---
title: "Insulating materials: classes, requirements and properties"
subject: "Electric Machine Design"
chapter: 1
key: "semester_5/electric_machine_design:insulating-material-properties"
source: https://www.bctengineer.com/q/semester_5/electric_machine_design/insulating-material-properties
site: BCT Engineering
---

# Insulating materials: classes, requirements and properties

Electric Machine Design · Chapter 1: Materials used in electrical equipment

- Asked in: 2072 Chaitra, 8 marks
- Video: [Insulating Materials | Electrical Machine Design #education #electrical #design #memes](https://www.youtube.com/watch?v=rT9JiS0hEow) — Prasant Decodes
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/electric_machine_design/insulating-material-properties
- Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machine_design/chapter-1

## Question

Discuss in brief about insulating material with their classification. What are the fundamental requirements of a good insulating material, electrical properties of insulating material?

## Short answer

Insulating materials have very high resistivity (10^9–10^20 Ωm) and are classified into solid, liquid, gas; organic/inorganic; and thermal classes Y to C. They must have high dielectric strength, high resistivity, low dielectric loss, high thermal endurance, good mechanical strength and be non-hygroscopic. Key electrical properties include insulation resistance, dielectric strength, permittivity, loss angle tanδ, tracking resistance and partial discharge resistance.

## Answer

An **insulating material** is a substance of very high resistivity that stops current flowing between parts at different potentials. In machines it insulates turns, coils and phases from each other and from the core and frame.

#### Classification
**(a) By state**
- **Solid:** mica, paper, pressboard, cotton, glass fibre, asbestos, porcelain, rubber, PVC, polyester film, epoxy and varnishes.
- **Liquid:** mineral transformer oil, silicone fluid, synthetic esters.
- **Gas:** air, nitrogen, hydrogen, SF₆.

**(b) By origin**
- **Organic** (paper, cotton, resins, plastics): flexible, but suited to low temperature.
- **Inorganic** (mica, glass, porcelain, ceramics): suited to high temperature, but brittle.

**(c) By thermal class (IS 1271 / IEC 60085)**

| Class | Max. temp | Typical materials |
|---|---|---|
| Y | 90 °C | Unimpregnated cotton, paper, silk |
| A | 105 °C | Impregnated cotton, paper; oil-immersed paper |
| E | 120 °C | Enamels, epoxy, polyester films |
| B | 130 °C | Mica, glass fibre with organic binders |
| F | 155 °C | Mica, glass fibre with epoxy/polyester binders |
| H | 180 °C | Mica, glass fibre with silicone binders |
| C | >180 °C | Mica, porcelain, glass, quartz, PTFE |

#### Fundamental requirements of a good insulating material
1. High dielectric strength and high resistivity.
2. Low dielectric loss and low power factor.
3. Able to withstand high temperature without deterioration; non-inflammable.
4. Good thermal conductivity and low thermal expansion.
5. Good mechanical strength, flexibility and resistance to vibration.
6. Non-hygroscopic; resistant to oil, chemicals and ozone.
7. Stable with age; easy to work; cheap.

#### Electrical properties of insulating materials
1. **Insulation resistance / resistivity.** Volume resistivity (through the body) and surface resistivity (along the surface) should be very high. Both fall with moisture and temperature.
2. **Dielectric strength.** The maximum voltage gradient (kV/mm) the material withstands before breakdown. It depends on thickness, temperature, moisture and the duration of voltage.
3. **Permittivity (dielectric constant $\varepsilon_r$).** In composite insulation the stress divides inversely as $\varepsilon_r$, so materials with similar $\varepsilon_r$ are preferred. A low $\varepsilon_r$ is preferred for cables.
4. **Dielectric loss and loss angle ($\tan\delta$).** The power lost in the dielectric is $P = \omega C V^2 \tan\delta$. A low $\tan\delta$ is needed to avoid heating and thermal breakdown.
5. **Surface tracking and arc resistance.** The ability to resist forming a conducting carbon track along the surface under discharge and contamination.
6. **Partial discharge resistance**, which matters in high-voltage machine insulation (mica is excellent here).

## Check yourself

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

1. What is the dielectric power loss?
   A. P = ωCV^2 tanδ
   B. P = V/R
   C. P = CV
   D. P = tanδ/ωC
   Answer: A. Dielectric loss is ωCV^2 tanδ, so low tanδ avoids heating. (Key point: Dielectric loss)

2. In composite insulation, how does electric stress divide between layers?
   A. Directly as εr
   B. Inversely as εr
   C. Equally
   D. Independent of εr
   Answer: B. Stress is inversely proportional to εr, so similar εr materials are preferred. (Key point: Permittivity in composite insulation)

3. How do moisture and temperature affect insulation resistivity?
   A. Both lower it
   B. Both raise it
   C. Moisture raises, temperature lowers
   D. No effect
   Answer: A. Both volume and surface resistivity fall with moisture and temperature. (Key point: Effect of moisture and temperature)

4. Which property makes mica excellent in HV machine insulation?
   A. Partial discharge resistance
   B. Hygroscopic nature
   C. High permittivity
   D. Low cost
   Answer: A. Mica resists partial discharge. (Key point: Mica)

## Related in Electric Machine Design

- [Good insulating material requirements and applications](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Ainsulating-requirements-application) — Electric Machine Design, chapter 1, asked 2×
- [Insulating material requirements and silicon steel](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Ainsulating-requirements-silicon) — Electric Machine Design, chapter 1, asked 2×
- [Insulating materials and their classification](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Ainsulating-material-classification) — Electric Machine Design, chapter 1
- [High conducting materials: requirements and classification](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Ahigh-conducting-materials) — Electric Machine Design, chapter 1, asked 3×
- [Insulating materials for different machines](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Ainsulator-materials-machines) — Electric Machine Design, chapter 1
- [Conducting versus insulating material characteristics](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Aconducting-vs-insulating) — Electric Machine Design, chapter 1

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