Apparatus for protecting and testing telephone network

Claim 1

Apparatus for linking together a pair of telephone networks, each having a pair of linking lines for interconnecting said networks to transmit signals therebetween, and one of said networks having testing means for selectively and momentarily applying to one of its linking lines a triggering signal having a potential greater than the maximum potential of the first-named signals to initiate a test for faults in said one network, comprising switch means normally connecting said pairs of lines together to enable the transmission of said first-named signals between said networks, means connected between the two linking lines of said one network to limit the AC power in said linking lines, when the two pairs thereof are connected, a circuit including a solid state element connected to said one linking line normally to be in a non-conductive state during the transmission of said first-named signals between said networks, and operative to be switched to a conducting state upon application of said triggering signal to said one linking line, switch operating means connected to said element to be energized, when said element is in its conducting state, and operative, when energized, to cause said switch means to disconnect said pairs of lines electrically from each other without adding any additional circuitry to said one network, and delay means connected to said element and said switch operating means and operative to maintain said switch operating means energized for a predetermined period of time necessary to conduct said test for faults in said one network, after said triggering signal is removed from said one line, said switch means being interposed between said power limiting means and the other of said networks, whereby said power limiting means remains connected across said lines of said one network during the test thereof.