According to IEC947-2 "low voltage switchgear and control equipment, low voltage circuit breaker" GBl4048.2, there are two standards for breaking capacity of low voltage circuit breaker, one is ultimate short-circuit breaking capacity  ICU, which means that after breaking the current according to the test cycle, the circuit breaker does not assess the ability to continue to carry the current, that is, it can no longer be used. The other is operating short-circuit breaking capacity ICs, which means that the circuit breaker needs to continue to carry the rated current after breaking the current. That is to say, it can continue to be used in the line after troubleshooting. These two values are the effective values of periodic components. Generally, ICS is expressed as% of ICU in manufacturer and design data. According to the manufacturing specification of the circuit breaker, there are four standard ratios of ICs =  25%, 50%, 75% and 100% ICU, which are rounded to the nearest integer. These two breaking capacity data are essential for us to select MCCB and verify breaking capacity. In the manufacturer's samples of circuit breakers, ICU is generally provided. According to the design specifications, the breaking current  IFD of low-voltage circuit breakers should be greater than the maximum short-circuit current  ID of load side. This raises the question of whether to choose ICU or ICs as IFD. The author thinks that when the circuit breaker is used in more important distribution lines or equipment, after the short circuit fault is eliminated, it should be considered according to ICS value if the power supply is required to be restored quickly. If it is a general distribution circuit, it can be assessed according to ICU value. MCCB with action time less than 0.02s can also be evaluated according to ICU value. In fact, according to the relevant design manual, the breaking capacity inspection requirements for the low-voltage circuit breaker at this time are that the breaking capacity of the circuit breaker  effective value of impulse current, or peak current should be greater than the effective value or peak value of full current in the first cycle of short circuit.