Types of dielectric breakdown

Dielectric breakdown is the sudden loss of insulating property of a dielectric when the applied field exceeds a critical value called the dielectric strength $E_{br}$; a large current flows and solids are usually damaged permanently.

Types of breakdown in solids

1. Intrinsic (electronic) breakdown
2. Thermal breakdown
3. Electromechanical breakdown
4. Discharge (partial-discharge) breakdown in internal voids
5. Electrochemical breakdown (slow chemical ageing of the insulation)

Intrinsic breakdown

• Depends only on the material itself, under pure, defect-free conditions; it occurs in about $10^{-8}$ s.
• The few free electrons in the conduction band gain energy from the field. When the field is high enough, they gain more energy than they lose to the lattice and can ionize atoms by impact, freeing more electrons. This chain reaction is an electron avalanche, and the current rises sharply.
• It sets the highest possible strength of a material (e.g. about $10^{8}$–$10^{9}$ V/m for good insulators). It is almost independent of temperature and thickness.

 e- -> * -> 2e- -> * -> 4e- -> 8e- ...
       ionising collisions (avalanche)

Thermal breakdown

• Leakage current and dielectric loss ($\propto \omega E^2\tan\delta$) heat the dielectric.
• Conductivity of insulators rises exponentially with temperature, so the current and heat increase further.
• If heat generated exceeds heat conducted away, temperature runs away until the material melts or chars.
• Slow (seconds to minutes); breakdown voltage falls with higher frequency, ambient temperature and longer stress. Common in power cables and capacitors.