a summation and level detecting circuit 7a and to a function element 8 common to all three phases which in a suitable manner combines the three input signals into a single reference signal this may for example as previously mentioned consist of a certain percentage of the signal appearing in the three phases which has the highest amplitude or for example consist of a certain part of the weighted sums or differences between two or three of the signals in the three phases of the network the factor which distinguishes the reference signal from the signal in the phases may be obtained for example by voltage division or via a multiplier this factor may also be changed during operation of the device by means of an external control input contr the absolute magnitude of the output signal from the function element 8 must in this embodiment of the invention be lower than the absolute magnitude of the greatest one of the signals appearing on the outputs from the peak value sensing circuits 6a 6b or 6c the output signal from the function element 8 is supplied to the inverted input of the above mentioned summation and level detecting circuit 7a the output signal from the circuit 7a is supplied in its turn to one input of an and gate 10a the output signals from the circuits 7 thus indicate if and in what phases the changes in the currents have exceeded the.