In case of serious overload or short circuit fault, the coil connected in series with the main circuit will generate strong electromagnetic attraction, which will attract the armature 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, the electromagnetic attraction attracts the armature and the main contact can be 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.
Extended data
C and d on the air switch represent the trip curve of the circuit breaker. There are four commonly used trip characteristics (curves) in the world: A, B, C and D.
A features: 2 times rated current, rarely used, generally used for semiconductor protection;
B characteristics: 2~3 times the rated current, generally used for transformer side secondary circuit protection;
C characteristics: 5~ 10 times the rated current, which is the most commonly used, generally used for building lighting electricity, etc.
D characteristics: 10~20 times the rated current, generally used for power distribution.
In addition, there is ABB's patent K feature, which is mainly used for motor systems with rated current below 40A (this is beyond our discussion).
References:
Baidu encyclopedia-air switch