Low voltage circuit breaker is a kind of electrical appliance with open circuit protection, overload protection, control and isolation functions. It has been widely used in terminal low-voltage distribution systems such as industrial and civil buildings. At present, intelligent low-voltage circuit breaker has also been introduced. In the distribution system, if it is not designed properly, it will affect the normal operation of the power supply circuit. The following problems should be paid attention to in the design of low-voltage circuit breaker in distribution system.

为了保证低压配电系统的正常工作，低压断路器的选择性也成为终端低压配电系统设计中的一项重要内容。在断路器所保护的配电系统中，当发生电气故障时，距故障点近的断路器动作将故障切除，而其他各级断路器不动作，从而将故障所造成断电限制在小范围内，使其无故障供电回路仍能保持正常供电，这就是对低压断路器所要求的择性。

In order to ensure the normal operation of low-voltage distribution system, the selectivity of low-voltage circuit breaker has also become an important content in the design of terminal low-voltage distribution system. In the distribution system protected by the circuit breaker, when an electrical fault occurs, the circuit breaker close to the fault point acts to remove the fault, while the circuit breakers at other levels do not act, so as to limit the power failure caused by the fault to a small range, so that the fault free power supply circuit can still maintain normal power supply, which is the selectivity required for the low-voltage circuit breaker.

非选择性的低压断路器，所指的是当发生电气故障时，距故障点近的低压断路器动作将故障切除，而其他各级断路器也动作，均处于打开状态，不能保证使其他无故障回路正常供电。低压断路器的选择性在低压配电系统的设计中占有十分重要的位置，它可以给用户带来便利，并能保证供电回路工作的连续性。所以，家用电器在无选择性保护下，一旦发生电气故障，配电回路的连续性就不能得到保证，使家用电器如电冰箱、排油烟机等处于停机待启动状态，影响了用户的日常生活。塑壳断路器能够在电流超过跳脱设定后自动切断电流。

The non selective low-voltage circuit breaker refers to that when an electrical fault occurs, the low-voltage circuit breaker close to the fault point acts to remove the fault, while other circuit breakers at all levels also act and are open, which can not ensure the normal power supply of other fault free circuits. The selectivity of low-voltage circuit breaker plays a very important role in the design of low-voltage distribution system. It can bring convenience to users and ensure the continuity of power supply circuit. Therefore, under the non selective protection of household appliances, in case of electrical failure, the continuity of distribution circuit can not be guaranteed, so that household appliances such as refrigerator and range hood are in the state of shutdown and waiting to be started, which affects the daily life of users. The molded case circuit breaker can automatically cut off the current when the current exceeds the tripping setting.