insulators?

Question

why do ceramic insulators have those funny repetitive disk shapes?

Answer 1

zee prime lists one very good reason for having the repetitive disk shape. The flashover length is effectively extended, thus increasing the voltage that the insulator can withstand. If the insulator were shaped like a smooth cylinder, there would be a more direct path for flashover. Flashovers occur at material interfaces (eg: ceramic against air), and the curvy disk shape helps to "break up" that material interface and so increase the flashover withstand voltage.

Another tidbit of info you might be interested in is that these disks are often referred to as "weathersheds". In outdoor conditions, these insulators have to put up with weather conditions (primarily rain and fog) that can reduce their ability to withstand high voltages. By having these umbrella-like weathersheds, the chance that the entire insulator gets wet is pretty small. A wet conducting path is not as likely to form, and so we have fewer losses to ground fault currents.

Answer 2

A good question. The ceramic is a better insulator than air, and a much better insulator than the layer of moisture which sometimes forms on the surface. So the most likely way the insulation will fail is arcing across the surface. Those repetitive disk shapes increase the distance the arc has toextend, so makes it less likely.