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neutral earthing resistors


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.

Technical Specifications

  • Operation Voltage : Up to 110 kV line to line systems
  • Rated Current : Up to 5000 A
  • Ambient Temperature : Up to 55oC (For higher ambient temperature, consult factory)
  • Resistance Alloy : Stainless-steel (CrNi or CrAl)
  • Protection Degree : IP 23 (outdoor) and others on demand
  • Standards : IEEE 32, IEC 60137 & 60273, EN ISO 1461, EN 10346, EN 12 ISO 129 44, IEC 60071, IEC 60060, IEC 60529
wiring for neutral earthing resistors

Advantages of NER’s include:

  • Reduce the single phase fault currents for securing each equipment in MV electrical networks
  • Reduce the transient overvoltage’s which can occur during an earth fault, and be monitored and used to activate the earth fault relay
  • Increase protection of generators, transformers and related equipment
  • Reduced operation/maintenance costs
  • Increase safety
  • Provide simple, reliable, selective means of protection
  • Allows the use of equipment, and in particular cables with lower insulation levels than for an insulated neutral scenario
  • Reduce the step voltage

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.