Field testing of D-C air circuit breakers on a high-capacity system

AN unusual opportunity for testing d-c air circuit breakers on a high-capacity system has been made possible with use of the mercury-arc rectifier system at Alcoa´s Rockdale Works. The system had a preliminary short-circuit current calculation of 880,000 amperes but the actual available current was determined at the test point, located mid-way on the d-c bus, to be slightly over 400,000 amperes. The maximum available current was obtained by connecting in parallel four potline busses without a short-circuit current-limiting test resistor. Under this extreme condition, a peak current of 220,000 amperes flowed through the breakers. These current values are greater than the interrupting rating listed in the standards for semi-high-speed d-c breakers normally used for high-capacity d-c application. However, sufficient current-limiting action of the interrupter was obtained and the clearing time was short enough to maintain a continuous power flow to the aluminum potline. Being the first known tests presented since the new rating structure was adopted, the results indicate that the standard1 is a conservative rating when used as the guide.