An electrical surge can be occurred in an electrical power transmission system due to various reasons. Surge in electrical system originated mainly due to lightning impulses and switching impulses. Electrical surge produces a large transient over voltage in the electrical network and system. The shape of the transient over voltage has a steeply rising front with slowly decaying tail as shown in the figure below. This steep voltage wave travels through the electrical network and causes over voltage stresses on all the electrical insulators and equipment come under its travelling path.
That is why all electrical equipment and insulators of power system must be protected against electrical surges. The method of protecting system from surge is normally referred as surge protection. The main equipment commonly used for this purpose is lightning arrester or surge arrester.
There are two types of surges one comes externally from atmosphere such as atmospheric lightning. Second type is originated from electrical system itself, such as switching surges.
When an electrically charged cloud comes nearby an electrical transmission line, the cloud induces electrical charges in the line. When the charged cloud is suddenly discharged, through lightning, the induced charged in the transmission line is no longer confined static. It starts travelling and originate dynamic transient over voltage. This transient over voltage travels towards both load and source side, on the transmission line because of distributed line inductance and stray capacitance. This surge voltage travels with speed of light. At the end of the transmission line, as the surge impedance changes, the surge voltage wave reflected back. This forward and backward travelling of surge voltage wave continues until the energy of the surge or impulse is attenuated by line resistance. This phenomenon causes voltage stress on the transmission system many times greater than normal rated voltage of the system. Hence, surge protection scheme must be provided to the electrical power transmission system to make reliable and healthy system. Lightning arrester is one of the main components to to protect the system from surge.