The circuit breaker is intended for installation in a low-voltage distribution board having a socket and having at least two busbars which are guided parallel, are held on the socket and are each associated with one pole conductor of a multiphase power supply system. This circuit breaker has an enclosure as well as a plug contact for making contact with one of the busbars. The plug contact can be positioned as a function of the position of the busbar with which contact is to be made, and is held on a contact mount, which can be moved transversely with respect to the busbars and can be locked to the circuit breaker enclosure. In a circuit breaker such as this, a pole conductor change from one busbar to another busbar can be carried out quickly and without the use of any tools.