What Is a Surge Arrester? What Does It Do, How Does It Work and What Types Are There?

A surge arrester is a protection device used in electrical systems to protect equipment insulation by limiting transient overvoltages caused by lightning impulses and switching events. This equipment is used especially to prevent damage to critical components such as transformers, circuit breakers, cable terminations, busbars, motors, generators and switchgear. In short, the answer to what a surge arrester is: it is protection equipment that suppresses sudden overvoltages occurring in the grid and safely directs these impulses to ground.

It would be incomplete to answer what a surge arrester does by saying only that it protects against lightning. Harmful overvoltages in power systems are not caused only by direct lightning strikes. Switching operations, line switching, impulse effects in the nearby environment and some transient system events can also create voltage rises that stress insulation levels. A surge arrester limits these sudden rises near a level the equipment can withstand and reduces insulation puncture, winding damage, equipment aging and sudden failures.

At the center of the surge arrester operating principle are metal oxide varistor, or MOV, blocks. Modern surge arresters mostly operate with these blocks, which have a nonlinear resistance characteristic. At normal operating voltage, the MOV structure behaves almost like an insulator and does not carry significant current on the system. However, when voltage rises to a dangerous level for a short time, this structure conducts, discharges the impulse current to ground and limits the voltage at the equipment terminals. When the overvoltage disappears, the surge arrester returns to a high-impedance state.

An important point here is that the surge arrester is connected in parallel, not in series, with the circuit. It is positioned near the equipment to be protected through phase-to-ground or another suitable connection arrangement and does not operate as the main current-carrying power path. Its main duty is to remain passive during normal operation and provide active protection at the moment of an impulse. Therefore, a surge arrester must not be considered like a circuit breaker, disconnector or fuse. It is a special protection device that directly performs overvoltage limitation.

The most common application areas of surge arresters include transformer substations, MV switchgear, distribution panels, overhead line entries, cable transition points, motor feeders, generator connections and special switchgear systems such as GIS. A correctly located surge arrester near equipment with high insulation cost, such as transformers and cables, significantly improves system reliability. A surge arrester installed in the wrong location or selected at an unsuitable level may fail to provide the expected protection.

Surge arrester types can be evaluated under different headings according to application. Distribution type surge arresters are widely used in medium-voltage networks and protection of distribution equipment. Station type surge arresters are preferred for heavier duty conditions and more critical equipment protection. Line surge arresters are used on certain transmission and distribution lines to improve lightning performance. In some applications, porcelain-housed solutions stand out, while in others silicone-housed solutions are preferred.

Looking only at system voltage is not sufficient when selecting a surge arrester. Continuous operating voltage, temporary overvoltage withstand, nominal discharge current, protection level, energy absorption capacity, short-circuit behavior, housing material and installation environment must be evaluated together. For example, in very polluted, humid or harsh outdoor facilities, housing material and surface behavior become more critical. Similarly, at points where special loads or high impulse energy are expected, a standard distribution type product may not be sufficient.

A surge arrester and a lightning protection system are not the same equipment, and this difference is often confused. A lightning protection system works by intercepting a direct lightning strike to a structure or facility and conducting it to ground. A surge arrester is a protection device inside the electrical system that limits overvoltages formed at equipment terminals. In other words, one is part of the external lightning interception system, while the other is a device that limits electrical impulse voltage. Therefore, although they may appear to serve a similar protection purpose, their duty definitions are different.

In MV and HV installations, surge arrester selection should usually be considered together with transformers, circuit breakers, cable terminations, current and voltage transformers and the busbar arrangement. The protection level depends not only on the product nameplate, but also on the distance to the equipment, the length of the connecting conductor and the field layout. The shorter and more correctly arranged the connection between the protected equipment and the surge arrester is, the better the protection effect becomes. Therefore, the correct installation point is as important as selecting a good product.

Surge arresters are equipment that may look passive in modern power infrastructure but have a very large effect. In many systems, they wait silently for years; their true value appears during a major impulse event. Especially in regions with high lightning activity, cable-overhead line transitions, open switchyards and sensitive load feeders such as motors and generators, the use of surge arresters contributes directly to operational continuity.

Before moving to maintenance, it should be noted that a surge arrester should not be considered a single-use element. A correctly selected and correctly applied MOV surge arrester can withstand many impulse events. However, over time, the impulse energy it is exposed to, environmental contamination, humidity, mechanical stress and aging may reduce its performance. Therefore, in systems with surge arresters, not only installation but also a periodic inspection approach is important.

Surge arresters are especially important parts of the main insulation coordination in transformer substations. In other words, they are not evaluated as standalone equipment but as part of the voltage withstand arrangement of the entire facility. If the protection level is not selected in harmony with the equipment withstand level, either unnecessary cost appears or the protection remains insufficient. For this reason, surge arrester selection is a serious design topic that requires engineering calculation.