fault on one circuit may result inconsiderable damage to equipment and interruption of service on other circuits,not to mention the increased difficulty in finding the original location of theproblem.

A neutral grounding resistor is designed to limit theground fault current to a safevalue while at the same time letting enough current to flow to operate theprotective relays that will alarm or clear the fault. While the disturbancelasts the resistor must be capable of absorbing and dissipating the energygenerated without exceeding the temperature limits established by IEEE-32Standards. In this way the fault is safely limited, isolated, and the powersystem is protected against over-voltages.

Advantages of using Neutral GroundingResistors:

• Reduced     operation and maintenance expenses.

• Fast     isolation of the original fault.

• Reduced     transient over-voltages.

• Reduced     physical damage on the equipment at fault.

• Simplification     of ground fault location.

• Increased     life and protection of transformers, generators and related equipment.

• Reduced     frequency of faults.

• Improved     service reliability.

• Increased     protection in the use of lightning arresters.

• Increased     safety for personnel.