Page "Insulator (electricity)" Paragraph 27

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 flashovers.

The flashover voltage can be more than 50 % lower when the insulator is wet.

High voltage insulators for outdoor use are shaped to maximize the length of the leakage path along the surface from one end to the other, called the creepage length, to minimize these leakage currents.

To accomplish this the surface is moulded into a series of corrugations or concentric disc shapes.

These usually include one or more sheds ; downward facing cup-shaped surfaces that act as umbrellas to ensure that the part of the surface leakage path under the ' cup ' stays dry in wet weather.

Minimum creepage distances are 20 – 25 mm / kV, but must be increased in high pollution or airborne sea-salt areas.