The following three types of breakdown are possible in Dielectrics:
(a) Electrothermal Breakdown - This is caused by the heat produced because of the dielectric loss which is proportional to the intensity of the electric field and the frequency.
(b) Purely Electrical Breakdown - This is basically caused because of the collision ionization by the electrons.
(c) Electrochemical Breakdown - This type of breakdown happens when the temperature is high and the surrounding air has huge humidity.