Why do surge arrest typically looks like this ?
I found one of the reason is to prevent ingress of water by using slippery rubber material but it does not explain the reason for the shape of the surge arrest.
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It is to increase the length of the surface path from one side to another. The surface is more conductive and has lower spark voltage than the middle of a material (because it can be coated with dust etc). The discs are there to make the surface path longer without increasing the length of the surge arrestor.