Distribution Box Fault Improvement

(1) Cause of failure

2. Malfunctions resulting from the impact of surrounding temperatures on low-voltage electrical equipment.

In the distribution box, you will find a range of low-voltage electrical appliances. These appliances include fuses, AC contactors, residual current action protectors, capacitors, and meters. All of these devices are designed and manufactured in accordance with the standards set by GB1497, which is the "Basic Standards for Low-Voltage Electrical Appliances". To ensure their proper functioning, specific working conditions have been laid out. For instance, the maximum ambient air temperature should not surpass 40°C, and the average temperature over a 24-hour period should not exceed 35°C. Similarly, the lowest limit for ambient air temperature should not fall below -5°C or -25°C. These guidelines guarantee the optimal performance of the low-voltage electrical appliances.

The outdoor distribution box of the rural power grid is subject to harsh environmental conditions. It is not just continually exposed to direct sunlight which generates high temperatures, but it also generates heat itself while in operation. As a result, during the peak of summertime, the temperature inside the box can go beyond 60°C. This is a temperature level that far exceeds the standard ambient temperature needed by electrical appliances. The excessive heat buildup can lead to failures due to the overheating of electrical components housed in the distribution box.

  2. Failure caused by product quality

The rural power grid transformation required a significant number of distribution boxes to be installed in a short period, necessitating the timely and voluminous supply of low-voltage electrical appliances from the distribution box factory. Unfortunately, some products failed quickly after installation. For instance, the contactor closing coil of certain types of AC contactors burned out soon after the distribution box was put into operation, rendering them inoperable.

Improper selection of electrical appliances in the distribution box can lead to various faults.

During the manufacturing process, AC contactors with inappropriate capacity were mistakenly installed for various outlet circuits. The critical aspect of considering the unbalanced three-phase load was overlooked. Consequently, some outlet contactors failed to maintain normal current levels and adjustments. To rectify this, a higher current level selection was implemented. Unfortunately, during the scorching summer months, the increased load resulted in the burning out of these AC contactors. It is evident that the hasty decision in model selection has proven detrimental in the hot weather conditions.

(2) Improvement plan

The distribution box with a distribution transformer capacity of 100kV A or higher is equipped with a temperature control relay (JU-3 or JU-4 ultra-small temperature relay) and a shaft flow fan. These components are positioned on the left side of the control board, above the box. Their purpose is to activate the exhaust fan automatically when the temperature inside the box reaches a predetermined value, typically 40°C. This activation helps in effectively dissipating heat from the box, ensuring optimal functioning.

One way to prevent failures in external circuits powered by distribution boxes is to use protection circuits. Installing a small and intelligent phase loss protector like the DA88CM-II motor phase loss protection module (Shanghai product) inside the distribution box can help prevent motor burnout caused by low voltage phase loss operation. This type of protector is especially useful in maintaining the integrity of the entire system even during unstable conditions. By using this protection module, you can ensure the longevity of all the external circuits that rely on your distribution box.

A recommended solution is to enhance the wiring method of the original low-voltage capacitor bank in the distribution box. This includes relocating the capacitor bank from the current installation position at the pile head of the AC contactor. Instead, place it between the low-voltage incoming line of the distribution box and the meter. This action helps prevent metering device deformation caused by inaccurate measurement or a phase fault in the capacitor circuit. This is a result of damage to the capacitor. Additionally, to guarantee safe operation and reliable quality of components, it is advisable to use BSMJ series products for capacitor selection models.

If a pole-mounted distribution station rack is installed, the distribution box shell can be constructed using a 2 mm thick stainless steel plate. To accommodate the rack, the size of the distribution box will be enlarged proportionally. For instance, the width of the original box will be increased by approximately 100 mm, resulting in a change from 680 mm to 780 mm. The overall dimensions of the improved distribution box will be 1300 mm×780 mm×500 mm. This expansion is done to enhance electrical safety by increasing the distance between the outgoing wires, as well as between the outgoing wires and the box shell. Moreover, it facilitates the operation and maintenance tasks of agricultural electricians, allows easier replacement of fuses, and improves heat dissipation.