18 n BRAKING RESISTORS May 2026 www.drivesncontrols.com What’s inside your braking resistors? Braking resistors are a familiar component in variable-speed drive systems, where they are used to dissipate excess energy during motor deceleration. Specifications often focus on their resistance, power rating and size. However, the internal construction of aluminium-housed resistors with similar ratings, and their installation provisions, can vary widely. While, externally, resistors may appear broadly comparable, internally, there can be differences in insulation systems, winding support and earthing arrangements – factors that are rarely shown on datasheets, but can influence real-world performance and reliability, and can impact electrical safety. Under EN 61800-3, braking resistors form part of a power drive system, alongside the drive, motor and cabling. This means that their design and installation contribute to both system performance and safety, and should be considered accordingly. The resistor should not be treated simply as an external accessory. Its construction, mounting and earthing all form part of the wider integrity of a drive installation. Aluminium issues Aluminium housings are widely used for their good thermal conductivity, which allows heat to be transferred efficiently away from the resistor element. As a conductive enclosure, however, the housing must also be connected reliably to protective earth to ensure safe operation. In the event of an internal insulation fault, a low-resistance earth path not only helps with EMC issues but also ensures that fault currents flow safely and that protective devices operate correctly. The key practical issue is that aluminium surfaces naturally form a thin oxide layer when exposed to air. Although extremely thin, this film is electrically insulating, with a typical electrical resistivity in the range 1012 – 1014 Ω·m. It can cause problems with the quality of an earth bond if the connection relies solely on surface contact, meaning that a resistor enclosure may appear to be securely mounted, yet still not provide an equally secure electrical bond. In practice, this makes earthing of aluminium housings more dependent on their mounting than you might assume. Internal insulation The insulation system between the internal resistor winding and its aluminium enclosure is critical, particularly at elevated operating temperatures. UL-listed materials are typically used because they are proven to withstand sustained thermal stresses and repeated cycling. Variations in insulation design can affect long-term reliability and the risk of electrical leakage. Where insulation is either absent or ineffective, the quality of the protective earth connection becomes even more important to the overall safety of the installation – Fig 1 shows a resistor with no insulation material, while Fig 2 shows gaps in insulation. Achieving a reliable protective earth connection in aluminium-housed braking resistors depends not only on the product design but also on the installation method. If no dedicated earth terminal is provided, installers often rely on mounting hardware or enclosure contact to complete the protective earth path. To improve consistency, good installation practice typically includes the use of serrated or star washers, adequate clamping forces, Aluminium-housed braking resistors are widely used in drive systems, yet their internal construction and installation requirements are rarely examined closely. As Steve Hughes, managing director of REO UK, explains, insulation design, earthing arrangements and surface oxidation can all influence drive safety and reliability. Fig 1: Internal view of an aluminium-housed braking resistor showing an exposed resistive element. Fig 2: A close-up of the internal construction of a resistor highlighting incomplete insulation coverage between the resistive element and the aluminium housing.
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