不断电电源设备系具备从交流电源(50)经由接触器(16、14)供电至负载(52)的节能模式。在节能模式中，切换控制部(17)系当交流电源(50)的电压下降度达5%时，令晶闸管开关(13)导通后令接触器(14)切断。电力转换控制部(19)系当交流电源(50)的电压下降度达10%时，令反向器(9)运行逆转换而将蓄电池(53)的直流电力转换成交流电力，并且以使从反向器(9)输出的交流电压与从交流电源(50)供给的交流电压同步之方式控制反向器(9)的逆转换。令接触器(14)切断后，于反向器(9)的逆转换运行时，切换控制部(17)系令晶闸管开关(13)切断。

This invention provides a fail-free power source device having a power saving mode (economy mode) which supplies power to load (52) from an AC power source (50) through contacts (16, 14). In the power saving mode, a switching control unit (17) causes a thyristor switch (13) to turn on first and then causes a contact (14) to turn off when the rate of a voltage drop of the AC power source (50) has reached to 5%. When the rate of the voltage drop of the AC power source (50) has reached to 10 %, a power conversion control unit (19) controls an inverse conversion operation of an inverter (9) such that the inverter (9) is caused to perform the inverse conversion operation to convert the DC power of a battery (53) to an AC power and the AC voltage outputted from the inverter (9) to synchronize with the AC voltage supplied from the AC power source (50). When the inverse conversion operation is performed by the inverter (9) after the contact (14) is turned off, the switching control unit (17) causes the thyristor to turn off.