From bubbles to breakdown, or vice-versa

A review of the present understanding of the electrical breakdown mechanism in dielectric liquids is presented. Two fundamental mechanisms, positive discharge and negative discharge, are emphasized. The positive discharge represents an electronic process occurring in the liquid. A highly overvolted negative discharge would follow a similar mechanism. However, at voltages nearer to the minimum breakdown voltage the negative discharge involves cavity formation, which then dominates subsequent events. In particular, liquid viscosity becomes an important parameter in the subsequent growth. The positive discharge exhibits negligible viscosity effect. By contrast the form of the negative discharge is dominated by the viscosity coefficient