Abstract: An arrangement for detecting disturbances influencing the network frequency in electric power supply systems is suggested and the use of such an arrangement in an adaptive automatic frequency control system for electric power systems is disclosed. A first measuring channel responsive to the frequency to be monitored comprises delay and sample circuit means operated by timer means connected in a second measuring channel responsive to the frequency change rate. Output signals of said delay and sample circuit means giving a history of the frequency change to be monitored, are compared and processed in an arithmetic unit to obtain information about the power flow in the network causing the frequency disturbance and the location of the disturbance, respectively.