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

# Insulating materials and their classification

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

- Asked in: 2076 Chaitra, 4 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-classification
- Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machine_design/chapter-1

## Question

Discuss in brief about insulating material with their classification.

## Short answer

Insulating materials are classified by state (solid, liquid, gas), origin (organic/inorganic), and thermal class (Y 90°C to C>180°C). Examples: mica, paper, pressboard for solids; transformer oil for liquids; air, SF6 for gases. Thermal classes set maximum working temperatures.

## Answer

An **insulating material (dielectric)** is a material with very high resistivity (about $10^{9}$–$10^{20}\ \Omega$m). It prevents the flow of current between conductors at different potentials, and between conductors and earthed parts.

#### Classification
**1. By physical state**
- **Solid:** mica, paper, pressboard, cotton, glass fibre, porcelain, rubber, PVC, polyester films, epoxy resins, varnishes.
- **Liquid:** mineral transformer oil, silicone oil, synthetic esters (askarels are no longer used).
- **Gaseous:** air, nitrogen, hydrogen (cooling of large alternators), SF₆ (switchgear).

**2. By origin/structure**
- **Organic:** paper, cotton, silk, wood, rubber, resins and plastics. Flexible and cheap, but limited to low temperatures.
- **Inorganic:** mica, glass, porcelain, asbestos and ceramics. They withstand high temperature but are brittle.

**3. By thermal class (IS 1271 / IEC 60085)**

| Class | Y | A | E | B | F | H | C |
|---|---|---|---|---|---|---|---|
| Max. temp (°C) | 90 | 105 | 120 | 130 | 155 | 180 | >180 |

Class Y and A are unimpregnated and impregnated organic materials. B, F and H are mica and glass with binders of increasing thermal stability. C covers pure inorganic materials.

## Check yourself

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

1. Which classes use mica and glass with binders of increasing thermal stability?
   A. Y, A and E
   B. B, F and H
   C. C only
   D. A and Y
   Answer: B. B, F, H are mica/glass with organic, epoxy-polyester and silicone binders. (Key point: Classes)

2. What is the drawback of inorganic insulators like mica and porcelain compared with organic ones?
   A. They are brittle
   B. They char at low temperature
   C. They are flammable
   D. They have low resistivity
   Answer: A. Inorganic materials withstand high temperature but are brittle; organic ones are flexible but low-temperature. (Key point: Organic vs inorganic)

3. What is the typical resistivity range of insulators?
   A. 10^-8 to 10^-6 Ωm
   B. 10^9 to 10^20 Ωm
   C. 1 to 100 Ωm
   D. 10^2 to 10^4 Ωm
   Answer: B. Insulators have very high resistivity, about 10^9 to 10^20 Ωm. (Key point: Insulator resistivity)

## Related in Electric Machine Design

- [Insulating materials: classes, requirements and properties](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Ainsulating-material-properties) — Electric Machine Design, chapter 1
- [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×
- [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
- [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
- [Temperature and life of insulating materials](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Atemperature-insulation-life) — Electric Machine Design, chapter 1

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