The low-voltage breaker is also called as automatic air switch or automatic switch. The functions of the breaker are: When there are dangerous faults in circuit such as short circuit, serious overload and loss of voltage. The low-voltage breaker can automatically switch off the circuit with faults, and effectively protect the electric equipment connected in series behind it. Therefore, the low-voltage breaker is a kind of very important protective electrical appliance in low-voltage power distribution network. Under normal condition, the low-voltage breaker is used for not frequently switching on and switching off circuit and controlling the motor . The low-voltage breaker has advantages such as safe operation, adjustable operating value and good breaking capacity, combination of all kinds of defensive function, so it can be widely used in electric engineering. 

According to different conditions, the low- voltage breakers can be classified as below:

The shape structure of the low-voltage breaker can be classified into two kinds, universal (DW series) and plastic shell type (DZ series). 

As per the movement velocity, they can be classified into two kinds, common type and rapid type. As per the use, they can be classified into the breaker used for power distribution, the breaker used for motor protection, the de-excitation breaker and the electric leakage breaker, etc. 

Working principle of the low-voltage breaker: 

The main components of low- voltage breaker consist of contact terminal system, protective devices, arc extinction devices, operation structure and so on. And the contact terminal system consists of main contact, arc contact and supplementary contact generally.

Arc extinction devices: The method of arc extinction by sheet is adopted, namely, the steel sheets with different length cross and form an arc chute, which is put in arc extinguish chamber made from insulating materials to form an arc extinction device. 

Protective devices: The various trippers realize the protection functions for short circuit, overload, loss of voltage etc.. According to the protection function, the trippers are classified into: overcurrent tripper, the tripper for loss of voltage and thermal tripper, etc. 

Working principle: The working principle of low-voltage fuse protector: The 3 pairs of main contacts of low-voltage breakers are connected in series in the protected triphase main circuit. When the green button is pressed, the 3 pairs of main contacts in main circuit grapple the agraffe by chain to overcome the pull of spring, and keep in closed status. 

When the circuit works normally, the suction produced in the coil of magnetic tripper is unable to pull in its armature. If there are short circuits and large overcurrent occur in circuit, the suction of magnetic tripper increases to pull in the armature and hit the lever, then push the agraffe upwards to switch off main contact. If the voltage in circuit declines or is lost voltage, the suction of the tripper for undervolatage will be decreased or lost, the armature will be pulled open by the spring to hit the lever, push the agraffe away and switch off the main contact. All of the above is the operation principle for the low-voltage breaker.