A lightning arrester is a device, essentially an air gap between an electric wire and the ground that protects the insulation and conductors in electric transmission and telecommunication systems from lightning damage. The alternate spelling is lightning arrestor (also known as lightning isolator). There are two terminals on a typical lightning arrester: a high-voltage terminal and a ground terminal. Current from a lightning surge (or switching surge, which is very similar) is diverted through the arrester, usually to earth, when it travels along the power line to the arrester.
A lightning arrester prevents damage to electronic instruments within a structure and ensures the safety of individuals near it in telegraphy and telephony. A surge arrester is a smaller version of a lightning arrester that connects each conductor of a power or communication system to the earth. In addition to blocking normal power or signal currents from flowing to ground, these devices provide a path over which high-voltage lightning currents can flow, bypassing the equipment when connected. The purpose of these devices is to limit the voltage rise caused by lightning strikes on communications or power lines.
Electrical systems can be seriously damaged by lightning strikes if protection fails or is absent, which can cause thousands of kilovolts to discharge into the transmission lines and transformers, among other items. Electrical appliances can be damaged or even killed by lightning-induced voltage spikes in incoming power lines.
Protecting electric fences with lightning arresters is a common practice. Sometimes a series inductor is included as well as a spark gap. A mast radiator is also protected by this type of equipment. Usually, such devices have just one winding for their series inductances.
Transformer ruptures can fracture lightning arresters, which are part of large electrical transformers. As per NFPA 850, high-voltage transformer fire barriers are required to defeat projectiles from transformer bushings as well as ballistics from small arms.