Automatic transfer switch (hereinafter call as MQ1) is suitable for two-path power supply system with AC 50 Hz/60Hz, rated working voltage 400V, rated current from 6A ~ 800A. It can automatically transfer power supply to reserve power or generator when common power occurring fault so as to guarantee the reliability and safe of supply power. MQ1 has the protective function of overload, under-voltage, short circuit and lack phase which is especially suitable for using in the place where does not allow power supply failure, such as fire fighting, hospital, marketplace, military project, high-rise building, bank, TV station and so on.