An insulating-material housing has first and second current path region next to each other and each having a first receiving space for a short-circuit tripping apparatus, a second receiving space for a switching contact and a third receiving space for an arc-quenching apparatus. The two first receiving spaces are arranged in a front side and a respective narrow side of the housing. The two second receiving spaces are arranged centrally between the two first receiving spaces. The third receiving spaces are arranged between a fastening side of the housing and the respectively associated first receiving space. A fourth receiving space for an additional functional assembly is next to the first and/or third receiving space of the first current path region in a normal direction of the broad sides. The compact configuration allows the circuit breaker to be expanded by an additional functional assembly in the fourth receiving space.