IEC 60034-18-31:2012 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. These test procedures 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 main technical changes with regard to the previous edition include: - Definition of the test method and sub-cycles required to establish a consistent standardized platform for thermal ageing of insulation systems for form-wound windings. - Recommendations for establishing a thermal life curve based on confidence intervals. - Comparison of candidate and reference system performance for specific requirements of thermal class, within feasible limits. Keywords: rotating machinery, insulation systems