A surge arrester is a device for protecting electrical devices against surge transitions caused by external (lightning) or internal (switching) events. This device class is also known as surge protection device (SPD) or surge protection device (TVSS) and is used to protect devices in power transmission and distribution systems. (Various products, known as surge protection, are used to protect consumer devices.) The energy criterion for different insulation materials can be compared based on the pulse ratio. A surge arrester should have a low pulse ratio so that a surge protector incident on the surge arrester can be redirected to the ground instead of passing through the device.
In order to protect a device unit against transients that occur on a connected conductor, a surge arrester is connected to the conductor immediately before entering the device. The surge arrester is also connected to ground and works by directing energy from a surge that is transient to ground while one occurs while the conductor is disconnected from ground at normal operating voltages. This is usually achieved by using a varistor that has significantly different resistances at different voltages.
Surge arresters are generally not designed to protect against direct lightning strikes on a conductor, but rather against electrical transients caused by lightning strikes near the conductor [citation required]. A lightning bolt that strikes the earth leads to earth currents that flow over buried conductors and can induce a transition that spreads outward to the ends of the conductor. The same type of induction occurs in overhead lines and above-ground conductors that experience the through energy of an atmospheric EMP caused by lightning. Surge arresters only protect against induced transients that are characteristic of the rapid rise time of a lightning discharge and do not protect against electrification caused by a direct blow to the conductor. Lightning-induced transients, for example from the error switching of a high-voltage system, can also be safely redirected to ground. Continuous overcurrents are not protected by these devices. The energy in a treated transition is much lower than that of a lightning discharge; However, it is still large enough to cause equipment damage and often requires protection.
Without very thick insulation, which is generally prohibitive, most conductors that travel more than minimal distances (more than about 50 feet) experience lightning-related transients at some point during use. Since the transition will normally be triggered sometime between the two ends of the conductor, most applications will install a surge arrester just before the conductor lands in each device to be protected. Each conductor must be protected as each induces its own transient, and each SPD must provide a path to earth to safely route the transient away from the protected component. The only notable exception where they are not installed at both ends is high voltage distribution systems. In general, the induced voltage is not sufficient to cause damage at the end of power generation on the lines. However, installation at the service entrance of a building is the key to protecting downstream products that are not as robust.