Rationalization of transmission-system insulation strength — III: Margins between basic insulation levels and protectable levels

SINCE the subject of “Rationalization of Transmission-System Insulation Strength”¹ was first presented in a comprehensive manner in 1928, a great deal of constructive study and analysis have been devoted to the problem, and outstanding progress has been made in placing the insulation of an electric system on a more rational basis from the lightning or impulse viewpoint. Three groups have been most active in this work; the equipment manufacturers, the laboratories, and the users of equipment. Particular attention has been given by the manufacturers to increasing and protecting the impulse strength of various types of commercial apparatus. The laboratories have worked on obtaining fundamental impulse data on equipment impulse strength, and have set up approved methods of testing. Both the above groups as well as the users of equipment have co-operated in field investigations to determine the characteristics of natural lightning. Finally, the users, in co-operation with both the other groups, have worked untiringly to co-ordinate the entire work into a rationalized insulation system. This principle of insulation rationalization should not be lost sight of in the individual or group efforts on insulation co-ordination to develop particular equipment to meet definite impulse strengths. That is, the fundamental problem is one of rationalizing system insulation strength by co-ordinating the insulation strength of the various items of equipment constituting or forming a part of the transmission or distribution system.