A low-voltage circuit breaker, comprising: - at least one first fixed contact (1, 3), which is electrically connected to a terminal for connection to an electric circuit; - a rotating moving contact, which comprises a central body from which at least one first arm protrudes, an active surface being provided at the end of said first arm, said active surface being associable/separable with respect to said fixed contact by means of a rotation of said moving contact; - a rotating contact supporting shaft, which is functionally connected to an actuation mechanism of the circuit breaker and is provided with a seat that accommodates the central body of the moving contact so that the first arm protrudes externally from said seat, at least one first spring being furthermore arranged in said contact supporting shaft, said first spring being functionally coupled to the moving contact and being suitable to ensure, when the circuit breaker is closed, an adequate contact pressure between the active surface and the first fixed contact; wherein at least one first abutment surface is provided on said central body of the moving contact and abuts, during a rotation of the moving contact caused by a short-circuit, against a complementarily shaped surface formed in said seat of the shaft, at least part of the kinetic energy accumulated by the rotating moving contact during its rotation being transmitted directly to the contact supporting shaft owing to the interaction between said first abutment surface and said complementarily shaped surface, characterised in that the kinetic energy transmitted by the rotating moving contact to the contact supporting shaft is used to overcome the inertia of said contact supporting shaft and trigger the rotation of said contact supporting shaft before the intervention of said actuation mechanism. - at least one first fixed contact (1, 3), which is electrically connected to a terminal for connection to an electric circuit; - a rotating moving contact, which comprises a central body from which at least one first arm protrudes, an active surface being provided at the end of said first arm, said active surface being associable/separable with respect to said fixed contact by means of a rotation of said moving contact; - a rotating contact supporting shaft, which is functionally connected to an actuation mechanism of the circuit breaker and is provided with a seat that accommodates the central body of the moving contact so that the first arm protrudes externally from said seat, at least one first spring being furthermore arranged in said contact supporting shaft, said first spring being functionally coupled to the moving contact and being suitable to ensure, when the circuit breaker is closed, an adequate contact pressure between the active surface and the first fixed contact; wherein at least one first abutment surface is provided on said central body of the moving contact and abuts, during a rotation of the moving contact caused by a short-circuit, against a complementarily shaped surface formed in said seat of the shaft, at least part of the kinetic energy accumulated by the rotating moving contact during its rotation being transmitted directly to the contact supporting shaft owing to the interaction between said first abutment surface and said complementarily shaped surface, characterised in that the kinetic energy transmitted by the rotating moving contact to the contact supporting shaft is used to overcome the inertia of said contact supporting shaft and trigger the rotation of said contact supporting shaft before the intervention of said actuation mechanism.