--- title: "Dielectric strength and types of breakdown" subject: "Electrical Engineering Material" chapter: 3 key: "semester_3/electrical_engineering_material:dielectric-strength-breakdown" source: https://www.bctengineer.com/q/semester_3/electrical_engineering_material/dielectric-strength-breakdown site: BCT Engineering --- # Dielectric strength and types of breakdown Electrical Engineering Material · Chapter 3: Dielectric Materials - Asked in: 2070 Asar, 6 marks - Video: [5 Insulator Breakdown & Dielectric Strength](https://www.youtube.com/watch?v=fn2BMuRzEow) — HVdesc - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/electrical_engineering_material/dielectric-strength-breakdown - Whole chapter: https://www.bctengineer.com/md/semester_3/electrical_engineering_material/chapter-3 ## Question What is dielectric strength of a dielectric material? Discuss in brief the different types of breakdown in dielectric material. ## Short answer Dielectric strength is the maximum field before breakdown, E_br = V_br/d (e.g., air ~3 kV/mm). Breakdown types: intrinsic (avalanche), thermal (runaway heating), electromechanical (compression), partial discharge (void erosion), electrochemical ageing, and surface tracking/flashover. ## Answer **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). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Dielectric strength of a material is best defined as: A. Maximum electric field it withstands without breakdown, V_br/d B. Its permittivity relative to air C. The minimum voltage that polarizes it D. Its resistance per unit thickness Answer: A. E_br = V_br/d, usually in kV/mm. (Key point: Definition) 2. What causes intrinsic (electronic) breakdown? A. Heating by leakage current B. Electron avalanche by collision ionization in a very strong field C. Gas voids discharging inside the solid D. Surface dirt and moisture Answer: B. Free electrons gain energy from the field and ionize atoms, multiplying in an avalanche. (Key point: Intrinsic breakdown) 3. Why does thermal breakdown run away? A. The electrodes attract and compress the material B. Ions migrate slowly over years C. Heat generated exceeds heat lost, and conductivity rises with temperature D. Voids have lower permittivity Answer: C. Heating raises conductivity, which gives more heating, until the material melts or burns. (Key point: Thermal breakdown) 4. Why do small gas voids in a solid ionize first? A. They have higher strength than the solid B. They are at higher temperature C. They carry no field D. They have lower permittivity and strength than the solid Answer: D. Voids take a higher field and break down early, and repeated discharges form treeing channels. 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