Telecommunication subscriber line access circuit with ground key and insulation fault indication

Claim 1

A telecommunications subscriber line access circuit for coupling a two wire subscriber station to a four wire interface to a system, said circuit comprising: (a) a two-to-four wire hybrid transformer; (b) a power supply bridge connected to the two wire side of said transformer for providing a potential difference across the two wires of said subscriber station, said power supply bridge including: (1) a first resistor connected between ground and a first winding of said transformer on the two wire side; (2) a second resistor connected between a source of potential and the second winding of said transformer on the two wire side; and (3) a capacitor connected between said first and second resistors on the sides thereof which are connected to said first and second windings; (c) an evaluating circuit connected to said first and second winding for producing an indicating signal upon ground key actuation of said subscriber; and (d) a cutoff circuit arranged in the power supply bridge, in series with at least one of said first resistor and said second resistor, said cutoff circuit including: (1) a cutoff switch which is opened, with a delay, upon ground key actuation and closed upon termination of ground key actuation; and (2) a third resistor bridging said cutoff switch and having a sufficiently high resistance value that, with the cutoff switch open, the residual current flowing through it due to ground key actuation does not damage the circuit components lying in the affected portion of the access circuit and having a sufficiently low resistance value that the current flowing through it, without ground key actuation and due to a leakage current between the wires of the subscriber line in excess of the permitted limit value, causes the evaluating circuit to respond in the same manner as with a closed cutoff switch and simultaneous ground key actuation.