In electrical machines of small sizes, Class “A” insulating materials are normally used.

Class “B” insulation system is mostly used in case of medium and large machines. The insulating materials belonging to class ‘B’ have hot spot withstand capability of 130^oC.

This insulation consist of materials (or) combinations of materials which by experience (or) accepted tests can be shown to be capable of operating at class E temperature (materials possessing a degree of thermal stability allowing to be operated at a temperature 15^oC higher than class A materials). i.e. 105^oC.

With the rapid progress in the material technology, new insulating materials such as class ‘F’ insulation system with match higher hot spot temperature capability on now available which can replace the traditional class ‘B’ materials which a substantial improvement in the Kw/Kg ratio, reduction in size of the machines thereby reducing the consumption of precious active material such as copper and sheet steel. The insulating materials belonging to class ‘F’ have hotspot withstand capability of 150^oC.

‘F’ class insulating materials consist of the combination of mica, glass, fibre, asbestos with suitable bonding substances.

i. Excellent dissipation factor

ii. Longer life to the machine

iii. Better chemical resistance

iv. Higher flame resistance characteristic

v. Imperviousness to moisture and humidity

vi. Reduction in insulation thickness

vii. Better heat transfer in the coil of the winding

viii. Improved radiation resistance properties.

This insulation consist of materials, such as silicon elastomer and combination of materials such mica, glass fibre, asbestos, etc., with suitable bonding substance such as silicon resins.

Class ‘C’ materials are not directly involved in machine design. Mica, ceramics, glass, quartz without binders or with silicon resins of higher thermal stability. The insulating belonging to class ‘C’, have hotspot withstand capability of above 180^oC.