To prevent generation of power supply short circuit, an electrical hazard, or a malfunction caused by back flow even if the thyristor of one AC power supply is in a short-circuit mode.
Thyristor switches 19, 20 are mounted respectively on two- system AD power supply supplied to a common load. Thyristor switches 19, 20 switch the two-system AD power supply. Input switches 8, 9 are mounted on the inputs of both the thyristor switches. A controller 30 is formed which turns on the one thyristor switch 19, when turning-off is conducted during a prescribed period from the zero cross point of the input voltage of the respective AD power supply and a prescribed voltage is generated at both ends of thyristor switches within a prescribed period, and turns on the other thyristor switch 20 and commands to turn off the input switch 8 mounted on the input of the thyristor switch 19, when the prescribed voltage is not generated within a prescribed period.
NISHIMURA MASANORI
ARAI TORU
