Insulating Material: Definition, Properties and Classification

Table Of Content:

1. Definition Of Insulating Materials
2. Properties Of Insulating Materials
3. Classification Of Insulating Materials

1. Definition Of Insulating Materials

Materials which are used for preventing the flow of current can be termed as non conducting or insulating materials, i.e. materials which offer such a high resistance that they do not allow current to pass through them are called insulating materials.

Their resistance is usually in Megaohm. They are non - metallic materials of high resistivity.

Insulating materials are available in a large variety e.g. mica, ceramics, rubber, glass, paper, porcelain, fibrous, plastic, transformer oil, air, SF6 gas, nitrogen, Freon etc.

2. Properties Of Insulating Materials

• High dielectric strength
• High insulation resistance
• Low dielectric constant or permittivity
• Low dielectric loss
• High mechanical strength
• High thermal or heat conductivity
• High degree of thermal stability. It should not be affected by change in temperature
• It should be uniform throughout.
• It should be free from gaseous not evaporate.
• Solid insulating materials should have high melting point.

3. Classification Of Insulating Materials

Insulating materials used in electrical machines and apparatus can be classified on the basis of following:

1. The physical state and
2. Thermal stability or maximum permissible temperature rise

(1) Classification on the basis of physical state:

(a) Solid insulating materials

Commonly used solid insulating materials are: 

• Mica
• Ceramics
• Glass
• Rubber
• Asbestos
• Porcelain
• Fibrous
• Plastic
• Silk
• Wood
• Epoxy resins
• ETC..

(b) Liquid insulating materials

Commonly used liquid insulating materials are:

• Varnish
• Transformer oil
• Silicone oils
• Cable oil
• Synthetic liquids
• ETC..

(c) Gaseous insulating materials

Commonly used Gaseous insulating materials are:

• Air
• Sulphur Hexafluoride
• SF6 gas
• Nitrogen
• Freon
• Halogens
• Hydrogen
• ETC..

(2) Classification on the basis of thermal stability or maximum permissible temperature rise:

Indian standards (IS: 271) has specified the temperature ranges for different insulating materials used in Electrical machines and apparatus is as follows:

(i) Class Y insulation

• Maximum working Temperature: 90° C

Example of class Y insulation:

• Cotton
• Paper
• Silk
• PVC and
• Materials which neither impregnated nor immersed in oil

(ii) Class A insulation

• Maximum working Temperature: 105° C

Example of class A insulation:

• Cotton
• Paper and 
• Silk

when impregnated with natural resins or immersed in a liquid dielectric ( oil ) mainly used in electrical machines.

(iii) Class E insulation

• Maximum working Temperature: 120° C

This class contains enamelled wire insulations on a base of

• Polyvinyl formal
• Polyurethane and epoxy resins
• Mouldings
• Powder plastics
• ETC...

(iv) Class B insulation

• Maximum working Temperature: 130° C

This class includes inorganic materials such as

• Mica
• Fiberglass
• Asbestos

which is impregnated with varnish and other compounds

(v) Class F insulation

• Maximum working Temperature: 155° C

This class includes

• Mica
• Polyester
• Epoxide varnishes and 
• Varnishes with a high heat resistance as compared with class B materials.

(vi) Class H insulation

• Maximum working Temperature: 180° C

This class includes composite materials of

• Mica
• Fiber glass and 
• Asbestos

Which bases impregnated with silicone rubber. These materials are mainly used in dry type transformer.

(vii) Class C insulation

• Maximum working Temperature: Above 180° C

This class includes

• Mica 
• Glass
• Teflon
• Quartz
• Ceramic