IEC 60370:2017 covers methods of test for the determination of thermal endurance (temperature index) of electrical insulating resins and varnishes for impregnation purposes. It is done by means of impregnating glass cloth and measuring electric strength or breakdown voltage before and after heat ageing. It covers the materials described in IEC 60455-3-5 and IEC 60464-3-2 and similar materials. This second edition cancels and replaces the first edition published in 1971. This edition constitutes a technical revision. This edition includes the following significant technical changes with respect to the previous edition:- this document is now describing two methods, the existing one, following ASTM D1932 and new a method following the requirements of IEC 60455-2 and IEC 60464-2;- the theoretical background and way of calculation were removed, and replaced by reference to IEC 60216;- the layout and numbering system was updated;- for better understanding and illustration purposes examples were added.Keywords: thermal endurance testing