This part of IEC 60034 gives test procedures for the thermal evaluation and classification of changes and insulation component substitution in insulation systems used or proposed for use in a proven insulation system used in wire-wound windings. The test procedures are comparative in that the performance of a candidate system is compared to that of a reference system which has previously been proved by experience or has been evaluated by one of the procedures given in 60034-18-21 and to which the change or substitution is intended.