Abstract: A circuit breaker-includes: a casing having a base and a main cover secured to the base, the casing accommodating a circuit breaking mechanism; a recesses formed in one surface of the main cover, the opposite surface of which is on the side of the base, the recess being adapted to receive an auxiliary device of the circuit breaking mechanism; and an auxiliary cover mounted on the main cover in such a manner as to close the recess. The circuit breaker is automatically tripped when the auxiliary cover is opened, thus permitting installation of the auxiliary device thereon with high security.

Abstract: A circuit interrupter comprising an automatic trip unit connected to an interrupting unit for tripping the interrupting unit to open separable contacts in response to an overcurrent, the automatic trip unit being a replaceable unit selected from a plurality of differing types of trip units including an electronic trip unit and a thermally responsive electromagnetic trip unit.

Abstract: A multi-pole circuit interrupter having a first pole unit provided with an operating mechanism and a second pole unit provided with no operating mechanism. Each of the pole units comprises a movable contact, a stationary contact, and a contact arm assembly for supporting the movable contact of the respective pole unit including a first contact arm and a second contact arm pivotally supported by a common pin. The first contact arm in the first pole unit have an elongated guide hole extending in a direction of movement of the movable contact and is directly connected to the operating mechanism. The second contact arm has an elongated hole extending substantially in a longitudinal direction thereof. A spring-biased sliding pin extends through the elongated hole and the guide hole.

Abstract: A circuit interrupter comprises a contact arm assembly for supporting the movable contact composed of a first contact arm and a second contact arm pivotally supported by a common pin, the first contact arm having an elongated guide hole extending in the direction of movement of the movable contact, the second contact arm having an elongated hole extending substantially in the direction of extension of the second contact arm. A second pin extends through the elongated hole and the guide hole, and a spring is provided for biasing the second pin. The elongated hole has a pin sliding surface on which the second pin slides and which forms an angle of 90.degree. or more with respect to a pin sliding surface of the elongated pin guide hole when the first contact arm is being reset after the second contact arm is separated by an electromagnetic repulsive force.