A mechanism for a multipole circuit breaker with high currents and high electrodynamic strength, comprising a toggle device associated to a trip hook and a switching bar, an opening ratchet cooperating with the hook to perform loading and tripping of the mechanism respectively in the locked or unlocked position of the ratchet. The opening ratchet comprises a disengageable actuator causing self-unlocking of the catch in the presence of a short-circuit current exceeding a calibration threshold defined by a flexible element, said self-unlocking being commanded from a mechanical reaction generated by the electrodynamic compensation effect and causing an ultra-fast rotation of the catch to unlock the opening ratchet before the tripping component operates.