The paper gives a practical explanation of the behaviour of power transformer windings subjected to incoming lightning and switching surges.A method of analysis is developed covering practical transformer windings, of various types and ratings, subjected to applied surges of various shapes. Stress distribution, and the ultimate breakdown strength of the complete transformer, are determined.This analysis leads to conclusions concerning methods of improving the surge strength, such as by electrostatic shielding or by stress control. These conclusions are substantiated by low-voltage surge-distribution tests, and by full-scale tests to destruction. Examples are described.Finally, so that the results of this work may be realized in practice, certain changes in purchasing specifications for transformers are suggested