1. A measuring device ( 10 - 13 ) for measuring at least one physical quantity of an electrical installation, the device comprising: at least one sensor ( 20 , 20 ′) being arranged to measure at least the physical quantity and provide at least one signal representative of the physical quantity, at least one measuring circuit ( 40 ) being arranged to receive the signal and process the signal to deliver at least one exploitable measurement value, the sensor and the measuring unit being connected to one another by at least one connection cable ( 50 ), the connection cable ( 50 ) comprising at least three pairs of conductors of which a first pair of conductors ( 51 +, 51 −), called a measurement pair ( 51 ), being arranged to transmit the signal representative of the physical quantity measured by the sensor ( 20 , 20 ′), a second pair of conductors ( 52 +, 52 −), called a power supply pair ( 52 ), being arranged to power electrically the sensor ( 20 , 20 ′) through the measuring circuit ( 40 ), and a third pair of conductors ( 53 a , 53 b ), called a communication pair ( 53 ), being arranged to transmit at least one complementary signal (DET 1 , DET 2 ) between the sensor ( 20 , 20 ′) and the measuring circuit ( 40 ), and the sensor ( 20 , 20 ′) comprising encoding circuitry ( 23 , 24 ; 60 ) arranged to characterize the sensor and transmit to the measuring circuit ( 40 ), through the communication pair ( 53 ), the at least one complementary signal (DET 1 , DET 2 ) that corresponds to at least one identification feature of the sensor ( 20 , 20 ′), wherein the measuring device ( 10 - 13 ) is modular and suitable for an analog or a digital sensor, the measuring circuit ( 40 ) is designed to be able to be connected to the sensor ( 20 , 20 ′) and form a module, the measuring circuit ( 40 ) comprises a processing unit ( 45 ) integrating an analog/digital converter, connected to the communication pair ( 53 ), to receive and process the at least one complementary signal (DET 1 , DET 2 ) transmitted on at least one of conductors ( 53 a , 53 b ) of the communication pair ( 53 ), whereby this signal can be either an analog signal or a digital signal and is representative of at least one identification feature of the sensor ( 20 , 20 ′) allowing the measuring circuit ( 40 ) to automatically recognize the sensor ( 20 , 20 ′) connected to the measuring circuit ( 40 ) and to provide an exploitable correlated value of the measured physical quantity, and the measuring circuit ( 40 ) comprises encoding circuitry ( 25 , 26 ) which is complementary to the encoding circuitry ( 23 , 24 , 60 ) provided in the sensor ( 20 , 20 ′).