Undervoltage release is in its terminal voltage drops below a predetermined range, there is a delay circuit breaker or disconnecting a delay-free release when the supply voltage drops (or even slow down) to the rated work within the range of 70 to 35% voltage, under-voltage release should operate undervoltage release trip equal to 35% of the rated voltage in the power source voltage, under-voltage release should be possible to prevent the circuit breaker is closed; when the power supply voltage is equal to or greater than 85% of the rated working voltage of the undervoltage release, under hot conditions, it should be able to ensure reliable circuit breaker is closed. Accordingly, when the power supply voltage constant voltage drop by the protective circuit, the circuit breaker can automatically disconnect the power supply so that the electrical load of the circuit breaker following electrical equipment from damage or undervoltage. In use, the undervoltage trip coil is connected to the power supply side of the circuit breaker, the undervoltage release is energized, the circuit breaker closing order, otherwise the circuit breaker not close the gate.