Protective circuit for the overvoltage protection of a subscriber line interconnect circuit

Claim 1

A protective circuit for providing overvoltage protection of a subscriber line interconnect circuit of a digital time multiplex-communication network having an electronic interface circuit and for providing overvoltage protection of the electronic switches associated with the subscriber line circuit for test accessibility, comprising: means providing for each conductor of a core pair of the connected subscriber line circuit for each corresponding connection of the subscriber line interconnect circuit an isolation of the separate conductors of the core pair from the connected subscriber line circuit; a separate test access connection to the conductors of the core pair to the corresponding connections of the active subscriber line interconnect circuits; a threshold setting element connected between the line side of an isolation switch and an access switch for each conductor of the core pair of the subscriber line circuit for accessing one conductor of the core pair, of identical construction to that of the threshold setting element associated with the other conductor of the core pair such that overvoltages are conducted to ground potential both via a grounded terminal of the circuit and through a second threshold setting element bridging said access switches for accessing one conductor of the the core pair, which is connected to ground potential via a third threshold setting element.