POWER SUPPLY SWITCHING DEVICE AND ITS METHOD

PROBLEM TO BE SOLVED: To provide a power supply switching device capable of safely and surely performing the switching work of a power unit, and realizing the stable and highly reliable switching work to a load. SOLUTION: A first AC power unit and a second AC power unit are connected to an input part 2 so as to be attachable and detachable. A load is connected to an output part 3 so as to be attachable and detachable. A changeover switch 4 applies either the first or second AC power inputted from the first and second AC power units to the input part 2 to an output part 3 as a power to be supplied to the load under the control of a control circuit 5. The control circuit 5 monitors the first and second AC powers, and switches the changeover switch 4 under a condition that synchronization is established between those AC powers.