NER integrity – do not underestimate the importance

The application of Neutral Earthing Resistors (NER) is one of the most effective ways to maximise personnel safety (limits touch and step potentials) and to minimise mechanical and thermal damage to transformers, cables and equipment associated with earth faults. Neutral Earthing Resistors (NER) have applications in many industries including mining (a requirement under AS/NZS 2081:2011), oil & gas, power generation and distribution.

However, the importance of monitoring the integrity of the NER often remains underestimated.

Along with the main purpose of limiting earth fault currents, the NER provides a link between the star point of a transformer (or generator) and earth, creating a return path for earth fault currents and allowing an earth fault detection relay to operate as intended (Figure 1).


The occurrence of loose connections between the NER and star point or earth, or the loss of integrity of the NER, results in the conversion of the resistance grounded system into an ungrounded system, rendering the current sensing earth fault detection ineffective and a false belief that the system is in a healthy functioning state (Figure 2).


To ensure that protection against earth fault currents in resistance grounded systems is not compromised and operates as intended, it is critical to continuously monitor the integrity of the NER (Figure 3).


Captech can offer advice and products to monitor the integrity of your NER. The Bender RC48N is available for high resistance grounded systems up to 5kV.

Contact Captech or find more information on the Bender range here.