A 500,000-kva 7.5-kv air circuit breaker for steel-mill service

WITH the advent of the first 15-kv power air circuit breakers for central station service in 1929,^1,2 it was soon recognized that an air circuit breaker was much more desirable for highly repetitive steel-mill motor switching than was the conventional equipment universally used at that time. Interrupting tests showed that the air circuit breaker could withstand many operations on considerable power without requiring maintenance, as compared to the high maintenance necessary on oil switching equipment of that period. This was somewhat to be expected in view of the fact that low-voltage air circuit breakers and contactors had demonstrated this ability for years. It was also believed that the air circuit breaker was highly desirable for steel-mill-substation service because of the additional severity of steel-mill operating conditions, as compared to the duty on corresponding equipment in central-station service.