The different classes are defined by NEMA,[1] Underwriters Laboratories (UL),[2] and IEC standards.
Older editions of standards listed materials to be used for the various temperature classes.
Modern editions of standards are proscriptive, only indicating that the insulation system must provide acceptable life at the specified temperature rise.
In large machines, different systems may be used according to the predicted temperature rise of the machine; for example, in large hydroelectric generators, stator windings may be Class B but the more difficult to cool rotor winding may be Class F. In IEC standards, the insulation system is a classification based on the level of electrical shock protection given to a user.
Basic insulation is any material added to protect a user from accidental contact with energized parts.