Circuit breaker is a switching device which can close, load and break the current under normal circuit conditions, and can carry and break the current under abnormal circuit conditions (including short circuit conditions) within a specified time. LV circuit breaker can be used to distribute electric energy, start asynchronous motors infrequently, protect power lines and motors, and automatically cut off circuits when they have serious overload or short-circuit or under-voltage faults. Their functions are equivalent to the combination of fuse switches and over-heated relays. And after breaking the fault current, there is no need to change parts. At present, it has been widely applied.

Circuit breakers are generally made up of contact system, arc extinguishing system, operation mechanism, release, shell and so on. LV circuit breaker are divided into miniature circuit breakers, moulded case circuit breakers and frame type circuit breakers according to their structures.

Working principle of low voltage circuit breaker

LV circuit breaker has a variety of protection functions (overload, short circuit, under-voltage protection, etc.), adjustable operating value, high breaking capacity, easy operation, safety and other advantages, so it is widely used.

The main contacts of low voltage circuit breakers are operated by manual operation or by electric switching. After the main contact is closed, the free tripping mechanism locks the main contact on the closing position. The coil of the overcurrent release and the thermal element of the thermal release are in series with the main circuit, and the coil and the power supply of the undervoltage release are in parallel. When the circuit is short-circuited or seriously overloaded, the armature of the over-current release closes and the free release mechanism acts, and the main contact disconnects the main circuit. When the circuit is overloaded, the heat element of the thermal release causes the bimetal sheet to bend and pushes the free release mechanism to move. When the circuit is under voltage, the armature of the undervoltage release is released. It also enables the free tripping mechanism to act. Split-excitation tripper is used for long-distance control. In normal operation, its coil is power-off. When distance control is needed, press the start button to make the coil energized and the armature drives the free tripping mechanism to disconnect the main contact.

Selection of low voltage circuit breakers
1) The rated voltage of the household circuit breaker is equal to or greater than the rated voltage of the line.
2) The rated current of the circuit breaker is equal to or greater than the rated current of the line or equipment.
3) The breaking capacity of the household circuit breaker is equal to or greater than the maximum short circuit current that may occur in the circuit.
4) The rated voltage of under voltage release is equal to line rated voltage.
5) The rated voltage of the shunt release is equal to the control power supply voltage.
6) The long delay current setting value equals the rated current of the motor.
7) Instantaneous Setting Current: For the household circuit breaker protecting cage induction motor, the instantaneous setting current is 8-15 times the rated current of the motor; for the circuit breaker protecting wound induction motor, the instantaneous setting current is 3-6 times the rated current of the motor.
8) The return time of the 6 times long delay current setting value is equal to or greater than the actual starting time of the motor.