When the phase sequence is correct, the relay of the phase sequence meter is engaged; If the phase sequence is incorrect, the relay of the phase sequence meter will not engage. There are A phase, B phase, and C phase in the three-phase power supply. If the motor is connected to the ABC phase sequence power supply and the motor rotates forward, then the ACB phase sequence power supply is connected to the motor and the motor rotates in reverse. To prevent the motor from reversing, a phase sequence table is added to prevent the incoming power supply from reversing the phase sequence, causing the motor to reverse. The phase sequence meter can detect five types of fault phenomena in industrial electricity, including phase loss, reverse phase, three-phase voltage imbalance, overvoltage, and undervoltage, and timely disconnect the electrical equipment to provide protection. The earliest phase sequence meter has an internal structure similar to a three-phase AC motor, with a three-phase AC winding and a very light rotor, which can rotate at a very small torque, The working voltage range of three-phase AC winding is very wide, ranging from tens of volts to 500 volts. During testing, determine the phase sequence based on the rotation direction of the rotor. It is also possible to display different signal lights for different phase sequences through a resistance capacitance phase shifting circuit