PROTECTING SYSTEM FOR POWER SUPPLY FAULT

PURPOSE:To prevent the supply of an extra call current by cutting off a power supply for call current with the output which detected a fault of a power supply for call control and preventing the supply of the call current. CONSTITUTION:In a normal state, a cut-off relay 12 for call current power supply operates to close a contact 3. When a subscriber on a line 1 delivers a call, a call current is supplied from a call current power supply 4 via a DC loop monitor relay 2 and the contact 3. Thus the relay 2 operates to close its contact 10, and a signal of a low level is supplied to a control circuit 11. Then the circuit 11 starts to process a call. When a calling process is over and a talking state is set, the sound signal is supplied to a 2-wire/4-wire converting and decoding circuit 7. Then the digital sound signal is sent to a time division switch network. The relay 12 is restored when a positive power supply 13 for call control is cut off by a fault. Then the power supply 4 is cut off by the contact 3. Thus no call current is supplied.