The life of electrical insulating materials gets adversely affected by the rise in temperature.
Average insulation life gets halved for every 10° C increase in temperature. Therefore, the classification of insulating materials is done on the basis of the maximum temperature that insulating material can withstand.
Insulating Materials Classifications
The insulating materials are grouped into the following categories.
| Insulation Class | Max Permissible temperature (° C) |
| Y | 90 |
| A | 105 |
| E | 120 |
| B | 130 |
| F | 155 |
| H | 180 |
| C | Above 180 |
Class-Y Insulation
The insulating materials of class-Y insulation can withstand temperatures up to 90° C. Examples of class-Y insulating materials are made of cotton, silk, paper, wood, and press wood.
Class-A Insulation
The temperature withstands-capacity of class-Y insulation can be improved by impregnation of class-Y insulation with varnish or insulating oil. The class-A insulating material can withstand temperature up to 105° C. Class-A insulation is used in oil filled transformer. Examples of class-A insulating materials are impregnated cotton, paper, oil, enamels, laminated wood, etc.
Class-E Insulation
The maximum temperature capacity of class-E insulating materials is 120° C.
Class-B Insulation
The maximum temperature withstands capacity of class-B insulating material is 130° Examples of class-B insulating materials are wire enamel based on polyvinyl formal, polyurethane or epoxy resin, phenol-formaldehyde moldings of cotton, paper, etc,, cellulose triacetate film, etc.
Class-F Insulation
The maximum temperature withstands capacity of class-F insulation is 155° C. Examples of class-F insulating materials are alkyd, epoxy polyester, silicon alkyd, silicon phenolic resin, impregnated glass fiber cloth, built-up mica with alkyd, epoxy cross-linked polyester and polyurethane resins with superior thermal stability silicone alkyd resins.
Class-H insulation
The temperature withstands-capacity of class-H insulating materials is 180° C. Examples of class-H insulating materials are silicon varnished impregnated glass fiber cloth, mica, and silicon. Resin bonded up mica.
Class-C Insulation
Class-C insulating materials can withstand temperatures above 180°. Examples of class-C insulating materials are mica, porcelain glass quartz, asbestos, and built-up mica-treated Glass fiber cloth.