Air circuit breakers in China, like those in other parts of the world, work based on the following general principles and mechanisms:

Basic Working Principle
**Electromagnetic Induction**: When the circuit is normal, the current passing through the air circuit breaker generates a certain magnetic field. The magnetic field generated by the normal current is not enough to cause the movement of the magnetic system in the circuit breaker. However, when a fault occurs and the current suddenly increases sharply, according to Ampere’s law, the increased current will generate a much stronger magnetic field.
**Thermal Effect**: In addition to the electromagnetic effect, the thermal effect also plays an important role. When an overcurrent passes through the conductor of the air circuit breaker, according to Joule’s law, the conductor will generate heat due to the resistance. The heat generated is proportional to the square of the current, the resistance of the conductor and the time. Under normal current conditions, the heat generated is within a tolerable range and will not cause the air circuit breaker to trip. But when an overcurrent occurs, the heat generated rapidly increases.

### Tripping Mechanism
**Electromagnetic Tripping**: The increased magnetic field due to the overcurrent causes the armature in the electromagnetic tripping device to be attracted, and the armature drives the tripping mechanism to move, disconnecting the circuit breaker’s contacts and cutting off the circuit. This is mainly used to protect against short-circuit faults, where the current increases instantaneously and significantly.
**Thermal Tripping**: The excessive heat generated by the overcurrent causes the bimetallic strip in the thermal tripping device to deform. The bimetallic strip is composed of two different metals with different coefficients of thermal expansion. When heated, the two metals expand at different rates, causing the bimetallic strip to bend. As the temperature rises due to the overcurrent, the bimetallic strip bends to a certain extent and pushes the tripping mechanism to trip the circuit breaker. This is mainly used to protect against overload faults, where the current is larger than the rated current but not as extreme as in a short circuit, and the overcurrent lasts for a certain period of time.

### Arc Extinction Mechanism
**Arc Generation**: When the contacts of the air circuit breaker are disconnected, an arc is generated between the contacts due to the ionization of the air and the high temperature. The arc can conduct electricity and may cause damage to the contacts and other components if it is not extinguished in time.
**Arc Extinction Principle**: Air circuit breakers usually use measures such as lengthening the arc, cooling the arc and blowing out the arc to extinguish the arc. For example, some air circuit breakers are equipped with arc extinguishing chambers. The arc extinguishing chamber is usually made of materials with good insulation and heat dissipation performance. When the arc enters the arc extinguishing chamber, it is lengthened and cooled by the baffles and grooves in the chamber. At the same time, the air flow generated by the movement of the contacts can also blow out the arc, so that the arc is quickly extinguished and the circuit is completely disconnected.

In general, Chinese air circuit breakers use a combination of electromagnetic, thermal and arc extinguishing principles to achieve the functions of overcurrent protection and circuit disconnection, ensuring the safety and stability of the power system.