The distribution box is a circuit distribution box for all users. The distribution box is a switchgear, measuring instrument, protective appliance and auxiliary equipment assembled in a closed or semi-closed metal cabinet or screen according to the electrical wiring requirements to form a low-voltage power distribution device. During normal operation, the circuit can be switched on or off with the help of manual or automatic switches. In the event of malfunction or abnormal operation, the protection circuit is used to cut off the circuit or give an alarm. Various measuring parameters can be displayed by the measuring instrument, and some electrical parameters can be adjusted to indicate or signal the deviation from the normal working state. Commonly used in various hair, distribution and substations.
Power distribution boxes, power distribution cabinets, power distribution boards, power distribution screens, etc., are complete sets of equipment that centrally install switches, comprehensive power distribution box wiring instructions, and instruments.
There are two types of commonly used distribution boxes: wooden and metal. Because metal protection boxes have a higher protection level, they are more often used in metal.
The purpose of the distribution box: reasonable distribution of electrical energy to facilitate the opening and closing operations of the circuit. It has a high level of safety protection and can directly show the conduction state of the circuit. It is easy to manage and facilitates maintenance when a circuit failure occurs.
Classified by structural characteristics and uses:
(1) Fixed panel switch cabinet, often called switch board or power distribution screen. It is an open-type switchgear with a panel cover. It has a protective effect on the front, and can still touch the live parts on the back and sides. The protection level is low. Substation centralized power supply.
(2) Protective (ie closed) switchgear refers to a type of low-voltage switchgear that is closed on all sides except the installation surface. The electrical components such as the switch, protection, monitoring and control of this cabinet are installed in a closed enclosure made of steel or insulation material, and can be installed on or off the wall. There can be no isolation measures between each circuit in the cabinet, and a grounded metal plate or insulation plate can also be used for isolation. Usually the door is mechanically interlocked with the main switch operation. In addition, there is a protective table-type switchgear (that is, a console), and the panel is equipped with control, measurement, signal and other electrical appliances. Protective switchgear is mainly used as a power distribution device at the process site.
(3) Drawer type switch cabinet. This type of switch cabinet is made of steel with a closed enclosure, and the electrical components of the incoming and outgoing line circuits are installed in a drawer that can be drawn out to form a functional unit that can complete a certain type of power supply task. The functional unit and the bus or cable are separated by a grounded metal plate or a functional board made of plastic to form three areas of the bus, functional unit and cable. There are also isolation measures between each functional unit. Drawer-type switch cabinets have higher reliability, safety and interchangeability, and are relatively advanced switch cabinets. Most of the currently produced switch cabinets are drawer-type switch cabinets. They are suitable for industrial and mining enterprises and high-rise buildings that require high reliability of power supply, as a centralized control power distribution center.
(4) Power and lighting distribution control box. Most are enclosed vertical installations. Due to different application occasions, the enclosure protection levels are also different. They are mainly used as power distribution devices on the production site of industrial and mining enterprises.
1. Failure cause
1) Failure 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. These low-voltage electrical appliances are designed and manufactured in accordance with GB1497 "Basic Standards for Low-Voltage Electrical Apparatus", and their normal working conditions are specified accordingly: the upper limit of the ambient air temperature does not exceed 40 ° C; the average value of the ambient air temperature for 24h does not exceed 35 ° C ; The lower limit of the ambient air temperature is not lower than -5 ° C or -25 ° C.
The distribution box reconstructed by the rural power grid operates outdoors. It not only generates high temperature when it is directly exposed to sunlight, but also generates heat during operation. Therefore, in the high summer season, the temperature inside the box will reach above 60 ° C. The temperature greatly exceeds the ambient temperature specified by these appliances, so failures due to overheating of electrical components in the distribution box can occur.
2) Failure caused by product quality
During the reconstruction of the rural power grid at that time, due to the large number of distribution boxes required and the short construction period, the distribution box factory needed to supply the low-voltage electrical appliances in a short time and with a large number, which resulted in a strict demand for product quality, resulting Some products failed shortly after being put into operation. For example, some models of AC contactors cannot run because the contactor closing coil is burned out shortly after the distribution box is put into operation.
