What is a Neutral Earthing Resistor?

Earthing systems play an important role in an electrical network. For network operators and end users, avoiding damage to equipment, providing a safe operating environment for personnel and continuity of supply are major drivers behind implementing reliable fault mitigation schemes.

What is a Neutral Earthing Resistor (NER)?

A widely utilised approach to managing fault currents is the installation of neutral earthing resistors (NERs). NERs, sometimes called Neutral Grounding Resistors, are used in an AC distribution networks to limit transient overvoltages that flow through the neutral point of a transformer or generator to a safe value during a fault event.

Generally connected between ground and neutral of transformers, NERs reduce the fault currents to a maximum pre-determined value that avoids a network shutdown and damage to equipment, yet allows sufficient flow of fault current to activate protection devices to locate and clear the fault.

NERs must absorb and dissipate a huge amount of energy for the duration of the fault event without exceeding temperature limitations as defined in IEEE32 standards. Therefore the design and selection of an NER is highly important to ensure equipment and personnel safety as well as continuity of supply.

The Importance of Neutral Grounding

Fault current and transient over-voltage events can be costly in terms of network availability, equipment costs and compromised safety.

Interruption of electricity supply, considerable damage to equipment at the fault point, premature ageing of equipment at other points on the system and a heightened safety risk to personnel are all possible consequences of fault situations. By installing NERs on the distribution system and controlling fault currents and transient overvoltages, the following benefits can be realised: „

  • Elimination or reduction of physical damage to equipment „
  • Extended life of connected distribution equipment such as transformers „
  • Reduced operation and maintenance expenses „
  • Simplification and fast isolation and clearing of the original fault „
  • Improvement in network security and reduction in unplanned shutdowns

Find out more about the methods of neutral earthing and what to consider when specifying an NER in our quick guide to NERs.