Composite insulators using silicone rubber (SiR) formulations have become commonly employed to reduce dry-band formation, because these can offer hydrophobicity transfer to control wetting of the layer. However, new fog-chamber tests on 11 kV SiR insulators manufactured in-house show that a salt/kaolin layer on the insulator can still cause a substantial fall of the flashover voltage [1]. Bright, mobile partial arcs are initiated at voltages well below flashover [2]. In the present paper, additional infrared (IR) recording reveals the formation and development of heated bands in the pollution layer which are found always to precede the arc initiation.

 Close-up visual photography shows that current continuity is maintained in the pollution layer by small streamer/spark discharges bridging the dry bands. The current associated with these discharges is limited by the low conductance of the pollution layer in series with the bands; the local surface temperature is raised to 5-10°C above ambient, which is sufficient to prevent re-wetting of the dry band. A higher voltage enables short, incipient spark channels to develop from the dry-band discharges, and sometimes to link separate dry bands. These can grow into the longer partial-arc channels that normally precede flashover.