Reasons for failure of explosion-proof distribution box and improvement plan

The explosion-proof power distribution box is assembled in a closed or semi-enclosed metal cabinet or screen panel according to the electrical wiring requirements, and the switchgear, measuring instruments, protection appliances and auxiliary equipment are assembled to form a low-voltage power distribution device. During normal operation, the circuit can be switched on or off by means of a manual or automatic switch. In the event of malfunction or abnormal operation, the circuit or alarm is cut off by means of a protective appliance. Measurement instruments can be used to display various parameters in operation, and certain electrical parameters can also be adjusted to indicate or signal deviations from normal operating conditions. Commonly used in all hair, distribution, substations. In the process of use, many explosion-proof distribution boxes are prone to a variety of failures. What are the causes of these failures? How to improve the explosion-proof distribution box to better achieve its function? The following explanation will be given on this issue.

First of all, I want to tell everyone that the reasons for the failure of the explosion-proof distribution box are mainly the following:

1.1 Failure caused by the influence of ambient temperature on the distribution box

Low-voltage electrical equipment in the explosion-proof distribution box consists of fuses, AC contactors, residual current protection devices, capacitors and meters. These low-voltage electrical appliances are designed and manufactured in accordance with GB1497 "Basic Standards for Low-Voltage Electrical Appliances" and their normal working conditions are stipulated accordingly: The upper limit of ambient air temperature does not exceed 40 °C; the average value of ambient air temperature 24h does not exceed 35 °C. The lower limit of ambient air temperature is not lower than -5°C or -25°C.

The power distribution box transformed by rural power grids is running outdoors. It not only receives direct sunlight to produce high temperatures, but also generates heat during its operation. Therefore, during the high summer season in midsummer, the temperature inside the cabinet will reach above 60°C. The temperature greatly exceeds the ambient temperature specified by these appliances, and failures due to overheating of electrical components in the distribution box can occur.

1.2 Product Quality Failures

During the reconstruction of rural power grids, the number of distribution boxes required at the time was large and the construction period was short. The distribution box manufacturers needed urgent and numerous supply times for low-voltage electrical appliances. As a result, the requirements for product quality were not strict, resulting in Some products failed shortly after they were put into operation. If some types of AC contactors were put into operation shortly after the distribution box was put into operation, they could not be operated because the contactor closing coil was burned out.

1.3 Failures Caused by Improper Selection of Electrical Equipment in the Distribution Box

Due to the improper selection of AC contactor capacity in manufacturing, AC contactors of the same capacity are installed for different outlet loops, and the unbalanced three-phase load is not taken into account, and the current rating of some outlet contactors cannot be normal. On the basis of the selected model, an increase in the choice of the current level results in the occurrence of AC contactor burnout in the summer hot season.

Above, we told you the reason why the explosion-proof power distribution box failed. Then how to improve the explosion-proof power distribution box? Next, I'll tell you about the improvement plan of the explosion-proof power distribution box, specifically the following points:

2.1 For distribution box with distribution transformer capacity of 100kV·A and above, installation of temperature control relays (JU-3 or JU-4 subminiature temperature relays) should be considered in the side wall of the heat dissipation window inside the box. The axial fan is installed on the box on the left side above the control electrical panel so that the exhaust fan can be automatically activated when the temperature inside the box reaches a certain value (for example, 40°C), and heat is forcibly exhausted to dissipate the heat of the cabinet.

2.2 The use of protection circuit to prevent the external box circuit breaker power supply from occurring. Choose a small-sized intelligent phase loss protector, such as the optional DA88CM-II motor phase loss protection module (Shanghai products) installed in the distribution box to prevent the motor from being burned out due to low-voltage phase-to-phase operation.

2.3 Improve the connection mode of the low-voltage capacitor bank of the original explosion-proof distribution box, and change the installation position from the AC contactor to the pile head and change it to the low voltage line of the distribution box and the meter. To prevent the metering device from measuring inaccurately due to a lack of phase fault in the capacitor circuit during operation or damage to the capacitor. In addition, the capacitor selection model should be BSMJ series products to ensure reliable and safe operation of components.

2.4 If a new distribution platform on the column is added, a 2 mm thick stainless steel plate can be selected and the size of the distribution box can be magnified proportionally (the JP4-100/3W type used in the agricultural reform project). On the basis of this, an increase of approximately 100 mm in the width dimension of the original tank, ie from the original 680 mm to 780 mm, the improved distribution box dimensions: 1300 mm × 780 mm × 500 mm, in order to increase the number of points The electrical safety distance between the outlets, outlets, and the enclosure of the enclosure is conducive to the maintenance and replacement of the fuses by the agricultural electricians, and it can also dissipate heat.

2.5 Select energy-saving AC contactor (similar to CJ20SI type) products, and pay attention to the AC contactor coil voltage connected to the corresponding terminal block of the selected residual current protection device, and pay attention to the correct load matching. When selecting AC contactors, products with insulation class A and above shall be used, and the rated current of the main circuit contact shall be guaranteed to be greater than or equal to the load current of the controlled line. The electromagnetic coil of the contactor has a rated voltage of 380V or 220V, and the coil can be used within the range of 80% to 105% of the rated voltage.

2.6 Selection of residual current action protector. It must be selected in accordance with GB 6829 "General Requirements for Residual Current Operated Protectors" and certified by the China Electrotechnical Product Certification Committee. Alternative LJM (J) series power-saving type and low sensitivity time-delay protectors are available. The protection device shall conform to the national standard GB13955-2005 "Installation and operation of residual current protection device". Leakage protection device breaking time, when the leakage current is rated leakage current, the action time should not exceed 0.2s.

2.7 Explosion-proof distribution box inlet and outlet selection of low-voltage cables, cable selection should meet the technical requirements. For example, for the distribution box of 30kVA and 50kVA transformers, use the VV22-35x4 cable for the power distribution box, the VLV22-35x4 cable with the same specifications for the branch and outlet, and the VV22-50 for the input cable of the 80kVA and 100kVA transformers. ×4, VV22-70×4 cable, branch outlets use VLV22-50×4, VLV22-70×4 cables respectively, the cable is crimped with the copper-aluminum connection nose and then bolted to the wiring pile head in the distribution box .

2.8 Selection of fuses (RT, NT type). The rated current of the low-voltage side overcurrent protection fuse of the distribution transformer shall be greater than the rated current of the low-voltage side of the distribution transformer, generally 1.5 times of the rated current. The rated current of the melt shall be in accordance with the overload tolerance and fuse allowed by the transformer. Determine the characteristics of the device. The rated current of the fuse of the outlet circuit over-current protection fuse shall not be greater than the rated current of the total over-current protection fuse. The rated current of the melt shall be selected according to the normal maximum load current of the circuit and shall avoid the normal peak current. The rated current of the shunt capacitor bank fuse can generally be selected according to the capacitor rated current of 1.5 to 2.5 times.

2.9 In order to analyze the reactive power of the rural low-voltage power grid, install a DTS(X) series active/reactive 2-in-1 multi-function energy meter (installed on the meter side of the meter) to replace the original installation. Three single-phase energy meters (DD862 series meter) to facilitate on-line monitoring of the load.

Today is the time to talk about it. The above is the cause of the failure of the explosion-proof distribution box and the improvement plan. I hope to be helpful to everyone.