What Are Arresters?

Arresters are protective devices installed in electrical power systems to limit the magnitude of voltage surges caused by lightning strikes, switching operations, or other transient disturbances, preventing insulation breakdown in transformers, cables, switchgear, and rotating machines. Also called surge arresters or lightning arresters, they operate by clamping the transient overvoltage at a level below the withstand rating of the equipment they protect, conducting the associated surge current safely to ground, and then returning to a high-impedance state once the transient has passed. Arresters are deployed across the full voltage range of power systems, from low-voltage distribution circuits to ultra-high-voltage transmission lines operating at 1000 kV and above. The predominant technology in use since the 1970s is the metal-oxide surge arrester (MOSA), which replaced earlier gapped silicon carbide (SiC) designs.

The protective function of an arrester depends on precise coordination with the equipment it protects: the arrester must conduct before the overvoltage reaches the equipment's insulation withstand level, yet it must withstand the continuous power-frequency voltage without conducting. This balance is expressed through two key parameters: the maximum continuous operating voltage (MCOV) and the protection level, defined as the maximum voltage appearing across the arrester terminals during a specified discharge current test.

Metal-Oxide Surge Arresters

Metal-oxide surge arresters use zinc oxide (ZnO) varistors as their active elements, exploiting the highly nonlinear current-voltage characteristic of polycrystalline zinc oxide ceramic: resistance falls by several orders of magnitude as voltage rises above a threshold, allowing large surge currents to pass at nearly constant voltage. Because the nonlinearity of ZnO is steep enough to clamp voltage effectively without requiring a series spark gap, MOSAs operate gaplessly, eliminating the re-ignition and power-follow problems that limited gapped SiC designs. A stack of ZnO varistor discs, each a few centimeters in diameter and optimized for energy absorption capacity and clamping voltage, is housed inside a porcelain or polymer composite insulator column. The IEEE Standard C62.11 for metal-oxide surge arresters defines the classification, testing, and application requirements for MOSAs on power circuits above 1000 V, establishing test procedures for energy handling, discharge voltage, and power-frequency withstand.

Power System Transients

Arresters are sized and selected based on the characteristics of the transients they must absorb. Lightning surges are the primary threat: a direct lightning strike on a transmission line or substation bus can inject tens of kiloamperes of current within a few microseconds. Arresters are rated by their discharge current capability (typically 5, 10, or 20 kA for station class devices) and their residual voltage at that discharge current, which determines the protective margin. Switching transients arise when circuit breakers open or close, generating overvoltages that are lower in magnitude than lightning surges but longer in duration and higher in total energy. The IEC standard IEC 60099-4 and its overview for IEC lightning protection guidelines address both lightning and switching surge protection for high-voltage systems. Temporary overvoltages, lasting from cycles to minutes, require that the arrester's MCOV be set above the highest steady-state line-to-ground voltage the system can experience, including during fault conditions when unfaulted phases may rise above normal voltage.

Insulation Coordination and Application

Arrester selection requires coordination with the basic lightning impulse insulation level (BIL) of the protected equipment. The protection margin is defined as the ratio of BIL to the arrester's lightning impulse protection level; IEEE and IEC guidelines recommend margins of at least 20 percent for direct connections. Arresters are located as close as possible to the protected equipment to minimize the additional voltage that propagates along the connecting lead inductance during the fast rise of a lightning surge. Polymer-housed arresters (silicone rubber or EPDM sheds on a fiberglass-reinforced polymer rod) have largely displaced porcelain in new installations because of their lighter weight and superior performance under pollution and mechanical loading. The Eaton fundamentals reference on surge arresters provides a practical treatment of selection and application criteria across voltage classes.

Applications

Arresters provide overvoltage protection across a wide range of power system equipment and locations, including:

• Substation power transformers and autotransformers
• Distribution and transmission lines at towers and riser poles
• Rotating machines such as generators and large motors
• Underground cable terminations and gas-insulated switchgear
• Renewable energy installations including wind turbine nacelles and photovoltaic inverter inputs