1.1. For the fault caused by the influence of environment temperature on low-voltage electrical appliances, the low-voltage electrical appliances in the distribution box are composed of fuse, AC contactor, residual current action protector, capacitor and meter. These low-voltage electrical appliances are designed and manufactured in accordance with gb1497 basic standard for low voltage electrical appliances, and their normal working conditions are specified accordingly: the upper limit of ambient air temperature is not more than 40 ℃; the average value of ambient air temperature for 24 hours is not more than 35 ℃; the lower limit of ambient air temperature is not less than - 5 ℃ or - 25 ℃. The distribution box of rural power grid transformation operates outdoors. It not only generates high temperature by direct sunlight, but also generates heat in operation. Therefore, in the hot summer season, the temperature in the box will reach more than 60 ℃, which greatly exceeds the specified ambient temperature of these electrical appliances. Therefore, the failure caused by overheating of electrical components in the distribution box will occur.

1.2. During the transformation of rural power grid, due to the large number of distribution boxes required and the short construction period, the distribution box factory needs a large number of low-voltage electrical appliances, which leads to the phenomenon that the requirements for product quality are not strict, resulting in the failure of some products shortly after they are put into operation. For example, some models of AC contactors cannot operate because the closing coil of contactors is burnt out soon after the distribution box is put into operation.

1.3 faults caused by improper selection of electrical appliances in the distribution box due to improper selection of AC contactor capacity during manufacturing, installation of AC contactor with the same capacity for different outgoing circuits, and failure to consider the imbalance of three-phase load, and failure to improve the current level selection of some outgoing contactors on the basis of normal selection of models, resulting in summer The AC contactor is burnt out during operation in high temperature season.