---
title: "Thermal and electromechanical dielectric breakdown"
subject: "Electrical Engineering Material"
chapter: 3
key: "semester_3/electrical_engineering_material:thermal-electromechanical-breakdown"
source: https://www.bctengineer.com/q/semester_3/electrical_engineering_material/thermal-electromechanical-breakdown
site: BCT Engineering
---

# Thermal and electromechanical dielectric breakdown

Electrical Engineering Material · Chapter 3: Dielectric Materials

- Asked in: 2078 Kartik, 4 marks; 2075 Chaitra, 4 marks; 2072 Chaitra, 4 marks; 2071 Chaitra, 4 marks (asked 4 times)
- Video: [Electromechanical breakdown of Solid Dielectrics|Solid Dielectric Breakdown Phenomenon|HVE Lecture](https://www.youtube.com/watch?v=ctY5nzzP_ig) — Simplified EEE Studies
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/electrical_engineering_material/thermal-electromechanical-breakdown
- Whole chapter: https://www.bctengineer.com/md/semester_3/electrical_engineering_material/chapter-3

## Question

Describe how thermal breakdown and electromechanical breakdown results in dielectric breakdown in solids.

## Short answer

Thermal breakdown: dielectric heating raises temperature, increasing conductivity and loss; thermal runaway occurs if heat generation surpasses cooling, causing melting or charring. Electromechanical breakdown: electrostatic compressive stress reduces thickness, increasing field; if elastic restoring force is insufficient, mechanical collapse leads to breakdown.

## Answer

**Dielectric breakdown** is the sudden large increase of current through an insulator when the applied field exceeds a critical value (the dielectric strength), so that the material loses its insulating property, usually permanently in solids.

#### Thermal breakdown

1. When an AC or DC voltage is applied, a small leakage current and dielectric loss ($W = \omega\varepsilon_0\varepsilon_r'E^2\tan\delta$ per unit volume) heat the insulator.
2. The conductivity of an insulator rises steeply (exponentially) with temperature, and $\tan\delta$ also rises.
3. More conductivity gives more current and more heat. If heat is generated faster than it can be conducted away to the surroundings, the temperature rises without limit (**thermal runaway**).
4. The material melts, chars or burns, forming a conducting path, usually at the hottest point in the middle.

Features: it takes seconds to minutes to develop. The breakdown strength falls with higher frequency, higher ambient temperature and longer application time, and also with thicker insulation, because the interior is cooled less well. It is the most common breakdown mode in power equipment insulation (e.g. cable insulation).

```text
 heat
  ^       generated
  |         /
  |        /   ...lost (cooling)
  |  .....'
  |  stable / unstable -> runaway
  |____________________> T
```

#### Electromechanical breakdown

1. A voltage $V$ across a soft solid dielectric of thickness $d$ produces an electrostatic attraction between the electrodes (Maxwell stress) of $\frac{1}{2}\varepsilon_0\varepsilon_r\frac{V^2}{d^2}$ per unit area.
2. This compressive force squeezes the material, so $d$ decreases.
3. The field $V/d$ then increases, the force grows further, and the material is compressed even more.
4. If the elastic restoring stress (Young's modulus $Y$) cannot balance it, the insulation collapses mechanically and then breaks down.

Balancing the stresses, the material becomes unstable when $d$ falls to about $0.6\,d_0$, which gives the highest apparent field

$$
E_{max} \approx 0.6\sqrt{\frac{Y}{\varepsilon_0\varepsilon_r}}
$$

It occurs mainly in soft materials such as polymers (polyethylene, rubber) and at high temperature, where $Y$ is low.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What causes thermal runaway in thermal breakdown?
   A. Heat generated by leakage and loss exceeds heat conducted away, and conductivity rises with temperature
   B. The electrons gain enough energy to ionize atoms by collision
   C. Electrodes attract each other and squeeze the insulator
   D. Voids inside the insulator discharge
   Answer: A. More heat raises conductivity, giving more current and more heat; if cooling cannot keep up, temperature rises without limit. (Key point: Thermal runaway)

2. In electromechanical breakdown, why does the field keep increasing?
   A. The voltage is increased automatically
   B. Compression reduces thickness d, so V/d rises and the force grows
   C. The Young's modulus increases
   D. The permittivity falls to zero
   Answer: B. Electrostatic attraction compresses the soft solid, decreasing d, raising E = V/d and the compressive force further. (Key point: Mechanism of electromechanical breakdown)

3. Electromechanical breakdown mainly occurs in:
   A. Hard ceramics at low temperature
   B. Gases at low pressure
   C. Soft materials such as polymers and rubber, especially when hot
   D. Pure defect-free crystals
   Answer: C. A low Young's modulus (soft or hot material) lets the material be compressed until it collapses at about 0.6 d0. (Key point: Where each mode occurs)

## Related in Electrical Engineering Material

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- [Types of dielectric breakdown](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Adielectric-breakdown-types) — Electrical Engineering Material, chapter 3, asked 2×
- [Dielectric strength and types of breakdown](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Adielectric-strength-breakdown) — Electrical Engineering Material, chapter 3
- [Breakdown mechanisms; electronic vs orientational polarization](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Abreakdown-and-polarization-types) — Electrical Engineering Material, chapter 3

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