The paper describes the principles of d.c. arc extinction using cold-cathode arc chutes, a type originally designed for use with a.c. circuit breakers. Design details are given for the coil-less cold-cathode arc-chute circuit breaker, illustrating the fundamental requirements of the arc chute, contacts, mechanism and withdrawable circuit breaker in its metal enclosed cubicle for use up to 3 kV d.c. The application of the d.c. circuit breaker to the various types of traction circuits is described, and particular reference is given to the rectifier and feeder circuits. The various types of basic protective devices, inherent in high-speed d.c. circuit breakers, are the direct-acting electromagnetic overcurrent releases, which are described in detail. Coverage is given to the testing of d.c. circuit breakers to meet the onerous conditions of a traction application, and these are supported by details of test and service experience.