Electrical apparatus for an electrical installation, able and intended to be connected to an electrical grid, comprising at least: - a reception module comprising at least a first reception area; - a primary functional module received in the first reception area and comprising a first connection pole able and intended to be connected to a first line of the electrical grid, a second connection pole able and intended to be connected to a second line of an electrical circuit of the electrical installation and a sixth connection pole, the primary functional module comprising a device (2") for interrupting the current, which device is arranged between the first connection pole and the sixth connection pole and is able, in an open position, to interrupt the current between the first connection pole and the sixth connection pole and, in a closed position, to allow the passage of the current between the first connection pole and the sixth connection pole, which electrical apparatus is characterized in that the reception module comprises a second reception area for receiving an additional functional module according to Claim 6, said additional functional module having means for performing at least one function requiring an electric power supply, at least a fourth connection pole and a fifth connection pole that are able to be electrically connected respectively to the second connection pole and to the sixth connection pole of the primary functional module in order to supply power to the additional functional module, and in that either the electrical apparatus comprises a first blocking element able to block the interruption device (2") in the closed position when the additional functional module is received in the second reception area or a first blocking element of the additional functional module is able to block the interruption device (2") in the closed position when the additional functional module is received in the second reception area. - a reception module comprising at least a first reception area; - a primary functional module received in the first reception area and comprising a first connection pole able and intended to be connected to a first line of the electrical grid, a second connection pole able and intended to be connected to a second line of an electrical circuit of the electrical installation and a sixth connection pole, the primary functional module comprising a device (2") for interrupting the current, which device is arranged between the first connection pole and the sixth connection pole and is able, in an open position, to interrupt the current between the first connection pole and the sixth connection pole and, in a closed position, to allow the passage of the current between the first connection pole and the sixth connection pole, which electrical apparatus is characterized in that the reception module comprises a second reception area for receiving an additional functional module according to Claim 6, said additional functional module having means for performing at least one function requiring an electric power supply, at least a fourth connection pole and a fifth connection pole that are able to be electrically connected respectively to the second connection pole and to the sixth connection pole of the primary functional module in order to supply power to the additional functional module, and in that either the electrical apparatus comprises a first blocking element able to block the interruption device (2") in the closed position when the additional functional module is received in the second reception area or a first blocking element of the additional functional module is able to block the interruption device (2") in the closed position when the additional functional module is received in the second reception area.