A differential electrical switchgear device housed in an insulated enclosure containing: - a first cut-off compartment comprising at least: • an electrical circuit with a fixed contact and a moving contact, • a first control mechanism for closing and opening the aforementioned contacts, operated manually by a joystick and automatically, - a second compartment with differential protection comprising: • means for detecting a differential fault in the circuit to be protected, • means for triggering a so-called second control mechanism controlled by the aforementioned detection means and capable of causing this second mechanism to open when such a fault occurs, • means for transmitting the triggering command to the first-mentioned control mechanism, and • electrical test equipment for the differential function designed to trigger the second mechanism when activated, said test means comprising a test circuit and being actuated by test control means, in which these transmission means comprise a return mechanically connected to the two aforementioned mechanisms so as to be able to transmit the triggering order sent by the differential triggering means to the second mechanism, to the first mechanism, characterised in that this apparatus comprises means (12, 24) for immobilising this transmission, activated when the aforementioned test is carried out, the immobilisation means being configured to bear against the transmission, so as to keep the transmission immobilised and thus prevent the transmission of this triggering order to the first mechanism for at least part of the duration of the test. - a first cut-off compartment comprising at least: • an electrical circuit with a fixed contact and a moving contact, • a first control mechanism for closing and opening the aforementioned contacts, operated manually by a joystick and automatically, • an electrical circuit with a fixed contact and a moving contact, • a first control mechanism for closing and opening the aforementioned contacts, operated manually by a joystick and automatically, - a second compartment with differential protection comprising: • means for detecting a differential fault in the circuit to be protected, • means for triggering a so-called second control mechanism controlled by the aforementioned detection means and capable of causing this second mechanism to open when such a fault occurs, • means for transmitting the triggering command to the first-mentioned control mechanism, and • electrical test equipment for the differential function designed to trigger the second mechanism when activated, said test means comprising a test circuit and being actuated by test control means, • means for detecting a differential fault in the circuit to be protected, • means for triggering a so-called second control mechanism controlled by the aforementioned detection means and capable of causing this second mechanism to open when such a fault occurs, • means for transmitting the triggering command to the first-mentioned control mechanism, and • electrical test equipment for the differential function designed to trigger the second mechanism when activated, said test means comprising a test circuit and being actuated by test control means, in which these transmission means comprise a return mechanically connected to the two aforementioned mechanisms so as to be able to transmit the triggering order sent by the differential triggering means to the second mechanism, to the first mechanism, characterised in that this apparatus comprises means (12, 24) for immobilising this transmission, activated when the aforementioned test is carried out, the immobilisation means being configured to bear against the transmission, so as to keep the transmission immobilised and thus prevent the transmission of this triggering order to the first mechanism for at least part of the duration of the test.