Effects of High Fault Currents on Circuit Breakers

This chapter examines some of the causes of circuit breaker failure. It also analyzes some of the factory testing procedures which help to ensure safe design and manufacture of circuit breakers. The most eminent reason for utilities to maintain or even reduce their fault current levels is to ensure proper functioning of circuit-interrupting devices such as circuit breakers and fuses. Circuit breakers in the United States are rated by the American National Standards Institute (ANSI) and the Institute of Electrical and Electronics Engineers (IEEE). While current-limiting fuses are enclosed in a sealed cylinder, and are usually contained in a metal-enclosed switchgear, limiting the danger even if their short-circuit rating is exceeded, expulsion fuse links are mounted in open tubes on utility poles. The chapter presents case studies that describe the replacement of air-magnetic circuit breakers whose interrupting ratings have been exceeded with new SF6 or vacuum interrupters.