There are three types of breakdowns which are possible in solid dielectrics:

• Electrothermal breakdown
• Purely electrical breakdown, and
• Electrochemical breakdown

Out of the above-mentioned breakdowns, the electrothermal and purely electrical breakdown is of significant importance in practice.

• Electrothermal breakdown – This breakdown occurs due to heat produced by the dielectric loss which is proportional to the intensity of the electrostatic field and the frequency. It is observed that the heat generated due to dielectric loss varies exponentially with the temperature whereas the heat dissipated to the cooler surrounding medium will be directly proportional to (1) temperature difference (2) the thermal conductivity of the dielectric, and (3) the coefficient of heat transmission of the surface of the dielectric to the ambient medium.
• Due to this low conductivity, the solid dielectrics are poor conductors of heat. At a given applied voltage, if the heat generated due to the losses is greater than the heat dissipated and if the voltage applied is for a long enough period then the dielectric will be unable to reach a state of internal thermal equilibrium which will result in an electrothermal breakdown of the dielectric.
• Following conditions are mainly responsible for the electrothermal breakdown of a dielectric:
• Large thickness of the dielectric material.
• The high temperature of both the dielectric material and the surrounding medium.
• Application of high voltage continuously.
• Large dielectric loss.
• Purely electrical breakdown – It is also known as intrinsic breakdown mechanism. It is caused due to collision ionization by electrons. When a strong electric field is applied, the free electrons in the crystals are accelerated and a part of the kinetic energy so obtained is lost through interaction with the lattice points. It has been experimentally observed that the lattice energy is directly proportional to the dielectric strength. So in order to observe a collision ionization, it is necessary that the accelerated free electrons should gain an amount of kinetic energy equal to a certain portion of the lattice energy.
• This breakdown can occur in the following conditions:
• Moderate temperature
• Low frequency
• The small thickness of the dielectric

The electric breakdown will occur only when impulse voltages are applied.

• Electrochemical breakdown – This type of breakdown usually occurs when the temperature is very high and the surrounding air has high humidity. The insulation resistance of a material is reduced by the dielectric processes developed in the dielectric material. This situation comes when a D.C. voltage is continuously applied to a dielectric whose leakage current due to ionic conductivity is adequately large.