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The Outdoor 11kV Neutral Earthing Resistors: Functionality and Calculation

In the realm of high-voltage power equipment, neutral earthing resistors play a crucial role in maintaining the safety and reliability of electrical systems. Australian Power Equipment can provide outdoor 11kV neutral earthing resistors that stand out for their ability to handle up to 250amps. In this article, we look at the concept of neutral earthing resistors, explore their purpose, mechanism and delve into the calculations involved in determining their optimal value.

What is a Neutral Earthing Resistor and its Purpose?

A neutral earthing resistor (NER) is an essential component in power distribution and transmission systems. Its primary purpose is to limit the fault current that flows through the neutral point of a power system when an insulation fault or ground fault occurs. By intentionally introducing resistance in the path of the fault current, the NER prevents excessive current from flowing and causing potential damage to equipment, electrical components, and the overall power network. This added resistance also serves to reduce the risk of fire and electrical hazards.

How does a Neutral Earthing Resistor Work?

When a fault occurs in a power system, such as a cable-to-ground fault, the fault current seeks a path of least resistance to return to the source. The NER is connected between the neutral point of the power system and the ground. This connection forces the fault current to pass through the resistor, which limits the magnitude of the current and prevents it from reaching dangerously high levels. The NER essentially provides a controlled path for the fault current to dissipate its energy as heat, thus safeguarding the system’s integrity.

Calculating a Neutral Earthing Resistor Value

The value of a neutral earthing resistor is a critical parameter that needs careful consideration. It depends on various factors such as the system voltage, fault current, and the desired fault current duration. The resistor value should be chosen such that it limits the fault current to a safe level without causing unnecessary disruption to the power system during normal operations.

To calculate the value of a neutral earthing resistor, the following formula can be used: NER Value (ohms) = System Voltage (V) / Fault Current (amps)

However, this simplified formula may not consider transient conditions, fault duration or other system-specific aspects. It’s important to work closely with electrical engineers and experts to perform detailed fault studies and simulations to accurately determine the optimal NER resistance value. It’s essential to note that while a lower resistor value provides faster fault detection and reaction, a higher value reduces stress on the equipment during fault conditions. At Australian Power Equipment we recommend NERs limit fault currents to a value that does not cause any further damage to switchgear, generators or transformers beyond what has already been caused by the fault itself.

Top 5 things to look for in an Outdoor 11kV Neutral Earthing Resistor

Outdoor 11kV Neutral Earthing Resistor, rated for 250 amps, must stand as a robust solution for reliable fault protection. Key features to consider include:

1. Durability: NERs must be built to withstand harsh outdoor conditions to ensure continued operation in diverse environments. Considerations such as Cyclone Wind loading rating and Seismic rating enclosures can be imperative.

2. Precise Resistance: It is critical the resistor’s precisely defined resistance value facilitates accurate fault current limiting and system protection.

3. IP23 enclosures: The resistor’s design should incorporate efficient ventilation that prevents overheating, ensuring prolonged functionality. It should also be designed to keep out birds and vermin.

4. Easy Maintenance: NERS should always be designed for easy maintenance, minimising downtime and optimising system performance.

5. Compliance: It should go without saying that it is imperative they are compliant with AS/NZ 2081 and relevant Mining Standard for Earth-fault current limiters


Neutral earthing resistors, such as the Outdoor 11kV NER rated for up to 250amps provided by Australian Power Equipment, play a crucial role in safeguarding power distribution systems.By limiting fault currents and preventing dangerous conditions, they contribute significantly to overall system safety. Understanding their purpose, operation, calculation and the features of specialised NER products empower industries to create resilient and secure electrical networks

Speak to Australian Power to ensure you have safeguarded your power distribution with the right Outdoor NER
Australian Power Equipment supply Outdoor Neutral Earthing Resistors


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