Surge arresters and surge limiters are used for protection of motors, generators and transformers against internal and external overvoltages. Thus they are an indispensable protection device for safe operation of medium-voltage witchgear. Their application makes sure that the arising overvoltages do not exceed the permissible withstand voltages of the equipment to be protected. So, their action in the power system is confined to the limitation of impulse and switching overvoltages. Apart from that they are passive devices.