Common Faults of Distribution Boxes
Causes
1.1 Faults Caused by the Impact of Ambient Temperature on Low-Voltage Electrical Appliances
Low-voltage electrical appliances in the distribution box include fuses, AC contactors, residual current operated protectors, capacitors, and metering meters. The upper limit of the ambient air temperature shall not exceed 40℃; the 24-hour average ambient air temperature shall not exceed 35℃; and the lower limit of the ambient air temperature shall not be lower than -5℃ or -25℃.
Distribution boxes renovated under the rural power grid project operate outdoors. They not only absorb heat from direct sunlight, leading to high temperatures, but also generate heat themselves during operation. Therefore, in the hot summer season, the temperature inside the box can reach over 60℃, which far exceeds the specified ambient temperature for these electrical appliances.
1.2 Faults Caused by Product Quality Issues
During the rural power grid renovation, due to the large demand for distribution boxes and tight construction schedules, distribution box manufacturers required a large quantity of low-voltage electrical appliances in a short time. This led to a lack of strict quality control for the products, resulting in faults occurring shortly after some products were put into operation. For example, the closing coils of certain types of AC contactors burned out soon after the distribution boxes were commissioned, rendering them inoperable.
1.3 Faults Caused by Improper Selection of Electrical Appliances in the Distribution Box
During manufacturing, the capacity of AC contactors was not selected appropriately. AC contactors of the same capacity were installed for different outgoing circuits, and the imbalance of three-phase loads was not taken into account. As a result, the current rating of contactors for some outgoing circuits was not upgraded by one level based on the normally selected model.
Improvements
2.1 For distribution boxes with a distribution transformer capacity of 100 kV·A or above, a temperature control relay (Model JU-3 or JU-4 ultra-small temperature relay) and an axial flow fan should be installed near the side wall of the heat dissipation window inside the box. They should be mounted on the box body on the left side above the control electrical panel. This allows the exhaust fan to start automatically when the internal temperature of the box reaches a certain value (e.g., 40℃), forcing heat out to cool the box.
2.2 Adopt protective circuits to prevent external circuit faults in the power supply of the distribution box. Choose a smaller intelligent phase loss protector, such as the DA88CM-II motor phase loss protection module (Shanghai product) installed in the distribution box to prevent the motor from burning out due to low-voltage phase loss operation.
2.3 Improve the wiring method of the low-voltage capacitor bank in the original distribution box. Change its installation position from the upper terminal of the AC contactor to the connection between the low-voltage incoming line of the distribution box and the metering meter. This prevents inaccurate metering by the metering device in case of phase failure faults in the capacitor circuit or capacitor damage during operation. In addition, the selected capacitors should be of the BSMJ series to ensure reliable component quality and safe operation.
2.4 When adding a new pole-mounted distribution rack, 2mm-thick stainless steel plates can be used to make the distribution box shell. The size of the distribution box should be appropriately enlarged in proportion. Based on the JP4-100/3W type used in the rural renovation project, increase the width dimension of the original box by approximately 100mm (from the original 680mm to 780mm). The external dimensions of the improved distribution box are: 1300mm × 780mm × 500mm. This helps increase the electrical safety distance between the outgoing lines of each branch, as well as between the outgoing lines and the box shell. It facilitates the operation, maintenance, and fuse replacement by rural electricians, and also improves heat dissipation.
2.5 Select energy-saving AC contactors (similar to Model CJ20SI). Pay attention to connecting the coil voltage of the AC contactor to the corresponding terminal of the selected residual current operated protector, and ensure correct load matching. When selecting AC contactors, those with an insulation class of Class A or above should be used. The rated current of the main circuit contacts must be greater than or equal to the load current of the controlled circuit. The rated voltage of the contactor's electromagnetic coil is 380V or 220V.
2.6 Selection of residual current operated protectors. Must select products that comply with GB 6829 General Requirements for Residual Current Operated Protectors and have passed the certification of the China Electrical Equipment Certification Committee. Optional products include energy-saving, low-sensitivity, time-delay protectors similar to the LJM (J) series. The installation method of the protector should comply with the national standard GB 13955-2005 Installation and Operation of Residual Current Operated Protective Devices. The breaking time of the leakage protector shall not exceed 0.1 seconds when the leakage current is the rated leakage current. (Note: The original text has a typo "0.s", which is corrected to "0.1 seconds" according to common industry standards for leakage protectors.)
2.7 Low-voltage cables should be used for the incoming and outgoing lines of the distribution box, and the selection of cables should meet technical requirements. For example, the incoming lines of distribution boxes for 30kVA and 50kVA transformers use VV22-35×4 cables, and the branch outgoing lines use VLV22-35×4 cables of the same specification; the incoming lines of distribution boxes for 80kVA and 100kVA transformers use VV22-50×4 and VV22-70×4 cables respectively, and the branch outgoing lines use VLV22-50×4 and VLV22-70×4 cables respectively. After crimping the cables with copper-aluminum cable lugs, they are connected to the terminal blocks inside the distribution box using bolts.
2.8 Selection of fuses (RT and NT types). The rated current of the main overcurrent protection fuse on the low-voltage side of the distribution transformer shall be greater than the rated current of the low-voltage side of the distribution transformer, generally 1.5 times the rated current. The rated current of the fuse link shall be determined according to the allowable overload multiple of the transformer and the fuse characteristics. The rated current of the fuse link for the outgoing circuit overcurrent protection shall not be greater than the rated current of the main overcurrent protection fuse. The rated current of the fuse link is selected based on the normal maximum load current of the circuit and shall avoid the normal peak current.
2.9 To analyze the reactive power of the rural low-voltage power grid, install a DTS (X) series multi-functional electric energy meter (integrating active and reactive power measurement) inside the box (mounted on the metering meter panel side) to replace the originally installed three single-phase electric energy meters (DD862 series meters). This facilitates online operation monitoring of the load.