JPH0546158B

PURPOSE:To decrease power feeding from a spare power supply by blocking calling at commercial power failure in a private branch of exchange driven by the commercial power supply and the spare power supply. CONSTITUTION:The power supply POW supplies a prescribed power by using a commercial power source AC and rectifier circuits PW1, PW2 and charges up batteries B1, B2 as the spare power supply by means by floating. When the power source AC is failed, power failure is informed to a processor by a power failure detecting circuit PD, the processor supervises the idle/busy state of extensions EXT1-n, brings a signal line PEi corresponding to an extension of idle state to a high potential, blocks power feeding and blocks calling. Moreover, the power supply fed to the processor is interrupted except for a storage circuit RAM. When the power failure is restored and the power of the commercial power source AC is restored, the power failure detecting circuit PD stops the information of power falure to the processor and the rectifier circuits PW1, PW2 supply again the prescribed power.