American practice in d.c. insulation testing

The testing of electrical machinery, so that it may be withdrawn from service and re-conditioned at a time when continued operation might result in breakdown, is of great importance. Various methods are used, and among them is the testing of the insulation by the application of a direct voltage. This has been the subject of much discussion, but such methods are used extensively in the United States, despite their admitted shortcomings. In the paper brief descriptions of two of these methods are given, together with examples of their applications. It is suggested that these methods could well repay further study.