A low-voltage circuit breaker that comprises: a containment structure; a control mechanism; a plurality of circuit breaking poles, each of which comprises a housing containing a sealed ampoule that in turn contains at least one fixed contact and at least one moving contact, which can be mutually coupled and uncoupled, said housing consisting of a first side and a second side that define an interior containing said ampoule, the outer side wall on said first side being complementary to and associable with the outer side wall of said second side, said poles being located side-by-side to form a set of poles complementary to at least a part of said containment structure; operative connection means between said control mechanism and said poles.