While designing electrode configurations in practice, it is tried to avoid extremely non-uniform fields, especially for solid insulations. Thus, one may say that solid dielectrics are normally not stressed with extremely sharp non-uniform fields in practice. But, the pre breakdown process generally begins at an extreme intensity or distortion of local fields. Under the action of this high local electric field, a solid dielectric may either globally or locally loose its electrical insulating property and mechanical stability, leading to its complete or partial disintegration. A review of the breakdown and pre breakdown phenomena in solid dielectrics made by Zeller [4.3] has brought out certain important conclusions described in the following.

At fields closer to local breakdown i.e. PB inception at the tip of a sharp electrode, massive space charge injection from the electrodes into the dielectric is inevitable. The local electric stress dominated by the space charge may increase considerably. The electromechanical action due to electric force causes dielectric instabilities in solids, giving rise to initiation and growth of electrical trees, a conductive path with the dielectric.