This question is a good answer, mainly from two aspects of voltage and current. The following mainly analyzes the selection of Moulded Case Circuit Breaker MCCB for distribution line protection. The selection of moulded case circuit breakers on motor protection is not within the scope of this discussion.

1. Rated voltage, that is, the voltage level of the distribution system where the Moulded Case Circuit Breaker MCCB is located, the common low voltage distribution system is 400V. If the ordinary people live in the residential district, the distribution transformer in the distribution room is 10/0.4kV, usually uses the TN-C-S or TN-S system, the neutral point of the voltage transformer is directly grounded; and of course, it does not exclude 690. V, for example, the low voltage distribution of the wind electric field is generally 690V., so the rated voltage of the plastic case circuit breaker is adapted to the system voltage, and the limit breaking capacity and operation breaking capacity of the Moulded Case Circuit Breaker MCCB are closely related to the rated voltage.

2. Rated limit short circuit breaking current Icu, according to the GB14048.2 low voltage circuit breaker standard, is the breaking capacity of the circuit breaker which is not required to load the rated current again after a short circuit breaker is broken, and the rated running short circuit breaking current Ics is the interruption that the circuit breaker needs to continue to load the rated current after breaking the short circuit current. The latter is generally 25,50,75100 of the former.

The rated limit short circuit breaking current Icu of the cm1 100 mccb switch and the rated running short circuit breaking current Ics are related to the rated voltage. The same type of plastic case circuit breaker is different at different rated voltage. The higher the voltage grade is, the more difficult the arc extinguishing is, the lower the short circuit current of the breaker can be broken. For example, the NSX100F molded case circuit breaker in the following figure, at 380/415V voltage, has a breaking capacity of 36kA, and its breaking capacity is 8kA at 660/90V rated voltage.

3. The rated current of the cm1 100 mccb switch, the rated current of the circuit breaker equal to the rated uninterrupted current Iu and equal to the agreed free air heating current Ith, that is, the circuit breaker can carry the current continuously without exceeding the temperature rise. The current of the cm1 100 mccb switch refers to the rated current of the maximum overcurrent release of the circuit breaker, for example, the NSX100 circuit breaker has two kinds of anti - time overcurrent tripping current rating 40A and 100A, so 100A is the NSX100F shell current. The setting current of the reverse time overcurrent release is based on the current value calculated by the circuit to calculate the current and the long-term permissible long-term running current of the conductor on the release.

According to the requirements of industrial and civil power distribution design manual, the shell frame current of the circuit breaker is more than the reverse time overcurrent release current rating.

The setting time of circuit breaker reverse time overcurrent release is greater than or equal to the calculated current of the circuit, which is less than the allowable continuous current carrying capacity of the conductor.

With the simple analysis of the rated voltage, short circuit breaking capacity, rated current, and setting current of the plastic case circuit breaker, we believe that you have a preliminary concept on how to choose the plastic case circuit breaker of the distribution line.