Metal-Clad 4,160-Volt 350-Mva Switchgear with Symmetrical Ratings

Air break power circuit breakers of the magnetic type have been used for 2,300-and 4,160-volt service for almost the last 20 years. These breakers have been built in interrupting ratings from 50 to 250 mva (megavolt-amperes). Within the last 2 years a need has been established for breakers of higher interrupting and continuous current-carrying capacity. The need is attributable to the larger sized transformers used for distribution substations and for auxiliary power for modern generating stations. These require continuous current ratings up to 3,000 amperes and interrupting ratings up to 350 mva. The breakers and switchgear described in this paper were developed to meet this requirement. Because of industry interest in the basis of rating power circuit breakers, design tests were made to demonstrate that these breakers would meet the 350-mva interrupting rating on a symmetrical basis and with a 5-cycle interrupting time rating.