Circuit breaker is a kind of switching device which can close, load and break the current under the condition of normal circuit and can close, load and break the current under the condition of abnormal circuit in the prescribed time. Household circuit breaker are divided into high-voltage circuit breakers and low-voltage circuit breakers according to their scope of use. The dividing line between high and low voltage is blurred. Generally, the circuit breakers above 3 kV are called high-voltage electrical appliances.

Household 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.

Low-voltage circuit breaker is widely used in building low-voltage distribution system. It is a protective electrical component with the functions of circuit breaking protection, overload protection, control and isolation. It is suitable for industrial and civil terminal low-voltage distribution system. Improper design of lv circuit breaker in the distribution system will affect the normal operation of the power supply loop. Therefore, in the design of low-voltage distribution system, attention should be paid to the selectivity of low-voltage circuit breakers and cascade protection and other details. The following circuit breaker manufacturers will analyze the application of low voltage circuit breakers in the distribution system.

In the distribution system protected by circuit breakers, when an electrical fault occurs, the household circuit breaker nearest to the fault point removes the fault while the other circuit breakers do not operate, thus limiting the power failure caused by the fault to a minimum, so that other fault-free power supply circuits can still maintain normal power supply. This is the low-voltage circuit breaker station. Selectivity of requirements. The selectivity of lv circuit breaker plays an important role in the design of low-voltage distribution system. It can bring convenience to users and ensure the continuity of the power supply circuit because the continuity of the distribution circuit can not be guaranteed in case of electrical failure in household appliances without selective protection. Appliances, such as refrigerators and lampblack exhauster, are in a state of shutdown, which affects the daily life of users.

Low voltage circuit breakers in low-voltage distribution systems can be classified into two categories: selective and non selective according to their protection performance. Selective lv circuit breaker have two-stage protection and three-stage protection. Instantaneous characteristics and short delay characteristics are suitable for short-circuit operation and long delay characteristics are suitable for overload protection. Non-selective low-voltage circuit breakers are generally instantaneous, only for short-circuit protection, and some for long-delay action, only for load protection.

In low-voltage distribution system, if the upper circuit breaker uses selective circuit breaker, the lower circuit breaker uses non-selective circuit breaker or selective circuit breaker, mainly using the difference of delay action or delay action time of short delay tripper to obtain selectivity. When we delay the action of the last circuit breaker, we should pay attention to the following problems:

1. Whether the next stage is a selective or non-selective circuit breaker, the setting current of the instantaneous overcurrent tripper of the upper stage circuit breaker should not be less than 1.1 times the maximum three-phase short circuit current of the outlet of the next stage circuit breaker.

2. If the next stage is a non-selective circuit breaker, in order to prevent short-circuit current from occurring in the protective circuit of the next stage, the short-delay overcurrent tripper of the upper stage acts first to make it lose selectivity because of the inadequate sensitivity of the instantaneous operation of this stage. The fixed current is not less than 1.2 times of the instantaneous overcurrent release of the next stage.

3. If the next stage is also a selective circuit breaker, in order to ensure selectivity, the short delay operation time of the upper stage circuit breaker is at least 0.1 seconds longer than the short delay operation time of the next stage circuit breaker.