The black boxes in the three places are air switches (main system), not AC contactors mentioned upstairs. There are several terminals on the AC contactor. The connection mode of the air switch is that the phase wires (live wires) of the three power lines are directly connected to the top connection of the switch, and the following three connection points are connected to the load. The zero line is directly connected to the zero line of the load. Its working principle is that the air switch, that is, the circuit breaker, makes on and off and carries the rated working current in the circuit, which can reliably protect the line and the motor under overload, short circuit and undervoltage conditions. The dynamic and static contacts and contact rods of the circuit breaker are designed in parallel, and the dynamic and static contacts are disconnected by using the electric repulsion generated by short circuit, which has high breaking capacity and strong current limiting characteristics. During short circuit, the aromatic insulation around the stationary contact is gasified, which plays the role of cooling and arc extinguishing, and the arc burning distance is zero. The arc-extinguishing chamber of circuit breaker adopts metal grid structure, the contact system has repulsion and current-limiting mechanism, and the circuit breaker has high breaking ability and current-limiting ability. Multiple releases. The inverse time action is that the bimetal is heated and bent to make the release move, and the instantaneous action is that the iron core street iron mechanism drives the release to move. There are three tripping modes: thermal tripping, electromagnetic tripping and compound tripping. When the line is short-circuited or the current is seriously overloaded, the short-circuit current exceeds the instantaneous tripping setting current value, and the electromagnetic release generates enough suction force to attract the armature to impact the lever, so that the hook rotates upward around the rotating shaft seat and disengages from the lock catch, and the lock catch disconnects three pairs of main contacts and cuts off the power supply under the action of the reaction spring.

When the line is generally overloaded, the overload current can't make the electromagnetic trip work, but it can make the thermal element generate a certain amount of heat, which will make the bimetal bend upward when heated, thus pushing the lever to disengage the hook from the lock, disconnecting the main contact and cutting off the power supply. If the main contact is closed by the operating mechanism (manual or electric), its contact system can not only connect or cut off the normal working current, but also quickly cut off the fault current several times larger than the normal working current when the fault occurs. When the main contact is closed by the operating mechanism, it is locked in the closed position by the locking hook. If there is a fault in the circuit, the related trip will release the lock hook in the trip mechanism, so the main contact will be quickly disconnected under the action of the trip spring. According to the different protection functions, the release can be divided into overcurrent release and pressure loss release. Under normal circumstances, the armature of the overcurrent release is released; When serious overload or short circuit fault occurs, the coil connected in series with the main circuit will generate strong electromagnetic attraction, which will attract the street rail downwards and push the unlocking hook to disconnect the main contact. The working principle of undervoltage release is just the opposite. When the voltage is normal, electromagnetic attraction attracts the armature and the main contact is closed. Once the voltage drops seriously or the power is cut off, the armature is released and the main contact is disconnected. When the power supply voltage returns to normal, it must be switched on again to work, thus realizing voltage loss protection.