1. Faults caused by the influence of ambient temperature on low-voltage electrical appliances

The low-voltage electrical appliances in the distribution box are composed of fuses, AC contactors, residual current operated protectors, capacitors and meters. 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.

2. Faults caused by product quality

In the transformation of rural power grid at that time, due to the large number of distribution boxes needed and the short construction period, the distribution box factory needed a large number of low-voltage electrical appliances, which resulted in the phenomenon that the requirements for product quality were not strict, resulting in the failure of some products shortly after they were 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. 3. Faults caused by improper selection of electrical appliances in the distribution box

Due to the improper selection of AC contactor capacity in manufacturing, the installation of AC contactors with the same capacity for different outgoing circuits without considering the imbalance of three-phase load, and the failure to improve the current level selection of some outgoing contactors based on the normal selection of models, resulting in the burning out of AC contactors during operation in high temperature seasons in summer Situation.