Why are high voltage insulators ribbed?
High voltage ceramic insulators are designed with a lower flashover voltage than puncture voltage, so they flash over before they puncture, to avoid damage.
Dirt, pollution, salt, and particularly water on the surface of a high voltage insulator can create a conductive path across it, causing leakage currents and flashovers².
The stacked-disc structures on high voltage power lines are used to insulate conductors end-to-end (not inside to outside, like traditional plastic insulators).
The shape makes it difficult for an electric arc to travel along the ceramic material, as opposed to along a solid cylindrical piece.