Contents I. Introduction
The wetted surface of a polluted insulator has a typical leakage conductance of a few microsiemens. This enables dry bands to divide the pollution layer because a leakage current of a few milliamperes in magnitude promotes evaporation from the thin moist layer [1]. The voltage difference across such bands, following their loss of conductivity, is sufficient to initiate electrical breakdowns that bridge the bands. These take the form of corona streamers [1, Fig. 1(a)] that are of low luminosity mainly in the ultraviolet (UV) spectral range. The streamer initiation across a dry band reduces the leakage current below that which was present prior to the band formation and causes negligible damage to the insulator. Nevertheless, the local energy dissipated by the streamers is shown by the infrared (IR) record to heat the insulator surface [Fig. 1(b)] which will prevent rewetting and so conserve the band. The temperature rise associated with dry-band formation is modest. In this case, where the insulator is in a clean fog environment at 14 °C, yet much of its surface is at 16 °C and the dry bands can reach 26 °C–27 °C.
Dry bands and partial arcs on the trunk section of a silicone rubber insulator. (a) Simultaneous visible and (b) IR images of dry band streamers. (c) Partial-arc inception and bridging of a dry band [1].
