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的額定分斷能力就是在保證斷路器不受任何損壞的前提下能分斷的大短路電流值?，F在，市場上制造廠商提供的產品主要有4.5kA，6kA，10kA等幾種額定分斷能力。

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.

我們在選用MCB時，應該像在選擇塑殼斷路器、框架式斷路器一樣，計算在使用場合的大短路容量，再選擇MCB。如果MCB的額定分斷能力小于被保護范圍內的短路故障電流，則在發生故障時，不但不能分斷故障線路，還有可能因MCB的分斷能力過小而引起MCB爆炸，危及人身和其他電器設備線路的安 全運行。

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.

低壓配電線路的短路電流與該供電線路的導線截面、導線敷設方式、短路點與電源距離長短、配電變壓器的容量大小等電器參數有關。一般工業與民用建筑配電變壓器低壓側電壓多為0.23/0.4kV，變壓器容量大多為1600kVA以下，低壓側線路的短路電流隨配電容量的增 大而增 大。對不同容量的變壓器，低壓饋線端的短路電流是不同的。

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.

一般來說，對于民用住宅、小型商場、公用建筑，由于由當地供電部門的低壓電網供電，供電線路的電纜截面較細，用電設備距供電電源距離較遠，選用4.5kA及以上分斷能力的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.

對于有有10kVA變電所的用戶，往往由于供電線路的電路截面較粗，供電距離較短，應選用6kA及以上分斷能力的MCB。而對于如變配電站(站內使用的照明、動力電源直接取之于低壓總母排)以及大容量車間變配電站等供電距離較短的類似場合，則必 須選用10kA及以上分斷能力的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.