PURPOSE: To improve the performance of maintenance by detecting automatically a line fault and switching automatically the line to a spare line in alarm sending line provided to a telephone exchange line.

CONSTITUTION: A fault class of a station where an alarm sender (ALSND) is installed is indicated by the number of brake pulses. Further, the said class is coincident with the number of brake pulses. The ALSND where the number of classes is taken as 1∼10 and a class 11 is taken as a pseudo fault class gives a start periodically to a pseudo fault class transmission section by a clock 3 and transmits the pulse "11" to an alarm receiver (ALRCV) in the sequence shown in the figure. On the other hand, the ALRCV is provided with a supervisory circuit 8, which lights up an AL lamp and raises an alarm to the maintenance personnel when a general fault class or a pseudo fault class is not incoming within a prescrived time, and also actuates a CH relay to switch the line to the spare line.