Selective circuit breaker arranged in an insulating case of modular type formed by two half-shells (1, 2) of inverted T-shaped volume forming a first volume of parallelepipedal shape surmounting a base and surmounted by a shoulder also of parallelepipedal shape, substantially centred with respect to the first volume, two portions (B, C), respectively left and right, of the said volume being furthermore distinct and defined by a transverse plane of central appearance, the said circuit breaker comprising: - connection devices (11, 12) for connecting conductors of the circuit to be protected via upstream and downstream terminals of the circuit breaker, between which is formed a main branch comprising a main thermal actuator, a pair of contacts, respectively main mobile and main fixed and a main magnetic actuator, capable of moving the said contacts (32, 28) apart in the event of a short circuit in the circuit to be protected and in parallel with these main contacts (32, 38), - a secondary branch comprising a pair of contacts, respectively secondary mobile and secondary fixed, a secondary thermal actuator and a resistor, - a mechanical lock actuated by the main and secondary thermal actuators, and/or by a connecting piece of an adjacent product, in order to open the main and secondary mobile contacts, - a control lever extending beyond the front of the case opposed to the base, capable of rearming the lock and rotatable between two positions corresponding respectively to closure and opening of the contacts (32, 28; 18, 20), and - an arc cut-off chamber, the circuit breaker comprising a plurality of distinct and autonomous subassemblies arranged in a case with a width of at most equal to 1.5 times the module: - a locking subassembly; - a main circuit subassembly comprising the main magnetic actuator, the main thermal actuator, the main mobile contact, the secondary fixed contact and an arc plate of the arc cut-off chamber; - an arc cut-off subassembly comprising a stack of ion plates, two lateral cheeks each provided with a screening plate, as well as the main fixed contact; - a secondary circuit subassembly including the resistor, the secondary thermal actuator and the secondary mobile contact. - connection devices (11, 12) for connecting conductors of the circuit to be protected via upstream and downstream terminals of the circuit breaker, between which is formed a main branch comprising a main thermal actuator, a pair of contacts, respectively main mobile and main fixed and a main magnetic actuator, capable of moving the said contacts (32, 28) apart in the event of a short circuit in the circuit to be protected and in parallel with these main contacts (32, 38), - a secondary branch comprising a pair of contacts, respectively secondary mobile and secondary fixed, a secondary thermal actuator and a resistor, - a mechanical lock actuated by the main and secondary thermal actuators, and/or by a connecting piece of an adjacent product, in order to open the main and secondary mobile contacts, - a control lever extending beyond the front of the case opposed to the base, capable of rearming the lock and rotatable between two positions corresponding respectively to closure and opening of the contacts (32, 28; 18, 20), and - an arc cut-off chamber, - a locking subassembly; - a main circuit subassembly comprising the main magnetic actuator, the main thermal actuator, the main mobile contact, the secondary fixed contact and an arc plate of the arc cut-off chamber; - an arc cut-off subassembly comprising a stack of ion plates, two lateral cheeks each provided with a screening plate, as well as the main fixed contact; - a secondary circuit subassembly including the resistor, the secondary thermal actuator and the secondary mobile contact.