Transient overvoltage protection system intended to be positioned in parallel to an equipment item to be protected, the protection system comprising at least three lines (L, N, M) consisting of a first line, a second line and a ground; the protection system comprising: - a first, a second and a third conductive branches which are intended to be connected respectively to the three lines (L, N, M) and of which each comprises a first end (14, 15, 16) intended to be connected to one of the three lines (L, N, M) and a second end (7, 8, 9) that can be connected to the second end (7, 8, 9) of the other two conductive branches (L, N, M); - the first and the second conductive branches being equipped with a first and a second (2, 4) voltage-sensitive protection elements; each of the first and second voltage-sensitive protection elements having two terminals (5, 10; 11, 12) and being arranged to allow a flow of current between the first and the second ends (7, 14) of the respective conductive branch when the voltage between said two terminals is higher than a voltage threshold and to inhibit a flow of current between the first and the second ends (7, 14) of the respective conductive branch when the voltage between said two terminals is lower than the voltage threshold; each voltage-sensitive protection element (2, 4) also being able to rise in temperature when the voltage between its terminals (5, 10) is higher than said voltage threshold; and - a thermally sensitive disconnection device comprising at least one thermofusible element which is in thermal contact with at least one of the first and second voltage-sensitive protection elements (2, 4); the thermofusible element being electrically conductive at ambient temperature and being capable of melting above a temperature threshold; the thermofusible element being arranged so as to keep the second ends (7, 8, 9) of the three branches in a connected position in which an electrical connection is assured between the second ends (7, 8, 9) of the first, second and third conductive branches; the thermally sensitive disconnection device comprising an elastic return device exerting a force tending to bring the first, second and third conductive branches to a disconnected position in which the second ends (7, 8, 9) of the first, second and third conductive branches are placed at a distance from one another so as to simultaneously disconnect the first, second and third branches from one another upon the melting of the thermofusible element. - a first, a second and a third conductive branches which are intended to be connected respectively to the three lines (L, N, M) and of which each comprises a first end (14, 15, 16) intended to be connected to one of the three lines (L, N, M) and a second end (7, 8, 9) that can be connected to the second end (7, 8, 9) of the other two conductive branches (L, N, M); - the first and the second conductive branches being equipped with a first and a second (2, 4) voltage-sensitive protection elements; each of the first and second voltage-sensitive protection elements having two terminals (5, 10; 11, 12) and being arranged to allow a flow of current between the first and the second ends (7, 14) of the respective conductive branch when the voltage between said two terminals is higher than a voltage threshold and to inhibit a flow of current between the first and the second ends (7, 14) of the respective conductive branch when the voltage between said two terminals is lower than the voltage threshold; each voltage-sensitive protection element (2, 4) also being able to rise in temperature when the voltage between its terminals (5, 10) is higher than said voltage threshold; and - a thermally sensitive disconnection device comprising at least one thermofusible element which is in thermal contact with at least one of the first and second voltage-sensitive protection elements (2, 4); the thermofusible element being electrically conductive at ambient temperature and being capable of melting above a temperature threshold; the thermofusible element being arranged so as to keep the second ends (7, 8, 9) of the three branches in a connected position in which an electrical connection is assured between the second ends (7, 8, 9) of the first, second and third conductive branches; the thermally sensitive disconnection device comprising an elastic return device exerting a force tending to bring the first, second and third conductive branches to a disconnected position in which the second ends (7, 8, 9) of the first, second and third conductive branches are placed at a distance from one another so as to simultaneously disconnect the first, second and third branches from one another upon the melting of the thermofusible element.