---
title: "Classes of insulation by thermal stability"
subject: "Electric Machine Design"
chapter: 1
key: "semester_5/electric_machine_design:insulation-thermal-classes"
source: https://www.bctengineer.com/q/semester_5/electric_machine_design/insulation-thermal-classes
site: BCT Engineering
---

# Classes of insulation by thermal stability

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

- Asked in: 2080 Bhadra, 5 marks; 2078 Bhadra, 5 marks; 2074 Chaitra, 4 marks; 2070 Chaitra, 5 marks (asked 4 times)
- Video: [What is Insulation class | Types of Insulation Class | V-12](https://www.youtube.com/watch?v=czv8zMtE-So) — Electrical Decodes
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/electric_machine_design/insulation-thermal-classes
- Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machine_design/chapter-1

## Question

What are the different classes of insulation used in electrical machine? Mention their maximum working temperature and the types of material used in such classes.

## Short answer

Classes with hot-spot temperature limits: Y (90°C, cotton), A (105°C, impregnated paper), E (120°C, epoxy enamels), B (130°C, mica/organic binder), F (155°C, mica/epoxy), H (180°C, silicone resins), C (>180°C, ceramics). Binder determines thermal class.

## Answer

Insulating materials are grouped into **thermal classes** by the maximum temperature they can withstand continuously and still give a reasonable service life. The classes are standardised in IS 1271 / IEC 60085.

| Class | Max. temperature | Typical materials |
|---|---|---|
| Y | 90 °C | Cotton, silk, paper, wood and pressboard **without** impregnation; PVC, natural rubber |
| A | 105 °C | Class Y materials **impregnated** with varnish or immersed in oil (transformer oil); enamelled (oleo-resinous) wire |
| E | 120 °C | Polyurethane and epoxy-resin wire enamels, cellulose triacetate film, PET fibre, phenol-formaldehyde laminates |
| B | 130 °C | Mica, glass fibre and asbestos with organic binders (shellac, bitumen, alkyd) |
| F | 155 °C | Mica, glass fibre and asbestos bonded with epoxy, polyester or alkyd resins of higher thermal stability |
| H | 180 °C | Mica, glass fibre and asbestos bonded with **silicone** resins; silicone rubber; polyimide (Kapton, Nomex) |
| C | above 180 °C | Mica, porcelain, ceramics, glass and quartz with or without inorganic binders; PTFE (Teflon) |

Newer editions of IEC 60085 also give classes of 200, 220 and 250 °C. These replace the open-ended class C.

#### Points to remember
- The temperature limit is the **hot-spot** temperature of the insulation, not the ambient. Permitted temperature *rise* = class limit − ambient (40 °C) − hot-spot allowance.
- Organic materials (cotton, paper, silk) char at high temperature, so they belong to the low classes Y and A. Inorganic materials (mica, glass, ceramics) belong to the high classes.
- In classes B, F and H the inorganic base material is the same. The class is set by the **binder** (varnish/resin), because the binder fails first.
- Insulation life roughly halves for every 8–10 °C rise above the class limit (Montsinger's rule). This is why the limits must be respected.

#### Use in machines
- Small motors and fractional-kW machines: class A or E.
- Most modern industrial induction motors: class F insulation, designed for class B temperature rise, which gives a thermal margin.
- Traction motors and machines in hot places: class H.
- Oil-immersed transformers: class A (paper and pressboard in oil).

## Check yourself

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

1. Which pairing of class and maximum temperature is correct?
   A. Class A - 90 °C
   B. Class B - 130 °C
   C. Class F - 180 °C
   D. Class E - 105 °C
   Answer: B. Y 90, A 105, E 120, B 130, F 155, H 180 °C. (Key point: Class temperatures)

2. What decides whether a mica/glass insulation is class B, F or H?
   A. The thickness of the mica
   B. The inorganic base material
   C. The binder (varnish/resin), since it fails first
   D. The ambient temperature
   Answer: C. The inorganic base is the same; the binder fails first, so it sets the class. (Key point: Binder sets class)

3. According to Montsinger's rule, what happens to insulation life for 8-10 °C rise above the class limit?
   A. It doubles
   B. It roughly halves
   C. It is unchanged
   D. It falls to zero
   Answer: B. Life roughly halves per 8-10 °C rise, so limits must be respected. (Key point: Life rule)

## Related in Electric Machine Design

- [Temperature gradient in slot conductors (thick insulation)](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Atemperature-gradient-slot-thick) — Electric Machine Design, chapter 2, asked 2×
- [Maximum temperature of slot conductor](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Amax-conductor-temperature-slot) — Electric Machine Design, chapter 2
- [Internal temperature of a homogeneous material](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Ainternal-temperature-homogeneous-material) — Electric Machine Design, chapter 2
- [Machine temperature after one hour of heating](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Amachine-temperature-after-hour) — Electric Machine Design, chapter 2
- [Temperature difference in slot copper strips](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Aslot-strips-temperature-difference) — Electric Machine Design, chapter 2
- [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

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