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。現在，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。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。

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. When we choose MCB, we should calculate the 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.

MCB的過載保護依靠熱脫扣器，通常，現有MCB的熱脫扣器額定電流是生產廠家根據IEC898標準在基準溫度為30℃條件下整定的，MCB的工作溫度一般推薦為－25℃～55℃，熱脫扣器由一種雙金屬片組成，當通過的電流達到某設定值并維持一定時間后使MCB脫扣。因此，熱脫扣器與溫度是息息相關的。如環境溫度變化將導致MCB的工作溫度變化，使熱脫扣器的工作特性相應變化。

The overload protection of MCB depends on the thermal release. Generally, the rated current of the thermal release of the existing MCB is set by the manufacturer under the reference temperature of 30 ℃ according to iec898 standard. The working temperature of MCB is generally recommended to be - 25 ℃ ~ 55 ℃. The thermal release is composed of a bimetallic sheet. When the current reaches a certain set value and maintains for a certain time, the MCB will release. Therefore, the thermal release is closely related to the temperature. If the ambient temperature changes, the working temperature of MCB will change, and the working characteristics of thermal release will change accordingly.

由于MCB通常安裝在配電箱內，使用環境溫度不可能恒定在30℃，實際使用時，終端配電箱內的MCB是緊密無間的安裝在一起的，且大多數場合又是嵌在墻內安裝，導致散熱效果差，使配電線路內的溫升很大，故MCB的實際工作溫度總比環境溫度高10～15℃左右。因此當環境溫度大于或小于校準溫度值時，我們必須根據相關制造廠商提供的溫度與載流能力修正曲線來調整MCB的額定電流值。一般來說，當環境溫度大于或小于溫度校正值10℃時，MCB的額定電流值需減小或增大5％左右。

Because MCB is usually installed in the distribution box, the ambient temperature can not be constant at 30 ℃, in actual use, the MCB in the terminal distribution box is tightly installed together, and in most cases, it is embedded in the wall, resulting in poor heat dissipation effect and large temperature rise in the distribution line, so the actual working temperature of MCB is always about 10 ~ 15 ℃ higher than the ambient temperature. Therefore, when the ambient temperature is greater than or less than the calibration temperature, we must adjust the rated current value of MCB according to the correction curve of temperature and current carrying capacity provided by relevant manufacturers. Generally speaking, when the ambient temperature is greater than or less than the temperature correction value of 10 ℃, the rated current value of MCB should be reduced or increased by about 5%.