The performance of small explosion-proof switches used underground in coal mines directly affects the quality and production safety of coal mine power supply. Article 432 of the Coal Mine Safety Regulations stipulates that underground low-voltage motors should have protection against short circuits, overloads, single-phase disconnections, and remote control devices. Therefore, it is of great significance to add a phase failure protection function inside the small explosion-proof switch, which realizes the detection of phase failure before power transmission and the protection of phase failure during operation, to achieve the goal of protecting phase failure throughout the entire period of time.

　　1. Function of protecting phase failure

　　The protection of phase failure has the functions of phase failure locking, phase failure protection, and fault location.

　　Phase failure lockout refers to the inspection of the power supply line and electrical equipment for phase failure before each power transmission. If a phase failure fault is found, the small explosion-proof switch cannot be closed for power transmission through signal lockout; If no phase failure is found, the circuit can be closed and powered on normally.

　　Phase failure protection refers to the process in which a small explosion-proof switch is closed and powered on, and the circuit and equipment operate normally. During this normal operation, if a phase failure or power supply phase loss fault suddenly occurs, the phase failure protection circuit will force the small explosion-proof switch to trip and cut off power. Fault localization is to determine the phase and location of a broken phase fault.

　　2. Principle of implementing phase failure protection function

　　The protection of phase failure is achieved through the following electrical principles based on the specific functions implemented.

　　⑴ Phase failure locking. The phase failure interlock detection adopts the additional DC power supply method. The circuit principle is shown in Figure 1. The isolation switch first connects the+5V power supply. As the vacuum contactor is not powered, the ZC and contacts do not close. Therefore, the+5V power supply forms a circuit through the motor winding, the normally closed contacts of the intermediate relay, and the resistor. Determine the integrity of the circuit by checking the signals at terminals 1 and 2.

　　⑵ Lack or phase failure protection. When there is a two-phase short circuit, phase failure, power phase loss, and three-phase load imbalance in the circuit, negative sequence current components are generated. Theoretical calculations show that there is a significant difference in negative sequence current between two-phase short circuit and phase failure; The negative sequence current generated by two-phase short circuit faults and phase failure faults is much greater than the unbalanced current of the line. Negative sequence current filter is used for phase loss and interruption protection.

　　⑶ Detection of phase failure points. The fault point of phase failure is detected using high-frequency signals.