Thermal breakdown:

This is due to the attainment of an excessive temperature in the dielectric. The electrical energy loss has to be dissipated as heat and if the heat dissipated is less than tha heat generated there is a progressive increase in the temperature of the dielectric which eventually results in local melting. During that time enormous current will flow through the material and immediately dielectric breakdown will occur.

Characteristics

1. This can occur only at high temperatures.

2. The strength of the electric field to create dielectric breakdown depends upon the materials size and shape.

3. The breakdown time is of the order of few milliseconds.

4. Since the dielectric loss is directly proportional to frequency, the electric field strength to create this dielectric breakdown  will be smaller for alternating fields and higher for D.C. fields.