The upper limit of ambient air temperature around the distribution box shall not exceed 40 ℃, the average ambient air temperature within 24 hours shall not exceed 35 ℃, and the lower limit of ambient air temperature shall not be lower than - 5 ℃ or - 25 ℃. The IEC 61439-1 sets the thermal limit in busbars working at the maximum working load. Here, 140°C (which is 105K over the ambient temperature of 35°C) is the upper safe temperature limit. Abnormal tests are performed to verify that the product will remain safe even when there is a fault condition, such as. The solutions are based on maintaining the internal temperature of the enclosure below 60°C. The distribution box of rural power grid transformation operates.