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The concept of the evaluation of the thermal ageing behaviour of machine winding insulations goes back to the work done by Arrhenius, Montsinger, Bussing and Dakin. Its basic principles are found in the chemical reaction kinetics but it also takes decades of experiences into account, which were made in order to make sure, that insulation systems for rotating electric machines are both: economic and reliable. Today there is a set of IEC and IEEE standards available, which define the procedures needed for the qualification of winding insulation systems. They always use the approach of comparative functional evaluation, which compares the behaviour of an unknown candidate system with a service proven reference system. As these standards are reviewed and revised, they will be even more practicable in the future.