Electric line protection device, of the differential switch type, comprising: - a cut-off device comprising a mechanical lock, a triggering device which is able to switch said lock, - a fixed contact and a mobile contact fixed to a rotary contact holder, - a control lever of the mobile contact driving said mobile contact away from or towards the fixed contact, the said lever being connected to the contact holder by means of the mechanical lock which is able to switch leading to the opening/closing of the contacts (1, 2) following a manual action on the lever; - the triggering device being able to switch the lock of the cut-off device to open the contacts (1, 2) in the case of a differential short-circuit current and able to leave the contacts (1, 2) closed in the case of a balanced short-circuit current; in a closed position of the contacts (1, 2), said mobile contact being held against said fixed contact at a contact point under the action of an elastic element arranged between the mobile contact and the contact holder and inducing a nominal contact force applied to the fixed contact, - means for transforming the balanced short-circuit current into an additional force of contact applied to the fixed contact and directed according to the contact force, the sum of the forces (F1 and F2) being at least equal to the electrodynamic repulsion forces (Fr) of the contacts generated by the balanced short-circuit current, - a connection terminal, the mobile contact being connected electrically to said connection terminal via a flexible conductor of the braid type, said transformation means consisting of a specific arrangement between the conductor, the mobile contact and the fixed contact, the mobile contact being arranged to pivot about a pivot point situated close to the contact point, the mobile contact comprising a first end proximal to the contact holder and a second end distal to the contact holder, the electric line protection device is characterised in that said conductor is connected to the second end of the mobile contact and at a distance from the pivot point, and in that the ratio between the distance of the pivot point to the second end for connecting to the conductor and the distance of the pivot point to the contact point is greater than 2. - a cut-off device comprising a mechanical lock, a triggering device which is able to switch said lock, - a fixed contact and a mobile contact fixed to a rotary contact holder, - a control lever of the mobile contact driving said mobile contact away from or towards the fixed contact, the said lever being connected to the contact holder by means of the mechanical lock which is able to switch leading to the opening/closing of the contacts (1, 2) following a manual action on the lever; - the triggering device being able to switch the lock of the cut-off device to open the contacts (1, 2) in the case of a differential short-circuit current and able to leave the contacts (1, 2) closed in the case of a balanced short-circuit current; in a closed position of the contacts (1, 2), said mobile contact being held against said fixed contact at a contact point under the action of an elastic element arranged between the mobile contact and the contact holder and inducing a nominal contact force applied to the fixed contact, - means for transforming the balanced short-circuit current into an additional force of contact applied to the fixed contact and directed according to the contact force, the sum of the forces (F1 and F2) being at least equal to the electrodynamic repulsion forces (Fr) of the contacts generated by the balanced short-circuit current, - a connection terminal,