Neutral Earthing Resistors (NERs) for Transformers: A Comprehensive Engineering Guide

Derrel Gerary
Sep 28
7 min read

At the heart of this decision lies the philosophy of system grounding—specifically, how the neutral point of a power transformer or generator is connected to the earth. While options range from leaving the neutral ungrounded to connecting it solidly to earth, both extremes present significant drawbacks. The engineered solution, designed to provide an optimal balance between safety and reliability, is the implementation of resistance grounding through a Neutral Earthing Resistor (NER).

This comprehensive guide from Leistung Energie provides a deep technical dive into the theory, design, specification, and application of Neutral Earthing Resistors for transformer neutral grounding.

It is intended as a definitive resource for substation designers, protection engineers, and industrial asset managers who are responsible for ensuring the resilience and safety of critical power infrastructure.

1. The Fundamentals of Transformer Neutral Grounding

Before delving into the NER itself, it is essential to understand the core reasons for grounding a system's neutral point and the different philosophies available.

Why Ground the Neutral Point? Grounding the neutral of a three-phase system serves two primary purposes:

Voltage Stabilization: It provides a stable reference to ground, which prevents the phase voltages from "floating" to dangerously high levels during certain fault conditions or switching operations.
Fault Current Path: It provides a defined, low-impedance path for earth fault currents to flow back to the source. This allows the fault current to be reliably detected by protective relays, which can then operate a circuit breaker to clear the fault.

The Spectrum of Grounding Methods: A Critical Comparison

Ungrounded Systems: In this configuration, the transformer neutral is not intentionally connected to the ground.
- Pros: The system can theoretically continue to operate during a single phase-to-ground fault, as the fault current is very small (limited to the system's capacitive charging current).
- Cons: This method suffers from severe disadvantages, including the potential for high transient overvoltages during intermittent faults (arcing grounds), which can cause catastrophic insulation failure across the network. Furthermore, locating the fault is extremely difficult.
Solidly Grounded Systems: Here, the transformer neutral is connected directly to the earth grid without any intentional impedance.
- Pros: It provides maximum voltage stability and is simple to implement.
- Cons: The primary drawback is immense and often unacceptable. A phase-to-ground fault becomes equivalent to a phase-to-phase short circuit, resulting in extremely high fault currents (often thousands of amperes). These currents can cause severe thermal and mechanical damage to transformers, switchgear, and cables, and create catastrophic arc flash hazards for personnel.
Resistance Grounded Systems: This is the engineered "middle path" that mitigates the risks of the other two methods. An NER is inserted between the transformer neutral and the earth grid. This approach is the focus of our guide.

2. What is a Neutral Earthing Resistor (NER)?

A Neutral Earthing Resistor (also known as a Neutral Grounding Resistor or NGR) is a high-power resistor designed for three-phase utility and industrial power systems.

Its purpose is to be connected between the neutral point of a transformer or generator and the system earth grid.

The primary function of an NER is to limit the current that flows through the neutral during an earth fault to a predetermined and safe level.

By inserting this resistance, we achieve several critical objectives simultaneously:

Limit Destructive Fault Current: The NER reduces the fault current to a level that prevents damage to the windings of the transformer and other system components.
Control Arc Flash Energy: By significantly lowering the fault current, the incident energy released during an arc flash event is drastically reduced, vastly improving personnel safety.
Reduce Transient Overvoltages: It effectively dampens the transient overvoltages that are a common and dangerous feature of ungrounded systems.
Provide a Reliable Fault Signal: While the current is limited, it is still high enough to be easily and reliably detected by standard protection relays, allowing for the selective and rapid clearance of the fault.

3. Low Resistance vs. High Resistance Grounding: A Key Distinction

Resistance grounding is not a one-size-fits-all solution. It is broadly categorized into two distinct philosophies based on the level of fault current permitted by the NER.

A. Low Resistance Grounding (LRG)

This is the most common method used in medium-voltage (MV) systems from 2.4kV to 35kV.

Typical Fault Current: A relatively high current, typically ranging from 100A to 1000A. A common value is 400A.
Guiding Philosophy: The goal of LRG is rapid fault clearance. The fault current is deliberately set high enough to be unambiguously detected by standard overcurrent protection relays. Upon detection, these relays command the appropriate circuit breaker to trip immediately, isolating the faulted section of the network. The system does not continue to operate with the fault present.
Advantages:
- Allows for selective coordination, meaning only the breaker closest to the fault opens.
- Uses standard and cost-effective overcurrent protection schemes.
- Effectively limits damage compared to a solidly grounded system.
Applications: General industrial plants, utility distribution substations, and commercial facilities where isolating a faulted feeder is the standard and acceptable protective action.

B. High Resistance Grounding (HRG)

HRG is a specialized method used in applications where process continuity is the absolute highest priority.

