Unstable heating of dielectric by preflashover current

A dynamical model of thermal instability of a surface of dielectrics heated by preflashover current is developed and analyzed. The instability is associated with nonlinear dependence of bulk conductivity of dielectric on its temperature. It occurs when preflashover current flows as a surface filament along the dielectric and heats its surface locally. Surface heating is shown to be a triggering process for unstable bulk Joule dissipation of electricity in a subsurface region of the material. The paper shows dynamics of the instability development for a cylindrical model of material heating by a long conducting filament in contact with the dielectric surface. Associated delay times are calculated for the typical physical properties of pink alumina ceramics. These purely thermal by their nature delay times were found to be virtually the main controlling parameter of flashover dynamics