[Distribution box] Cause analysis of common faults

cause of issue:

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 action protectors, capacitors and meters. The upper limit of the ambient air temperature shall not exceed 40°C; the average value of the ambient air temperature for 24 hours shall not exceed 35°C; the lower limit of the ambient air temperature shall not be lower than -5°C or -25°C.

The distribution box transformed from the rural power grid operates outdoors. It is not only exposed to direct sunlight to generate high temperature, but also generates heat itself during operation. Therefore, in the midsummer high temperature season, the temperature inside the box will reach above 60°C. temperature significantly exceeds the ambient temperature specified for these appliances

2. Faults caused by product quality

During the transformation of the rural power grid, due to the large number of distribution boxes required and the short construction period, the distribution box factory needed to supply low-voltage electrical appliances in a short time and in large quantities. Some products failed shortly after being put into operation. For example, some types of AC contactors cannot operate due to the burnout of the contactor closing coil shortly after the distribution box is put into operation.

3. Faults caused by improper selection of electrical appliances in the distribution box

Due to the inappropriate selection of AC contactor capacity during manufacture, AC contactors of the same capacity are installed for different outlet circuits, and the unbalanced three-phase load is not considered, and the current level of some outlet contactors cannot be adjusted to normal Based on the selected model, increase a current level selection

Corresponding improvement measures:

1. For the distribution box with a distribution transformer capacity of 100kV·A and above, the temperature control relay (JU-3 or JU-4 ultra-small temperature relay) should be installed near the side wall of the heat dissipation window in the box. And the axial flow fan is installed on the box on the left side above the control electrical board, so that when the temperature in the box reaches a certain value (such as 40°C), the exhaust fan can be automatically activated to forcibly discharge heat to dissipate heat from the box.
2. The protection circuit is used to prevent the external circuit failure of the distribution box power supply. 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 being burned due to low voltage phase loss operation.
3. Improve the wiring method of the low-voltage capacitor bank of the original distribution box, and change its installation position from the pile head of the AC contactor to be connected between the low-voltage incoming line of the distribution box and the meter. To prevent the inaccurate measurement of the metering device due to a phase failure of the capacitor circuit during operation or damage to the capacitor. In addition, the capacitor selection model should be BSMJ series products to ensure reliable quality and safe operation of components.
4. If the distribution rack on the pole is added, when making the distribution box shell, 2 mm thick stainless steel plate can be selected, and the size of the distribution box will be enlarged in proportion (JP4—100/3W used in the agricultural reform project) On the basis of the type, the size of the width direction of the original box is increased by about 100 mm, that is, the original 680 mm is changed to 780 mm. The overall size of the improved distribution box is: 1300 mm×780 mm×500 mm), in order to increase the The electrical safety distance between the outlets of the shunts and between the outlets and the casing of the box is conducive to the operation and maintenance of agricultural electricians and the replacement of fuses, and it can also dissipate heat.
5. Select an energy-saving AC contactor (similar to CJ20SI type), and pay attention to the connection of the coil voltage of the AC contactor with the corresponding terminal of the selected remaining current-operated protector, and pay attention to the correct load matching. When selecting an AC contactor, you should choose a product with an insulation class of A and above, and you must ensure that the rated current of the main circuit contact should be greater than or equal to the load current of the controlled line. The solenoid coil of the contactor is rated at 380V or 220V
6. Selection of residual current action protector. Products that meet the standard of GB 6829 “General Requirements for Residual Current Operated Protectors” and have been certified by China Electrical Product Certification Committee must be selected. A power-saving and low-sensitivity time-delay protector similar to LJM (J) series can be selected. The way of the protector device must comply with the national GB13955-2005 “Installation and Operation of Residual Current Action Protection Device” standard. The breaking time of the leakage protector, when the leakage current is the rated leakage current, its action time should not be greater than 0.s.
7. Low-voltage cables are used for the incoming and outgoing lines of the distribution box, and the selection of cables should meet the technical requirements. For example, the incoming lines of the distribution boxes of 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 the distribution boxes of 80kVA and 100kVA transformers respectively use VV22-50 ×4, VV22-70×4 cables, VLV22-50×4 and VLV22-70×4 cables are used for branching out, and the cables are crimped with the copper-aluminum wiring nose and then connected with the wiring pile head in the distribution box with bolts .
8. Selection of fuses (RT, NT type). The rated current of the total overcurrent protection fuse on the low-voltage side of the distribution transformer should be greater than the rated current on the low-voltage side of the distribution transformer, generally 1.5 times the rated current, and the rated current of the melt should be in accordance with the allowable overload multiple of the transformer and fusing The device characteristics are determined. The rated current of the melt of the overcurrent protection fuse of the outlet circuit should not be greater than the rated current of the total overcurrent protection fuse. The rated current of the melt is selected according to the normal maximum load current of the circuit, and should avoid the normal peak current.
9. In order to analyze the reactive power of the rural low-voltage power grid, install a DTS (X) series active and reactive two-in-one multifunctional electric energy meter (installed on the side of the meter board) in the box to replace the original Three single-phase watt-hour meters (DD862 series meters) are installed to facilitate online monitoring of loads.