Abstract: The present heavy duty automatic circuit breaker is compact in construction, since all elements are coordinated to each other in an efficient manner. Thus, an upper arc guide plate of a quenching stack is extended to form the stationary contact member. The movable contact member is a single arm pivot contact arranged directly in front of said quenching stack. A lower arc guide plate of the quenching stack is extended and electrically connected to the movable contact member. The circuit breaker includes means for either manual or automatic circuit breaking, the latter in response to overload and/or excess temperature conditions.