At present, the general power supply of MCCB made in China can only be upper terminal incoming line  power terminal, lower terminal outgoing line  load terminal, and its rated breaking capacity is also measured under this condition. This is determined by the arc extinguishing performance of mc-cb. Some foreign products are greatly improved with the traditional MCCB in terms of arc extinguishing system, which can realize the exchange of power supply end and load end without affecting the breaking capacity. If it is required to enter, go up and go out or be used as tie circuit breaker under special conditions in the project, the author thinks that the interchangeable products at the power supply end and load end should be selected first. As a last resort, the qualified mc-cb must be selected in strict accordance with the conversion of capacity reduction coefficient provided by the manufacturer, which is generally reduced by about 20% - 30%.