Although the impulse voltage testing of transformers has long been recognized as a necessity, to ensure satisfactory design, there has hitherto been no entirely satisfactory method for detecting the occurrence, during such tests, of insulation failures—particularly of the type which impulse tests are especially liable to produce; namely between sections or between turns of the winding. Attention is drawn to a new fault-detection technique recently proposed in the U.S.A., and a summary is given of published information on this subject from various American and European sources.Details are presented of some exploratory experimental work done by the authors, with a view to ascertaining the usefulness of the new technique, and a brief outline is given of the theoretical principles on which it depends. This is followed by a review of the authors' experiences in the practical application of the new methods to the routine impulse testing of 3-phase power transformers over a period of about 2 years. The various practical difficulties encountered are discussed, and means by which they may be avoided are indicated. Tentative test circuits for various types of 3-phase transformers are proposed.It is concluded that the technique described permits the condition of the transformer after impulse tests to be established with confidence. In the authors' opinion the results obtained merit the introduction of the new technique as a standard procedure in the impulse testing of transformers.