Dry Band formation and Flash overs in Insulators

When an insulator gets wet a thin film of water forms on the surface and a small leakage current starts to flow.  When this film of water evaporates due to rise in atmospheric temperature a "dry band" forms on the surface.  This dry band is formed around the insulator.  When this dry band is formed, the current flow is interrupted and a voltage gradient appears across the dry band. 

This voltage gradient exerts electrostatic stress across the surface and causes further evaporation and an increase in the width of the dry band.  This increase in width of the band causes a higher voltage gradient which causes minor arcing and can lead to flash over.

Dry band formation can be prevented by designing the insulators with  semiconductor glazing the insulator surface.  and by designing the insulator to have increased leakage distance.