Neutral Earthing Resistor

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 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 the ground and neutral of transformers, NERs reduce the fault current to a predetermined maximum that avoids a network shutdown and equipment damage, yet allows sufficient fault current to activate protection devices to locate and clear the fault. NERs must absorb and dissipate a large amount of energy during the fault event without exceeding the temperature limits defined in IEEE-32 standards.

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 over-voltages, the following benefits can be realised:

• Elimination or reduction of physical damage to equipment

• An extended life of connected distribution equipment, such as transformers

• Reduced operation and maintenance expenses

• Simplification, fast isolation, and clearing of the original fault

• Improvement in network security and reduction in unplanned shutdowns