IEC 60172:2020 specifies, in accordance with the provisions of IEC 60216-1, a method for evaluating the temperature index of enamelled wire, varnished or unvarnished with an impregnating agent, and of tape wrapped round and rectangular wire, in air at atmospheric pressure by periodically monitoring changes in response to AC proof voltage tests. This procedure does not apply to fibre-insulated wire or wire covered with tapes containing inorganic fibres. NOTE The data obtained according to this test procedure provide the designer and development engineer with information for the selection of winding wire for further evaluation of insulation systems and equipment tests. This fifth edition cancels and replaces the fourth edition published in 2015. This edition constitutes a technical revision. This edition includes the following significant technical changes with respect to the previous edition: - revision of 3.1, definition of thermal index; - revision of 3.3, time to failure; - revisions to 5.1.1 for clarity and to reduce the range wire size range to which the test applies; - revisions to 5.1.2 for tape wrapped round and enamelled or tape wrapped rectangular wire for clarity; - revision to Clause 9 to add the correlation coefficient, r to the report.