GB1452428A - System for detecting earth or alternating voltages in telephone exchanges - Google Patents

Abstract

1452428 Automatic exchange systems INTERNATIONAL STANDARD ELECTRIC CORP 10 Dec 1974 [27 Dec 1973] 53325/74 Heading H4K A subscriber's line is tested for false earth potentials and for interfering A.C. potentials each time the line is taken into use, the tests being performed at a "common unit" to which the line is connected during one stage of a call. In a first embodiment the test equipment is incorporated in a digit receiver (Fig. 2, not shown). When a calling line has been connected through to the receiver, central control (UC) interrogates an A.C. detection circuit in the receiver to determine whether an A.C. voltage in excess of 50 Volts is present on either wire. If such a voltage is present the receiver is dropped and tested in undisclosed manner for damage. The calling line and line circuit are also put out of service and a teleprinter is used to output a fault message. If no A.C. voltage is present dial tone is sent, and during the dialling phase an earth detection circuit is used to monitor the wires for earth potential on one, the other, or both of the line wires. Should the central control detect an earth fault condition, the line is put out of service or is parked. In the embodiment of Fig. 3, the test circuits are incorporated in a local junctor. Earth potential detection circuits TC1, TC2 are provided respectively on the calling and called sides of the coupling capacitors Ca, Cb and line split contacts KC3, KC4, whereas an A.C. detection circuit TD is only supplied on the called side. During a call establishment the called line is checked for the presence of an A.C. voltage, and if no such voltage is detected, ringing current is supplied to the line by circuit CD. Circuit TD can then be used to test that the ringing current is present. During the call, the circuits TC1, TC2 monitor the earth conditions of a respective one of the two lines whilst circuit TD monitors the A.C. voltage on both lines. In the presence of a high A.C. voltage both lines are put out of service whereas for an earth fault only the corresponding line is parked. No details of the test circuits are given. It is also stated that path check circuitry may be provided for checking the path through the network RC, but again no details are given. The arrangements have particular application to exchanges with electronic switches.