The invention relates to a circuit breaker having two parallel current paths each with a contact point (15, 18; 15, 22) which, as the main and secondary current paths (19, 21), are connected, via a contact device (15, 17, 18, 22), in series with a coil (11) of a magnetic trip (10), the armature (12) of which, in the event of a fault, opens the contact point (15, 18) of the main current path (19) which is formed exclusively only by the series circuit of the contact device (15, 17, 18) with the coil (11). In order to provide a further circuit breaker which, in the case of energy losses which are minimized during normal continuous operation, prevents the formation of an arc when the circuit is interrupted in the event of a fault and therefore also largely prevents contacts from burning away, provision is made, in the case of a circuit breaker according to the invention, for an interrupter and trip insert (24) to be arranged in the secondary current path (21), which insert, in the event of a short circuit, interrupts the secondary current path and trips a switching lock (30) to open and secure the contact device (15, 17, 18, 22).