Flashover along cylindrical insulating surfaces in a nonuniform field under positive switching impulse voltages

Results are presented concerning discharge development and breakdown in air and along cylindrical insulating surfaces in a 12 cm long rod-plane gap under positive switching impulse voltages. Discharge development and breakdown probability curves were obtained. The times taken for the discharge to cross the full gap and for breakdown were measured and the corresponding instantaneous voltages were calculated. The gap was over stressed so as to study the effect of the applied peak voltage on the discharge characteristics. In the presence of an insulating surface the discharge consists of a ´surface´ and an ´air´ component; the former hinders the development of the latter. At voltages causing 50% breakdown probability only the ´air´ component traverses the full gap and breakdown occurs always in free air. For higher applied voltages the ´surface´ component may become able to cross the full gap and breakdown may occur also along the insulating surface; under these conditions both the arrival of the discharge at the plane and breakdown occur at shorter times and at lower voltages than in air alone, depending on insulating surface material.