A three-phase automatic power switch was designed and implemented using three voltage comparators (LM741 AH1883), 3-input Y gate (4073), two AC 108 transistors and 12V, 30mA relay, as well as some polarization resistors . The voltage comparators (LM741 AH1883) were polarized to detect the unregulated voltage, one for each of the three phases (Rø, Yø, Bø) and then couple the analog outputs to the 3-input AND gate (4073). The AND gate produces an output of "0" (OFF) when all three-phase input voltages are all within the normal (preset) range, but produces an output of "1" (ON) which implies a voltage drop or phase in at least one of the phases compared. The output of the door when coupled to the base of the switching transistors (BC 108) determines its states (OFF or ON). Since the transistors are configured in a Darlington pair arrangement, the second is only on when the first one is off. This activates the public power supply due to the normal phase voltage. In contrast, when the first transistor is ON, the 12V battery produces a potential that activates the alternate power source (generator) through the 12 V and 30 mA relays, thus breaking the contact of the public power supply to the side of the generator. The switch has been tested to have an optimal function within ± 5% rated voltage of 220 or 415V supply at the switch point to an alternate power supply. Therefore, this device may be of industrial or domestic use, where the three-phase power supply is available with a standby power supply.