on lead 723 indicating the completion of a test sequence wait flip flop 703 provides control outputs on leads 724 for use in the remainder of the circuit the one outputs of flip flops 701 and 702 are combined in and gate 725 with a ctrt signal on lead 726 indicating the status of the link between the control terminal and the first remote terminal the switch from spare is thus permitted only if this first link is operative if the system is operating on the spare line and trouble occurs on the spare line and if the main line is operative at all and gate 725 will produce an output signal on offsp lead 727 to automatically initiate a switch back to the main line and a testing sequence of the remote links of the main line the one output of wait flip flop 703 is combined with an lb control signal on lead 728 in or gate 729 to generate a lock up lu signal on lead 730 to among other things clear the fault count shift register and the fault alarm flip flop 608 in fig6 the signal is also used to lock up the current assignments of channels to subscribers and locks up switchhook information to prevent calls from being dropped while a testing sequence takes place this output also lights a fault alarm lamp 731 to indicate to attendant personnel that a fault.