1. The Concept of Low Voltage Circuit Breaker

Low-voltage circuit breaker is also known as automatic air switch or automatic air circuit breaker, referred to as circuit breaker. It is not only a manual switch, but also can automatically protect against voltage loss, undervoltage, overload and short circuit. It can be used to distribute electric energy, start asynchronous motors infrequently and protect power lines and motors. When they have serious overload or short circuit and undervoltage faults, household circuit breaker can automatically cut off the circuit. Its function is equivalent to the combination of fuse switch and overheating relay. Moreover, after breaking the fault current, there is no need to change parts, which has been widely used.

2. Structure of Low Voltage Circuit Breakers

LV circuit breaker is composed of contacts, arc extinguishing device, operating mechanism and protective device.

2.1 Contact system
Contacts (static contacts and dynamic contacts) are used in circuit breakers to realize circuit switching on or off.
The basic requirements of contacts are:
(1) It can safely and reliably connect and break the limit short circuit current and the circuit current below;
(2) Long-term working current;
(3) Within the specified number of times of electrical life, no serious wear will occur after switching on and breaking off.
The action sequence of household circuit breaker contacts is that when the circuit breaker closes, the arc contacts first close, then the secondary contacts close, and finally the main contacts close. When the circuit breaker breaks, on the contrary, the main contacts bear the load current, and the secondary contacts protect the main contacts. The arc contacts are used to bear the arc ignition and arc when the current is cut off. Only on the arc contacts, thus ensuring that the main contacts are not ablated by the arc, long-term stable work.

2.2 Arc extinguishing system
The arc extinguishing system is used to extinguish the arc between contacts when the circuit is disconnected. The arc extinguishing system consists of two parts: one is a strong spring mechanism, which makes the contacts of circuit breakers separate rapidly; the other is that an arc extinguishing chamber is arranged above the contacts.

2.3 operating mechanism
The operating mechanism of LV circuit breaker includes transmission mechanism and release mechanism.
(1) Transmission mechanism:
According to the different operation modes of circuit breaker, it can be divided into manual drive, lever drive, electromagnet drive and motor drive. According to the closure mode, it can be divided into energy storage closure and non-energy storage closure.
(2) Free release mechanism:
The function of free release mechanism is to realize the connection between transmission mechanism and contact system.

2.4 protection device
The protection device of household circuit breaker is realized by various trippers.
The types of circuit breaker release are: undervoltage release, overcurrent release, excitation release and so on. Overcurrent tripper can also be divided into overload tripper and short circuit tripper.
Undervoltage tripper is used to monitor the fluctuation of working voltage. Undervoltage tripper with delayed action can prevent voltage fluctuation caused by steep load rise and cause inappropriate breaking of circuit breaker. The delay time can be 1s, 3S and 5S.
Excitation tripper is used for remote control or thermal relay action breaker.
Overcurrent tripper is used to prevent overload and short circuit at load side.
Common circuit breakers also have short-circuit locking function, which is used to prevent circuit breakers from closing again after breaking due to short-circuit fault.

3. Function of Low Voltage Circuit Breakers

LV circuit breaker (formerly called automatic switch, air switch) is a kind of switching device which can not only connect and break normal load current and overload current, but also switch on and break short-circuit current. Low-voltage circuit breaker not only plays a control role in the circuit, but also has certain protective functions, such as overload, short circuit, under-voltage and leakage protection. Such as:
1) Used for non-frequent on-off control of low-voltage distribution circuit;
2) When the circuit is short-circuit, overload or under-voltage, the circuit will be automatically disconnected.