103 b of current measurement device 103 and is connected with an output a 77 to additional measurement value output a 103 b of current measurement device 103 an additional input e 78 b of control unit 78 is connected to the one measurement value input e 103 a of current measurement device 103 an additional measurement value input e 78 c of control unit 78 is connected with additional measurement value input e 103 b of current measurement device 103 using measurement unit 73 current ib u2032 in conductor end 123 is measured and a current measurement value ib corresponding to current ib u2032 is formed this value arrives at arithmetic unit 76 in which a summation current measurement value ia ib is formed from this value and from the one measurement value ia at the one measurement value input e 103 a and this summation current measurement value is transmitted as a measurement value to the one measurement value output a 103 a of current measurement device 103 current measurement value ib also arrives at additional summation element 77 in which an additional summation current measurement value ib ic id is formed from additional measurement value ic id at additional measurement value input e 103 b of current measurement device 103 and from current measurement value ib and this additional summation current measurement value ib ic id is outputted as a measurement value at the additional measurement value output a 103 b.