When specifying a neutral earthing resistor (NER), attention often focuses on fault current levels and overall protection. However, performance depends on the resistor element, which absorbs fault energy, manages heat, and ensures reliable operation. Here, Mike Torbitt, managing director of resistor manufacturer Cressall, explains the different element technologies for resistors and how choosing the right one ensures safe, dependable performance.
Credit: Cressall
Neutral earthing resistors (NERs) are critical components in modern power systems, helping to limit earth fault currents and protecting equipment from damage. NERs improve system stability, reduce the risk of outages and support the safe operation of electrical networks.
Their use is expanding as networks become more complex with the growth of renewable generation, large-scale data centres and other high-demand infrastructure. This increasing demand is reflected in a recent UK Parliament briefing, which notes that “data centres currently consume around 2.5 per cent of the UK’s electricity. The sector’s electricity consumption is expected to rise fourfold by 2030.”
Within NER systems, three element technologies have proven particularly effective: edge-wound coils, wire-wound coils and expanded mesh, each offering distinct advantages depending on the requirements of the application.
Edge-wound coils: high current, high energy performance
Edge-wound elements are manufactured from continuous stainless steel strip wound edgewise and mounted on rigid ceramic insulators. This design allows a large mass of resistance material to be packaged into a compact space.
One of the defining characteristics of the edge-wound coil is its ability to accommodate thermal expansion. Operating temperatures can reach up to 1000 degrees Celsius during short-duration faults, yet the element can expand and contract freely without imposing mechanical stress.
This makes it well suited to systems that must absorb significant fault energy without risk of deformation or premature ageing. For example, edge-wound elements are commonly used in high-voltage utility substations where NERs must safely dissipate large fault currents during transient ground faults.
Edge-wound coils also offer exceptional short-term overload capability and can be supplied with fixed or adjustable tapping options, providing flexibility. For high fault current ratings and demanding duty cycles, this construction remains a robust and proven choice.
Wire-wound coils: compact and economic solutions
For applications requiring higher resistance values and lower power ratings, wire-wound coils provide an efficient alternative. Manufactured using chrome-aluminium or nickel-chrome wire wound onto ceramic formers, these elements are compact and cost-effective.
Wire-wound designs are particularly suitable for applications where space is limited and the duty is less severe. They offer low inductance and negligible audible noise in installations where electromagnetic performance and acoustic considerations are critical. For instance, they are often used in smaller industrial power systems or indoor electrical rooms where compact construction and quiet operation are important.
While they are not typically used for the highest energy NER duties, wire-wound coils remain an important part of the resistor manufacturer’s toolkit. In the right context, they deliver dependable performance and excellent value.
Expanded mesh: rapid cooling and mechanical resilience
Expanded mesh elements are cut from flat sheets to create an open construction with a large surface area, promoting unrestricted airflow and efficient convection cooling. They can cool from 600 degrees Celsius to ambient temperature approximately six times faster than traditional stamped grid elements.
The open structure also results in low inductance, even up to one megahertz, because current flows through a wide, flat lattice rather than tightly wound coils, reducing stored magnetic energy and unwanted reactance. Lightweight yet flexible, the construction provides inherent shock absorption, making it suitable for rail, offshore, and other vibration-prone environments. In many cases, no forced cooling fans are required.
Cressall’s Omega-shaped mesh elements enhance rigidity and limit displacement under load. Testing at temperatures above 500 degrees Celsius showed maximum displacement of less than two millimetres, maintaining safe spacing between parallel elements and minimising short-circuit risk. This established design continues to deliver dependable performance under fault conditions.
Credit: Cressall
The right element for the right NER
There is no universal answer when selecting an element for an NER. Fault current level, duration, ambient conditions, footprint constraints and maintenance philosophy all play a role. The key is understanding how each element type behaves electrically, thermally, and mechanically under fault conditions.
As a resistor manufacturer with over 100 years of experience, Cressall designs and builds NER systems in accordance with standards such as IEEE C57.32 and IEC 60076-25. The company offers solutions ranging from 500 watts to 86 megawatts and beyond, working closely with customers to ensure each NER is engineered around the application rather than adapted from a standard catalogue part.
If you are reviewing your earthing strategy or specifying a new NER, our engineering team would be pleased to advise on the most suitable element technology for your system.