IEC 60076-3:2013 specifies the insulation requirements and the corresponding insulation tests with reference to specific windings and their terminals. This International Standard applies to power transformers as defined by IEC 60076-1. It also recommends external clearances in air. It gives details of the applicable dielectric tests and minimum dielectric test levels. Recommended minimum external clearances in air between live parts and between live parts and earth are given for use when these clearances are not specified by the purchaser. For categories of power transformers and reactors which have their own IEC standards, this standard is applicable only to the extent in which it is specifically called up by cross reference in the other standards. This third edition of IEC 60076-3 cancels and replaces the second edition published in 2000, and constitutes a technical revision. The main changes from the previous edition are as follows: - Three categories of transformer are clearly identified together with the relevant test requirements, these are summarised in Table 1. - Switching impulse levels are defined for values of Um > 72,5 kV. - The procedure for Induced voltage tests with PD has been revised to ensure adequate phase to phase test voltages. - The AC withstand test has been redefined (LTAC instead of ACSD). - Induced voltage tests are now based on Ur rather than Um. - New requirements for impulse waveshape (k factor) have been introduced. - Tables of test levels have been merged and aligned with IEC 60071-1:2010. - Additional test levels have been introduced for Um > 800 kV. - A new Annex E has been introduced, which sets out the principles used in assigning the tests, test levels and clearances in air.