---
title: "Temperature and life of insulating materials"
subject: "Electric Machine Design"
chapter: 1
key: "semester_5/electric_machine_design:temperature-insulation-life"
source: https://www.bctengineer.com/q/semester_5/electric_machine_design/temperature-insulation-life
site: BCT Engineering
---

# Temperature and life of insulating materials

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

- Asked in: 2072 Chaitra, 4 marks
- Video: [Insulation importance and factors affecting the life of insulator](https://www.youtube.com/watch?v=QTej22r4BRg) — Dipak Yeole
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/electric_machine_design/temperature-insulation-life
- Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machine_design/chapter-1

## Question

What is the importance of temperature as a factor in the life of insulating materials?

## Short answer

Temperature governs insulation life by accelerating chemical degradation (oxidation, depolymerization), causing brittleness and loss of dielectric strength. Montsinger’s rule states life halves for every 8–10°C rise above the rated limit; hot-spot temperature must stay within the insulation class to achieve 20–25 year life.

## Answer

Temperature is the **most important factor** deciding the life of insulating materials, and so the life of the machine. A machine's rating is in fact limited by the temperature its insulation can safely withstand.

#### Why temperature matters
- At high temperature the organic materials in insulation (paper, cotton, varnish, resins) undergo slow **chemical changes**: oxidation, loss of volatile matter and depolymerisation.
- The insulation becomes **dry, brittle and cracked**. Its mechanical strength falls first; the vibration and electromagnetic forces of normal service then crack it, and its dielectric strength is lost. Breakdown follows.
- The rate of these chemical reactions follows the **Arrhenius law**, so it rises rapidly with temperature:
$$
L = A\, e^{B/T}
$$
where $L$ is life, $T$ is absolute temperature, and $A$, $B$ are constants of the material.

#### Montsinger's rule (10-degree rule)
A practical form of the Arrhenius law: **the life of insulation is roughly halved for every 8–10 °C rise** in operating temperature above its rated limit.

$$
L = L_0 \times 2^{-(\theta-\theta_0)/10}
$$

For example, class A insulation running at 115 °C instead of 105 °C would last only about half as long.

#### Consequences for design
- Insulation is grouped into thermal classes (Y 90, A 105, E 120, B 130, F 155, H 180, C >180 °C). Each class's limit gives an economic life of about 20–25 years.
- The design must keep the **hot-spot** temperature (not the average) below the class limit. Permissible temperature rise = class limit − ambient (40 °C) − hot-spot margin.
- Short overloads are allowed only while the temperature stays within limits. Thermal time constants decide how long these overloads can last.

## Check yourself

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

1. How does high temperature shorten the life of organic insulation?
   A. It slowly causes chemical changes (oxidation, depolymerisation) that make it brittle
   B. It makes insulation stronger
   C. It only affects the conductor
   D. It increases dielectric strength
   Answer: A. Heat causes slow chemical degradation, then cracking by vibration and forces and loss of dielectric strength. (Key point: Cause of ageing)

2. According to Montsinger's rule, what happens to the insulation life for each 8-10 °C rise above the rated limit?
   A. It is unchanged
   B. It falls by 10 %
   C. It doubles
   D. It is roughly halved
   Answer: D. Life is roughly halved for each 8-10 °C rise, a practical form of the Arrhenius law. (Key point: Montsinger rule)

3. Which expression describes life L against absolute temperature T?
   A. L = A/T^2 only
   B. L is independent of T
   C. L = A e^(B/T), so life falls rapidly as T rises
   D. L = A T
   Answer: C. Reaction rate rises rapidly with temperature, so life falls sharply. (Key point: Arrhenius law)

4. Which temperature must be kept below the class limit in design?
   A. The ambient temperature only
   B. The hot-spot temperature
   C. The average temperature
   D. The surface temperature of the frame
   Answer: B. Insulation near the hottest point ages fastest, so the hot-spot value governs. (Key point: Design limit)

## Related in Electric Machine Design

- [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
- [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
- [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
- [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×
- [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×
- [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

## Related in other subjects

- [Insulating materials in brief](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Ainsulating-materials) — Civil Engineering Materials, chapter 10, asked 3×
- [Temperature difference across insulating layer, 1600 W](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Ainsulating-layer-temp-difference-1600w) — Fundamental of Thermodynamics & Heat Transfer, chapter 7
- [Sound insulating materials](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Asound-insulating-materials) — Civil Engineering Materials, chapter 10, asked 2×
- [Gumbel design flood, factor of safety 1.3, 45-year life](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Agumbel-25yr-life45-fos1-3) — Engineering Hydrology, chapter 6