Uninterruptible power supply using vacuum circuit breaker

A power supply apparatus comprising:

• an input terminal that receives AC power supplied from an AC power supply;
• an output terminal connected to a load;
• a plurality of mechanical switches and a bypass circuit connected in series between the input terminal and the output terminal;
• a control device that (i) turns on both of the plurality of mechanical switches and the bypass circuit when the AC power supply is normal, and (ii) turns off a mechanical switch different from a mechanical switch turned off on occurrence of previous power failure of the plurality of mechanical switches and turns off the bypass circuit when the AC power supply fails;
• and
• a power converter that converts DC power supplied from a DC power supply into AC power and provides the AC power to the output terminal when the AC power supply fails.