Miniature circuit breaker (MCB) is a kind of terminal protection appliance widely used in building electrical terminal distribution equipment. Although MCB is a kind of terminal electrical equipment, it often causes serious losses if improper MCB is selected. Therefore, it is very important to choose the right MCB for protection equipment.

MCB的额定分断能力就是在保证断路器不受任何损坏的前提下能分断的大短路电流值。现在，市场上制造厂商提供的产品主要有4.5kA，6kA，10kA等几种额定分断能力。

The rated breaking capacity of MCB is the large short-circuit current value that can be broken without any damage to the circuit breaker. At present, the main products provided by manufacturers in the market are 4.5ka, 6ka, 10kA and other rated breaking capacity.

我们在选用MCB时，应该像在选择塑壳断路器、框架式断路器一样，计算在使用场合的大短路容量，再选择MCB。如果MCB的额定分断能力小于被保护范围内的短路故障电流，则在发生故障时，不但不能分断故障线路，还有可能因MCB的分断能力过小而引起MCB爆炸，危及人身和其他电器设备线路的安 全运行。

When we choose MCB, we should calculate the large short-circuit capacity in the use situation just as we choose moulded case circuit breaker and frame circuit breaker, and then choose MCB. If the rated breaking capacity of MCB is less than the short-circuit fault current within the protected range, the fault line can not be broken in case of fault, and the MCB explosion may be caused by the too small breaking capacity of MCB, which may endanger the safe operation of personal and other electrical equipment lines.

低压配电线路的短路电流与该供电线路的导线截面、导线敷设方式、短路点与电源距离长短、配电变压器的容量大小等电器参数有关。一般工业与民用建筑配电变压器低压侧电压多为0.23/0.4kV，变压器容量大多为1600kVA以下，低压侧线路的短路电流随配电容量的增 大而增 大。对不同容量的变压器，低压馈线端的短路电流是不同的。

The short-circuit current of the low-voltage distribution line is related to the electrical parameters of the power supply line, such as the cross-section of the wire, the laying mode of the wire, the distance between the short-circuit point and the power supply, and the capacity of the distribution transformer. The voltage at the low voltage side of the distribution transformer in general industrial and civil buildings is mostly 0.23/0.4kv, and the transformer capacity is mostly less than 1600KVA. The short-circuit current of the line at the low voltage side increases with the increase of the distribution capacity. For different capacity transformers, the short circuit current of low voltage feeder terminal is different.

一般来说，对于民用住宅、小型商场、公用建筑，由于由当地供电部门的低压电网供电，供电线路的电缆截面较细，用电设备距供电电源距离较远，选用4.5kA及以上分断能力的MCB即可。

Generally speaking, for residential buildings, small shopping malls and public buildings, due to the power supply from the low-voltage power grid of the local power supply department, the cable section of the power supply line is relatively thin, and the distance between the electrical equipment and the power supply source is relatively long, the MCB with 4.5ka and above breaking capacity can be selected.

对于有有10kVA变电所的用户，往往由于供电线路的电路截面较粗，供电距离较短，应选用6kA及以上分断能力的MCB。而对于如变配电站(站内使用的照明、动力电源直接取之于低压总母排)以及大容量车间变配电站等供电距离较短的类似场合，则必 须选用10kA及以上分断能力的MCB。

For users with 10KVA substation, the MCB with 6ka and above breaking capacity should be selected because the circuit section of power supply line is thick and the power supply distance is short. For similar occasions with short power supply distance, such as power transformation and distribution station (the lighting and power supply used in the station are directly from the low-voltage main bus bar) and large capacity workshop power transformation and distribution station, MCB with 10kA and above breaking capacity must be selected.