Although impulse-voltage acceptance tests are, by assumption, non-destructive when successful, it is not easy in practice to establish working criteria of insulation failure and to co-ordinate these criteria with observations. A tentative definition of impulse failure is used as a basis for considering the significance of indications given by electrical methods of fault detection applied to transformer windings. Exploratory tests on high-voltage motors are described, in which it was found that fault indications were likely to be obscured by disturbances arising from non-cyclic magnetization of the iron and from other causes not determined. Acceptance impulse testing for high-voltage capacitors and cables is briefly discussed. It is concluded that the techniques for acceptance testing of transformer windings are broadly satisfactory, but that, for motor windings, existing methods are inadequate.