Abridgment of high-speed circuit breakers

High-speed circuit breakers have been successfully applied to d-c. railway systems for a number of years, but designs suitable for application on 12,000-volt single-phase systems have not been available until recently. A description of an air circuit breaker design suitable for 12,000-volt applications and having a speed of operation comparable with the d-c. type has been included. The method of applying the magnetic type of mechanism previously used for d-c. breakers to the a-c. type breaker has been described. The theory of the operation of the saturated transformer type of trip circuit used for this purpose has been given. Typical oscillograms of short-circuit tests, showing the high speed of operation on a-c. circuits, are included. The method of obtaining selective operation in connection with railway feeder circuits has also been outlined. The use of this type of breaker on a 12,000-volt single-phase system makes possible the same degree of protection and selective operation as is now being realized on d-c. systems.