Proposed Standards for Outdoor Bushings

The AIEE joint committee on bushings has attempted to determine the electrical characteristics of bushings on an engineering basis. The recommendations in this report are based on the theory that an ample margin of insulation strength should be provided above the voltages to which the bushings may be subjected in service, including dynamic overvoltages, switching surges, and lightning surges up to the level of protection which can be provided with modern lightning protective devices. This should make it possible to use bushings having the same electrical characteristics for transformers, circuit breakers, and all types of apparatus of corresponding voltage ratings because, if bushings have an ample margin of insulation strength above the level of protection which can be provided, there should be no need for any higher insulation strength for bushings in circuit breakers than in transformers or other apparatus. Lower values of insulation strength may be assigned for bushings for small circuit breakers, distribution transformers, etc., where space or other limitations may require smaller bushings and where a smaller margin of protection may be satisfactory. The joint committee is continuing the study of bushings and will make additional recommendations covering indoor bushings as soon as agreement can be reached. They may also make recommendations covering bushings for lower-voltage ratings and possibly other characteristics of outdoor bushings.