What Are Surge Arresters?

Surge arresters are protective devices installed in electrical power systems to limit the amplitude of transient overvoltages caused by lightning strikes, switching operations, and system faults, thereby protecting transformers, cables, rotating machines, and other insulation-limited equipment from dielectric breakdown. A surge arrester acts as a voltage-dependent resistor: at normal operating voltage its impedance is very high and negligible current flows through it, but when the terminal voltage rises above a defined clamping level, the device becomes conductive, diverting the surge current to ground and clamping the voltage across the protected equipment. After the surge passes, the arrester recovers its high impedance and normal system operation resumes.

Surge arresters are a fundamental component of high-voltage transmission and distribution systems, deployed at generating stations, substations, and on overhead distribution lines. The technology evolved from the early silicon carbide gapped arrester, which required a series spark gap to interrupt power-frequency follow current, to the modern metal oxide surge arrester (MOSA), which eliminates the series gap entirely and offers substantially better energy absorption capability and clamping performance. IEEE standards govern arrester application and testing, with IEEE C62.11 covering the standard for metal oxide surge arresters for AC power circuits and related guides addressing selection and application methodology.

Metal Oxide Varistor Technology

The active element of a modern surge arrester is a column of zinc oxide (ZnO) ceramic discs, each approximately 40 to 90 millimeters in diameter and 20 to 30 millimeters thick, sintered with small additions of bismuth oxide, manganese oxide, and other dopants that create highly nonlinear grain boundary junctions. The current-voltage characteristic is described by a power law, I = k·V^α, where the exponent α typically ranges from 25 to 50, compared to 2 to 6 for earlier silicon carbide elements. This steep nonlinearity means the arrester conducts only microamperes at normal operating voltage but can pass tens of kiloamperes of lightning current while maintaining a nearly constant clamping voltage. The energy absorption capability, rated in kilojoules per kilovolt of continuous operating voltage, determines the arrester's ability to survive multiple lightning strokes or prolonged switching surge events. Technical details on ZnO element characteristics and application guidance appear in the Hubbell/Chance application guide for metal oxide surge arresters.

Arrester Classes and Ratings

Surge arresters for AC power systems are classified by duty cycle rating and by application class. Station class arresters serve generating stations and major substation equipment, providing the lowest protective ratios and the highest energy handling capability. Intermediate class arresters are applied to distribution substations and large industrial feeders. Distribution class arresters, the most numerous type, are installed on pole-mounted transformers, cable risers, and sectionalizing equipment throughout overhead distribution systems. A normal duty arrester suits areas of low lightning incidence, while a heavy duty arrester is specified where ground flash density exceeds four flashes per square kilometer per year. The riser pole arrester addresses the specific overvoltage hazard at the transition from overhead line to underground cable, where traveling wave reflections can produce voltage doubling.

Installation and Protection Coordination

Effective surge protection depends on close physical proximity of the arrester to the protected equipment, since voltage reflections on the lead inductance between arrester and equipment terminal add to the clamped voltage. A rule of thumb limits the total lead length to 1.8 meters for distribution applications. Protection coordination requires selecting arrester protective levels that are lower than the basic insulation level (BIL) of the protected equipment by a margin of at least 20 percent, with guidance provided by the NEMA Arresters organization's application resources. Polymer-housed arresters, which replaced the porcelain-housed designs that dominated through the 1990s, resist vandalism and seismic loading better and weigh significantly less, simplifying installation on distribution poles and compact substations.

Applications

Surge arresters have applications throughout the electrical power system and in related industries, including:

• Transmission substations protecting power transformers rated from 69 kV to 765 kV
• Distribution systems protecting pole-mounted transformers, reclosers, and cable terminations
• Industrial facilities with large motors, capacitor banks, and variable frequency drives susceptible to switching surges
• Railway traction systems where pantograph separations and switching generate steep-fronted overvoltages
• Renewable energy generation sites, particularly wind farms and solar installations with long cable runs