It is used in the control circuit of the motor composed of switching appliances such as AC contactors in circuits with a main circuit of 50Hz and voltage up to AC1000V. When the main circuit of the motor has phase failure, overload, blocked rotor, three-phase current imbalance, etc., abnormal operation occurs. state, promptly disconnect the electrical switch contacts, disconnect the three-phase power supply of the motor, and quickly protect the motor.