This part of IEC 60034 describes thermal endurance test procedures for classification of insulation systems used in a.c. or d.c. rotating electrical machines with indirect cooling and form-wound windings. The test performance of a candidate insulation system is compared to the test performance of a reference insulation system with proven service experience. The test procedures described in IEC 60034-18-31 are intended to compare the thermal endurance performance of the mainwall insulation between conductor(s) and ground and,where required by the design of the coil or bar, the insulation between the turns. The test is not intended to simulate the in-service mechanical stresses experienced by the endwinding bracing or support materials. It does not include the evaluation of thermomechanical deterioration by expansion and contraction of insulation during temperature cycling. IEC 60034-18-1 describes general testing principles applicable to thermal endurance testing of insulation systems used in rotating electrical machines. The principles of IEC 60034-18-1 are followed unless otherwise stated in IEC60034-18-31. The thermal class for the insulation system refers to its maximum allowed (“hot spot”) temperature. The average temperature measured in service should not exceed the allowed temperature rise according to IEC 60034-1. NOTE 1 Large machines, especially synchronous generators using bars, may require special thermal evaluation test procedures which are not included in this part. NOTE 2 Recommended parameters for the diagnostic test may be applied according to IEC 60034-18-42 to formwound coils designed with Type II insulation systems for use in converter applications.