The Indian standards of insulating materials form a convenient classification for electrical machinery and apparatus in relation to their thermal stability.

Class y. It includes materials or combinations of materials such as cotton, silk, paper, press board, wood, PVC with or without plasticizer vulcanized natural rubber. These materials can withstand temperature up to 90⁰C relining their insulating properties.

Class A. It includes materials or combinations of materials such as cotton, silk and paper when suitably impregnated or coated or when immersed in dielectric liquid such as oil. These can withstand temperature up to 105⁰C without loosing appreciable their insulating properties.

Class E. It includes materials or combinations of materials which by experience or accepted tests can be shown to be capable of operating at temperature up to 120⁰C.

Class B. It includes materials or combinations of materials such as mica, glass, fibre, asbestos etc, with suitable bonding substances. These can withstand temperature upto 130⁰C.

Class F. It includes materials of class B with more thermally resistant bonding materials. These can withstand temperature up to 155⁰C

Class H. It includes materials such silicon elastomer and combinations of materials such as mica, glass fibre, asbestos etc. with suitable bonding substances such as appropriate silicon resins. There can withstand temperature upto 180⁰C.

Class C. It includes materials or combinations of materials such as mica, porcelain, glass and quartz with or without an inorganic binder. They can operate at temperature above 180⁰C.

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