<p>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. <IMAGE></p>
