The insulation characteristics of protective relays and their associated auxiliaries have been the concern of relay engineers since the early 1920's. Until about 1954, relays utilized principally electromechanical construction. Some investigations were made during this period on the effects of surges on these relays. This has been reported in the literature. The development of the transistor resulted in semiconductor components being used in both relay and communication equipment. Field experience indicated that these components are sensitive to certain electrical surges. Since about 1960, considerable amount of investigation has been done relating to the nature of these surges and methods for their control in relation to static relaying. The results of some of these studies are given in Refs [4] through [17]. These investigations and others were used as a basis by the IEEE Relay Committee's Working Group on Static Relay Surge Protection to develop the surge withstand capability (SWC) test for relays.