Lists the required test procedures for qualification of modifications of an established electrical insulation system (EIS) with respect to its thermal classification. Applicable to EIS used in wire-wound electrotechnical devices with input voltage up to 1000 V. The test procedures are comparative in that the performance of a candidate EIS is compared to that of a reference EIS, which has proven service experience in accordance with IEC 60791 or has been evaluated by one of the procedures given in IEC 61857 series.