In low voltage and electronics Leakage Current is any current that flows when the ideal current is zero.
In medium and high voltage applications it is the current that flows either through the body or over the surface of an insulator. Leakage current often arises when outdoor insulation is submitted to conductive fog (sea mist/spray, industrial smogs) or when a pollution layer containing soluble salts is formed on the insulator surface and then wetted, for example by dew or mist. These currents can reach several hundred mA and may lead to flashover of the insulation.