is assured by the selection circuit 42 such switch naturally being able to assume an electronic form by means of switching transistors for instance the selection circuit may be equipped with an indicator 42a of the channel connected for the measurement the circuit 42 may be manually controlled or automatically controlled according to a predetermined selection program in addition circuits 40 and 42 are galvanically insulated from one another by means of an optocoupler 44 the electronic channel selection circuit 40 is connected to another processing circuit 46 for the current induced in coil 34 36 or 38 such induced current processing circuit 46 comprises an amplifier 46a a filter 46b and impedance matching means 46c such processing circuit 46 thus enables furnishing a processed signal giving an image of the current circulating in the channel with which the second coil 34 36 or 38 as selected for measurement is associated the induced current processing circuit 46 itself is coupled to a receiving unit 48 for the processed signal such receiving unit being capable of interpreting such signal and giving thus the value of one or several predetermined parameters it will be noted that there has also been provided an optocoupler 50 in order.