An electromagnetic trip device comprising a shell and a moving core sliding due to the action of a coil, the shell comprising a radial surface having an opening through which the moving core passes, superposition of a radial crown of the moving core and the radial surface forming a magnetic flux transfer surface enabling flow of an axial magnetic flux. The trip device comprises intercalary adjustment means of said transfer surface respectively positioned between the moving core and the opening, said intercalary adjustment means comprising two calibrated elements adjoined surface against surface and being respectively formed by an alternation of magnetic sectors and non-magnetic sectors; movement of a calibrated element with respect to the other enabling a variation of said transfer surface to be obtained.