Since it develops very rapidly, the intrinsic breakdown is assumed to have electronic origin. The most simple theoretical approach for intrinsic breakdown has been to derive a theory for an electronic instability in the dielectric subjected to a uniform field, O'Dwyer [4.2]. While measuring the intrinsic strength, if there exists a cause which leads to other than electronic instability, the value of electric strength measured may be anywhere below intrinsic strength. One such cause is 'thermal instability', particularly in the dielectrics having sufficient conductivity. The conductivity is found to increase with electric stress, but the rate of increase may not be linear, Lawson [4.1]. The 'hot electron' theory of breakdown is solid dielectrics developed by Frohlich and Paranjape [4.2] also predicted a stress-enhanced conductivity, which may cause thermal failure before a true instability (intrinsic failure) occurs. This theory involves lattice temperature rise processes and is derived for intrinsic or near intrinsic breakdowns.

The experimental concept of 'thermal breakdown' is different as it is based upon self heating of the dielectric due to power losses. Because of power loss due to conductivity, polarization or other forms of dielectric losses, heat is produced continuously in electrically stressed dielectrics. Depending upon the magnitude of the applied voltage, its period of application and the conduction of heat the dielectric temperature rises. If the heat generated within a dielectric system equals the dissipation of heat to the surroundings by thermal conduction, the temperature rises to an equilibrium value, and a thermally balanced and stable operation of the insulation system takes place. But in practice, the power losses in a dielectric are some function of temperature also. Since the ionic conductivity increases with increasing temperature, the power losses increase with temperature. If the dissipation of heat by cooling processes is not adequate, it is possible that an unstable state may arise in which the temperature increases without limit causing a breakdown.