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.

為了保證低壓配電系統的正常工作，低壓斷路器的選擇性也成為終端低壓配電系統設計中的一項重要內容。在斷路器所保護的配電系統中，當發生電氣故障時，距故障點近的斷路器動作將故障切除，而其他各級斷路器不動作，從而將故障所造成斷電限制在小范圍內，使其無故障供電回路仍能保持正常供電，這就是對低壓斷路器所要求的擇性。

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.

非選擇性的低壓斷路器，所指的是當發生電氣故障時，距故障點近的低壓斷路器動作將故障切除，而其他各級斷路器也動作，均處于打開狀態，不能保證使其他無故障回路正常供電。低壓斷路器的選擇性在低壓配電系統的設計中占有十分重要的位置，它可以給用戶帶來便利，并能保證供電回路工作的連續性。所以，家用電器在無選擇性保護下，一旦發生電氣故障，配電回路的連續性就不能得到保證，使家用電器如電冰箱、排油煙機等處于停機待啟動狀態，影響了用戶的日常生活。塑殼斷路器能夠在電流超過跳脫設定后自動切斷電流。

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.