D-C circuit breakers for steel mill service

With several large motors and generators operating in parallel in a system that is coupled very closely electrically, short-circuit currents in d-c steel mill circuits may be as high as several hundred thousand amperes with a rate of rise of from 10 to 20 million amperes per second. Circuit breakers for this service therefore must have high current interrupting capacity and quick response. In this paper the circuit breakers and associated equipment for a typical modern strip mill are described. The circuit breakers are tripped not only by overload current, but also by current having too high a rate of rise.