Electrical breakdown of dielectric liquids

The nature of the breakdown process in insulating liquids is very complex and over the years many theories of breakdown have been put forward. The most widely accepted theory, and the only one which is given prominence in the existing reviews on breakdown of liquids, is the cumulative ionization theory. This theory has its origins in the early interpretations of results of high-field conduction current measurements in liquids and assumes that breakdown is due to a Townsend-like ionization of liquid molecules by electrons emitted from the cathode. The relevance of the cumulative ionization theory to the available data on breakdown of liquids is questioned by the following facts: 1. The theory is incapable of explaining the dependences of breakdown strengths of liquids on hydrostatic pressure, on the degree of filtration, on stress conditioning and on spark energy in spark conditioning. 2. The theory is contradicted by studies of prebreakdown processes in viscous liquids 1 which showed that breakdown results from formation and growth in the liquid of a bubble of vapor. 3. It has been shown experimentally 2 that no charge multiplication occurs in liquids at fields of up to 1.2 MV/cm. 4. Significant emission of electrons from the cathode into a liquid is unlikely at fields of up to the breakdown strength. 3, 4