Abstract: A method of and a circuit for determining current criteria in a subscriber's loop of a telephone system are described. Two constant-current sources are symmetrically connected to two wires of the loop and are controlled by an adjustable reference voltage source. Voltage signals on the voltage source are evaluated in a detecting circuit to determine the actuation of a ground button on the subscriber's set or the occurrence of a ground short-circuit. A testing device is connectable to the subscriber's loop to simulate various conditions of the subscriber's set.

Abstract: A method of energizing a subscriber's circuit in a telephone system having two separate constant-current sources arranged symmetrically to the subscriber's station is disclosed. Current flowing through the two lines of the circuit is measured by a shunt, and differences between the line currents are compensated by a regulating device which applies such reference voltages to respective current sources as to supply a maximum permissible current in each line. If this maximum value is exceeded, then the regulating device automatically turns off, and a reduced reference voltage is applied to the current sources so as to return to the maximum permissible value of the curent.