Many investigations have confirmed benefit of applying arresters to Transmission Lines. In case of non-sufficient or non-existing shielding systems, arresters are generally placed on the upper most phase. The frequency of application viz., in every structure or once in two or three structures varies depending on the levels of protection expected. In case where lines have good shielding but poor grounding, the arresters are used on the lowest phase which has a maximum potential for back flash over in the event of lighting striking shield wire.
Overhead transmission lines are the most vulnerable for lightning strokes. More than 50% of electrical faults of overhead lines are known to be caused by lightning induced voltages reaching the tower and causing the line insulator to flash over, resulting in an interruption. Back flash over occurs during the lightning discharge current, flowing through the tower and due to tower footing impedance producing potential differences across the line insulator. Back flash over is most prevalent when tower footing impedance is high. Installation of ZnO arresters across the line insulators (at frequent intervals) would minimize/eliminate the possibilities of insulation flashover.
Transmission line arresters are designed with or without series gap. From the reliability view point, non gapped configuration with disconnecting device is preferred. Besides, usage of the transmission line arresters allows to reduce the insulation level (BIL) of the substation equipments, e.g. power transformers, as the lightning surges are suppressed in the line itself by the line arresters. Hence, the no. of surges experienced by the substation arresters would be reduced and hence the reliability of the entire system would be enhanced.