Lightning arresters are designed to protect electrical power equipment from the sudden high voltage or medium voltage. They are made of raw material which is doing the good insulating funciton in the transformers and conductors of the power systems when the light comes. The typical one has a ground and high voltage terminals. While the surge of lightning or switch comes, it will go along the line of power  reaching to the arrester. And the current of the lightning surge or switch surge will then be diverted from the arrester to the ground.