energy surge. Degradation resulting from energy surge absorption is therefore negligible in practice and has almost no effect on the life of the elements. However, high energy surge applied to ZnO elements exhibiting localised spots
of conduction could melt the material [66, 99]. Switching surge degradation is pronounced under unidirectional surges but can be reduced to some extent by applying surges of opposite polarity [67]. Only 1 per cent degradation was
observed [68] on the direct voltage which produces 1 mA discharge of positive and negative switching surges. After absorbing two rectangular surges of 600 A and 2 ms duration followed by power frequency, less
than −4 per cent maximum variation in voltage, V, was observed [4], and the ZnO element could return to its initial working conditions of voltage and temperature after just one hour. As can be seen on Figure 5.6a, V
1mA undergoes as little as 5 per cent change when the arrester is subjected to 1000 switching
surges of 600 A magnitude and 2 ms duration [4].1mA 1mA), after the