3) Failure caused by improper selection of electrical appliances in the distribution box
Due to the improper selection of the capacity of the AC contactor at the time of manufacture, the AC contactors with the same capacity were installed for different outgoing circuits, and the imbalance of the three-phase load was not considered, and the current level of some of the outgoing contactors was not normal Based on the selection of the model, a higher current level is selected, which causes the AC contactor to burn out during high-temperature summer operation.
2. Improve the plan
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 considered at the side wall of the cooling window inside the box. The axial fan is installed on the box on the left side above the control electrical board, so that when the temperature inside the box reaches a certain value (such as 40 ° C), the exhaust fan can be automatically started to forcibly discharge heat to dissipate the box.
2) The protection circuit is used to prevent the external circuit failure of the power supply from the distribution box. Choose a smaller intelligent phase loss protector. For example, you can use DA88CM-II motor phase loss protection module (Shanghai product) to install in the distribution box to prevent the motor from being damaged 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 on the AC contactor to be connected between the low-voltage incoming line of the distribution box and the meter. To prevent inaccurate metering by the metering device due to phase failure or capacitor damage due to capacitor circuits during operation. In addition, the capacitor selection model should be BSMJ series products to ensure reliable component quality and safe operation.
4) If a new distribution stand on the column is added, when making the distribution box shell, a 2 mm thick stainless steel plate can be selected, and the size of the distribution box is appropriately enlarged (JP4-100 / 3W used in agricultural reform projects Based on the model, the size of the original box width direction is increased by about 100 mm, that is, from 680 mm to 780 mm. The improved distribution box dimensions are: 1300 mm × 780 mm × 500 mm) in order to increase The electrical safety distance between the outlets of the shunts, the outlets and the casing of the box, which is conducive to the operation and maintenance of agricultural electricians and the replacement of fuses, and it can also dissipate heat.
5) Select energy-saving AC contactor (similar to CJ20SI type), and pay attention to the AC contactor coil voltage is connected to the corresponding terminal of the selected residual current protective device, and pay attention to correct load matching. When selecting an AC contactor, select a product with an insulation level of A or higher. The rated current of the main circuit contact must be greater than or equal to the load current of the line being controlled. The contactor's solenoid coil has a rated voltage of 380V or 220 V, and the coil is allowed to be used within the range of 80% to 105% of the rated voltage.
6) Selection of residual current action protector. Products that meet the GB 6829 "General Requirements for Residual Current Operated Protectors" and are qualified by the China Electrotechnical Product Certification Commission must be selected. Can choose similar to LJM (J) series of power-saving type, and is a low-sensitivity delay type protector. The method of the protector device shall conform to the national GB13955-2005 "Installation and Operation of Residual Current Action Protection Device". The breaking time of the leakage protector, when the leakage current is the rated leakage current, its action time should not be greater than 0.2s.
7) The low-voltage cables are used for the incoming and outgoing wires of the distribution box. The selection of the cables should meet the technical requirements. For example, the inlets of the distribution boxes of 30kVA and 50kVA transformers use VV22-35 × 4 cables, and the outlets of the branches use the same specifications of VLV22-35 × 4 cables; the inlets of the distribution boxes of 80kVA and 100kVA transformers use VV22-50. × 4, VV22-70 × 4 cables, and the outgoing cables are VLV22-50 × 4, VLV22-70 × 4 cables, and the cables are crimped with copper and aluminum wiring noses, and then bolted to the terminal heads in the distribution box. .
8) Selection of fuse (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 of the low-voltage side of the distribution transformer, which is generally 1.5 times the rated current. The rated current of the melt should be based on the overload multiple and the fuse allowed by the transformer. Device characteristics. The rated fuse current of the overcurrent protection fuse of the outgoing circuit should not be greater than the rated current of the total overcurrent protection fuse. The rated current of the fuse should be selected according to the normal maximum load current of the circuit and should avoid the normal peak current. The rated current of the shunt capacitor bank fuse can generally be selected according to 1.5 to 2.5 times the rated current of the capacitor.
9) In order to analyze the reactive power of rural low-voltage power grids, install a DTS (X) series of active and reactive two-in-one multi-function energy meters (installed on the meter board side) in the box to replace the original Three single-phase watt-hour meters (DD862 series meters) installed to facilitate online monitoring of loads.
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