What is lightning arrestor ?

Lightning arrestor is a device which is mainly installed across each phase and earth at the entry of the transmission line to the sub - station yard. It can also be seen at HV and LV sides of all power transformer installed at the sub - station. The modern LA is generally made of gap less ZnO. In this type of Lightning Arrestor required number of ZnO or zinc oxide discs are kept inside a hollow cylinder of insulated material such as porcelain. The column of ZnO discs is kept under spring pressure at its top or bottom under the hollow cylinder to ensure no gap between the discs in the column.

ZnO has such a property that it behaves as short circuit during transient surge over voltage and over frequency of the power system and becomes like normal insulator when transient surge voltage is over. So when lightning strikes on the overhead transmission line, the over voltage impulse travel toward both ends of the line and bypasses through the lightning arrestor to ground at the entry of the sub - station. As the transformer is the costliest equipment installed in the system, for better protection of each transformer is equipped with lightning arrestor at its both HV and LV sides.