Typical Fault Current: A very low current, typically between 5A and 10A. The NER is sized so that the fault current is just slightly greater than the system's total capacitive charging current.
Guiding Philosophy: The goal of HRG is process continuity and alarming. The fault current is limited to such a low level that it is not immediately damaging to equipment. Therefore, upon detecting a fault, the system does not trip. Instead, it triggers an alarm to notify operators that an earth fault is present. This gives them time to locate the fault and arrange for an orderly, planned shutdown for repair, preventing a sudden and costly interruption to their process.
Advantages:
- Maximizes system uptime and avoids catastrophic process interruptions.
- Virtually eliminates arc flash hazards associated with earth faults.
Applications: Critical process industries where a sudden shutdown is unacceptable, such as oil and gas refineries, pulp and paper mills, data centers, and continuous manufacturing facilities.

4. Anatomy of a Neutral Earthing Resistor: Components and Construction

An NER is a robust piece of equipment designed to withstand extreme thermal and electrical stress.

The Resistor Elements: This is the active and most critical part of the NER. The elements are made from high-grade, corrosion-resistant stainless steel alloys (such as AISI 304, 316, or 430) chosen for their high resistivity and stability at extreme temperatures. Common constructions include wire-wound elements on ceramic cores, edgewound resistor ribbons, or stamped steel resistor grids.
The Enclosure: The resistor bank is housed in a protective enclosure. For outdoor applications, this is typically a hot-dip galvanized steel or stainless-steel structure with a sloped roof and a specific Ingress Protection (IP) rating (e.g., IP23 or higher) to protect against weather while allowing for natural ventilation. The enclosure features ventilation louvers covered with vermin-proof mesh.
Insulators and Bushings: High-voltage porcelain or composite polymer insulators are used to support the resistor elements and to provide the main input terminal (bushing) where the connection from the transformer neutral is made. These must have adequate creepage distance for the system voltage and environmental pollution level.
Optional Integrated Components:
- Current Transformer (CT): It is common practice to house the neutral CT, which measures the fault current for the protection relay, inside the NER enclosure.
- Disconnector Switch: An off-load disconnector can be included to safely isolate the NER from the transformer neutral for testing or maintenance.
- Elevating Stand: NERs are typically mounted on a galvanized steel stand to provide the necessary electrical safety clearance from the ground.

5. Engineering and Sizing an NER: Key Specification Parameters

Specifying an NER is a precise engineering task. The following parameters are essential:

Rated Voltage (VLN): The line-to-neutral voltage of the system. The NER must be insulated to withstand this voltage continuously during a fault.
Rated Fault Current (Amps): The desired current limit. This is a critical choice determined by the grounding philosophy (LRG or HRG) and the requirements of the protection system. For HRG, it must be greater than the system's capacitive charging current.
Rated Time (Seconds): The maximum duration for which the NER is designed to carry the full rated fault current without exceeding its temperature limits. A 10-second rating is the most common standard, as it provides ample time for protection systems to operate. Longer ratings (e.g., 30 seconds, 60 seconds, or continuous) are specified for systems where faults may persist for longer.
Maximum Temperature Rise: The maximum allowable temperature of the resistor elements at the end of the rated time. This is defined by standards such as IEEE C57.32 (e.g., 760°C rise over a 40°C ambient).
Material and Enclosure Specification: The materials for the resistor elements, enclosure (galvanized vs. stainless steel), and insulators must be specified based on the environmental conditions of the site (e.g., coastal, corrosive industrial).

6. Clarifying a Related Concept: Neutral Grounding Transformers (NGTs)

It is important to distinguish an NER from a Neutral Grounding Transformer (NGT), also known as a Grounding Transformer.

An NGT is used to create an artificial neutral point on a three-phase system where a neutral is not naturally available, such as a Delta-connected system.

The NGT is typically a dedicated transformer with a zig-zag or Wye-Delta winding.

The NER is then connected to the neutral of this NGT. In short: the NGT creates the point for grounding, and the NER provides the impedance for grounding.

Conclusion

The method of transformer neutral grounding is a foundational design choice in any electrical network. While ungrounded and solidly grounded systems have their place, they present significant risks in terms of overvoltages and destructive fault currents, respectively.

Resistance grounding, through the application of a correctly specified Neutral Earthing Resistor, provides the optimal, engineered solution for managing earth faults in the vast majority of medium-voltage systems.

By precisely controlling the magnitude of fault currents, NERs prevent catastrophic equipment damage, vastly improve personnel safety by mitigating arc flash hazards, and ensure the reliability and stability of the power supply.

Specifying the right NER requires a thorough understanding of your system's parameters, your operational needs, and your protection philosophy. It is a decision that directly impacts the safety, resilience, and lifecycle cost of your entire electrical network.

At Leistung Energie, we specialize in the engineering, design, and manufacturing of high-quality Neutral Earthing Resistors that meet the stringent demands of Australian standards and harsh operating environments. Our team of expert engineers is ready to assist you in analyzing your requirements and engineering the precise protection solution your system deserves.