Neutral Earthing Resistors (NER’s) are used to limit the fault current for safety of equipment and personnel in industrial systems.
In solid grounding, the system is directly grounded and the fault current is limited only by the soil resistance. The fault current can be very high and can damage the transformers, generators, motors, wiring and other equipment in the system. The NER’s are inserted between neutral and ground in order to increase the nett resistance in the event of an earth fault and limit the current to a safe level.
The fault current value should be limited to a value that can be safely handled by the machine or transformer. It also needs to be high enough to be sensed by the earth fault protection relays. If the NER resistance value is too high, the fault current will be very low and will not be able to activate the earth fault protection relay during earth fault conditions.