The insulating classes differ insulating materials into temperature areas for which these materials suit. The insulating materials become less effective because of the permanent existent thermal loss of the electric motor, transformer, generator and electrical devices.
This chemical ageing process is sped up exponential when the temperature increases. Because of that insulating materials and varnishes of the equivalent classes have to be used, to ensure a sufficient life expectancy of the insulating.
More information to the insulating classes can be found in the IEC 85, the IEC 216 as well as VDE 0530, part 1.