Neutral Grounding Resistor

NGR protects the electrical system and equipment by limiting the ground fault currents. Because of the use of resistance between the neutral and the ground, the ground fault current is minimized for the single line to ground fault (most common type of fault) and hence there is less damage to the system as well as the faulty equipment.

There are two types of NGR: High Resistance Grounding (HRG) and Low Resistance Grounding (LRG)

High Resistance Grounding (HRG): It is used where service continuity is vital, such as process plant motors. With HRG, the neutral is grounded through a high resistance so that very small current flows to the ground if ground fault occurs. In the case of ground fault of one phase, the faulty phase goes to the ground potential but the system doesn't trip. This system must have a ground fault monitoring system. The use of line to neutral (single phase) is prohibited (NEC, 250.36(3)) in HRG system, however, phase to neutral is used with using the additional transformer having its neutral grounded. When ground fault occurs in HRG system, the monitoring systems gives alarm and the plant operators start the standby motor and stop the faulty one for the maintenance. This way, the process plant is not interrupted.

Low Resistance Grounding (LRG): This is used for limiting the ground fault current to minimize the impact of the fault current to the system. In this case, the system trips for the ground fault. In this system, the use of line to neutral (single phase) is prohibited here as well.

See these links for further help:

http://ecmweb.com/power_quality/electric_ground/?smte=wl

- MS