What are Neutral Grounding Resistors?
The purpose of a neutral grounding resistor is to limit ground fault currents to safe levels so that all the electrical equipment in a power system is protected. The resistor should be the only current path between the neutral of power transformers or power generators and ground.
When the neutral of a system is not grounded it is possible for destructive transient overvoltages to appear from line to ground during normal switching of a circuit having a line-to-ground fault. Experience has proved that these overvoltages cause aging and failure of insulation at locations on the system other than at the point of fault. In this way, a relatively unimportant line-to-ground fault on one circuit may result in considerable damage to equipment and interruption of service on other circuits, not to mention the increased difficulty in finding the original location of the problem.
A neutral grounding resistor is designed to limit the ground fault current to a safe value while at the same time letting enough current to flow to operate the protective relays that will alarm or clear the fault. While the disturbance lasts the resistor must be capable of absorbing and dissipating the energy generated without exceeding the temperature limits established by IEEE-32 Standards. In this way the fault is safely limited, isolated, and the power system is protected against over-voltages.
Neutral Grounding Resistors are also commonly referred to as Neutral Earthing Resistors and Earth Fault Protection.
High Resistance or Low Resistance?
High Resistance Grounding Resistors are recommended for installations that require continuous service even after a phase ground fault occurs.
A phase to ground fault will not cause a large current to flow and will not trip the breakers because the neutral grounding resistor will limit the current to a very low value, typically 5 Amps.
For added security it is required that a suitable ground detection device, or ground fault relay, be used to indicate the presence of a ground fault.
Pulsing devices can be used with high resistance systems to reduce the time required to find and remove the ground fault.
This type of Neutral Grounding Resistor should limit the fault to a value greater than the capacitive charging current of the system to avoid over-voltages caused by intermittent faults. While the disturbance lasts the resistor must be capable of absorbing and dissipating the energy generated without exceeding the temperature limits established by IEEE-32 standards.