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。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。

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的过载保护依靠热脱扣器，通常，现有MCB的热脱扣器额定电流是生产厂家根据IEC898标准在基准温度为30℃条件下整定的，MCB的工作温度一般为－25℃～55℃，热脱扣器由一种双金属片组成，当通过的电流达到某设定值并维持一定时间后使MCB脱扣。因此，热脱扣器与温度是息息相关的。如环境温度变化将导致MCB的工作温度变化，使热脱扣器的工作特性相应变化。

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通常安装在配电箱内，使用环境温度不可能恒定在30℃，实际使用时，终端配电箱内的MCB是紧密无间的安装在一起的，且大多数场合又是嵌在墙内安装，导致散热效果差，使配电线路内的温升很大，故MCB的实际工作温度总比环境温度高10～15℃左右。因此当环境温度大于或小于校准温度值时，我们必须根据相关制造厂商提供的温度与载流能力修正曲线来调整MCB的额定电流值。一般来说，当环境温度大于或小于温度校正值10℃时，MCB的额定电流值需减小或增大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%.