A protective electrical apparatus in modular format, of parallelepiped general shape with two main faces, respectively a left face and a right face, and lateral faces extending from one to the other of the main faces (11, 12), respectively a back face, an upper face, a front face and a lower face, with a width, that is to say the spacing between the left face and the right face, equal to an integer number of times a predetermined distance, called module, the back face being configured to be fastened to a support rail disposed horizontally, the upper face having two insertion apertures (20, 21) giving access respectively to a first upper terminal for a first electrical pole and a second upper terminal for a second electrical pole different from the first electrical pole, the lower face having two insertion apertures (24, 25) giving access respectively to a first lower connection terminal for the first electrical pole and to a second lower connection terminal for the second electrical pole, each said connection terminal (22, 23, 26, 27) being configured to receive a stripped end section of an electric cable or a tooth of a horizontal comb-like busbar; which apparatus comprises: - a first circuit for conveying current between the first upper terminal and the first lower terminal, comprising a fixed contact and a mobile contact; - a second circuit for conveying current between the second upper terminal and the second lower terminal; - an actuating mechanism of the mobile contact having two stable positions, respectively a cutting-off position in which the mobile contact is away from the fixed contact and an engaging position in which the mobile contact bears against the fixed contact; - a lever, projecting from the front face, to manually act on the actuating mechanism in order to pass from the cutting-off position to the engaging position or from the engaging position to the cutting-off position; - at least one triggering member (30, 33, 45) configured to act automatically on the actuating mechanism in order to pass from the engaging position to the cutting-off position when predetermined current conveying conditions occur; and - an arc quenching chamber associated with the first current conveying circuit with a first conductive horn bearing the fixed contact and with a second conductive horn electrically connected to the mobile contact, disposed opposite the first conductive horn; characterized in that at least part of an annular armature (40, 103) of an electrical transformer (35, 102) sensitive to said predetermined conveying conditions is situated, in relation to a section of the second conductive horn that is inclined towards the back face and towards the arc quenching chamber: - towards the back face; and - opposite said inclined section of the second conductive horn, in a front-to-back direction, that is to say a direction which is parallel to the main faces (11, 12) as well as to the upper face and the lower face. - a first circuit for conveying current between the first upper terminal and the first lower terminal, comprising a fixed contact and a mobile contact; - a second circuit for conveying current between the second upper terminal and the second lower terminal; - an actuating mechanism of the mobile contact having two stable positions, respectively a cutting-off position in which the mobile contact is away from the fixed contact and an engaging position in which the mobile contact bears against the fixed contact; - a lever, projecting from the front face, to manually act on the actuating mechanism in order to pass from the cutting-off position to the engaging position or from the engaging position to the cutting-off position; - at least one triggering member (30, 33, 45) configured to act automatically on the actuating mechanism in order to pass from the engaging position to the cutting-off position when predetermined current conveying conditions occur; and - an arc quenching chamber associated with the first current conveying circuit with a first conductive horn bearing the fixed contact and with a second conductive horn electrically connected to the mobile contact, disposed opposite the first conductive horn; - towards the back face; and - opposite said inclined section of the second conductive horn, in a front-to-back direction, that is to say a direction which is parallel to the main faces (11, 12) as well as to the upper face and the lower face.