Dielectric strength and types of breakdown

Dielectric strength is the maximum electric field a dielectric can withstand without breakdown, $E_{br} = V_{br}/d$, usually in kV/mm or MV/m. Examples: air ≈ 3 kV/mm, transformer oil ≈ 10–15 kV/mm, mica ≈ 100 kV/mm. It depends on thickness, temperature, frequency, moisture, impurities and time of voltage application.

Types of breakdown

1. Intrinsic (electronic) breakdown

   • A few free electrons gain enough energy from a very strong field to ionize atoms by collision.
   • The number of electrons multiplies in an avalanche, and current rises suddenly.
   • Very fast (~$10^{-8}$ s), occurs in pure, defect-free material; gives the highest (ideal) strength.

2. Thermal breakdown

   • Leakage current and dielectric loss heat the material.
   • Conductivity rises with temperature, so heating increases further.
   • If heat generated exceeds heat lost, temperature runs away and the material melts or burns.
   • Depends on frequency, ambient temperature and cooling; takes seconds to minutes.

3. Electromechanical breakdown

   • Electrostatic attraction between electrodes, $\tfrac12\varepsilon_0\varepsilon_rE^2$, compresses soft materials (polymers).
   • Thickness falls, field rises, and the material collapses mechanically.

4. Discharge (partial discharge) breakdown

   • Small gas voids inside the solid have a lower permittivity (so the field in them is higher) and a lower dielectric strength, so they ionize first.
   • Repeated discharges erode the walls of the void and slowly form a conducting channel (treeing).

5. Electrochemical breakdown / ageing

   • Long-term chemical changes (oxidation, moisture, ion migration) caused by the field and temperature lower the insulation resistance until it fails.

6. Defect (surface) breakdown

   • Moisture, dirt or cracks on the surface create a leakage path along the surface (tracking, flashover).