1、 Working principle of distribution box

The distribution box is to assemble the switchgear, measuring instruments, protective appliances and auxiliary equipment in a closed or semi closed metal cabinet or panel according to the electrical wiring requirements to form a low-voltage distribution device. During normal operation, the circuit can be connected or disconnected with the help of manual or automatic switch. In case of fault or abnormal operation, cut off the circuit or alarm with the help of protective appliances. The measuring instrument can display various parameters in operation, adjust some electrical parameters, and prompt or send signals for deviation from the normal working state. It is commonly used in various power generation, distribution and substation.

2、 Structure and application of distribution box

（1） Structure

Distribution box, distribution cabinet, distribution panel, distribution panel, etc. are complete sets of equipment for centralized installation of switches, integrated distribution box wiring guidance, integrated distribution box wiring guidance, instruments, etc.

（2） Use

Purpose of distribution box: reasonably distribute electric energy to facilitate the opening and closing operation of the circuit. It has high safety protection level and can intuitively display the conduction state of the circuit. It is convenient for management and maintenance in case of circuit failure. Replace the elevator distribution box with AC contactor. Replace the elevator distribution box with AC contactor

（3） Category

The commonly used distribution boxes are made of wood and metal. Because the protection grade of metal distribution boxes is higher, metal distribution boxes are more used.

Classification by structural features and uses:

(1) Fixed panel switchgear, often called switch board or distribution panel. It is an open switch cabinet with panel blocking. The front has protective effect, and the back and side can still touch the live parts. The protection level is low. It can only be used for centralized power supply in the substation in industrial and mining enterprises with low requirements for power supply continuity and reliability.

(2) Protective (i.e. closed) switchgear refers to a low-voltage switchgear whose sides are closed except the mounting surface. The switch, protection, monitoring and control and other electrical components of the cabinet are installed in a closed shell made of steel or insulating materials, which can be reliably installed on or off the wall. No isolation measures can be taken between each circuit in the cabinet, or grounded metal plate or insulating plate can be used for isolation. Generally, the door is mechanically interlocked with the main switch. In addition, there is a protective platform type switch cabinet (i.e. console), which is equipped with control, measurement, signal and other electrical appliances. The protective switch cabinet is mainly used as the power distribution device on the process site.

(3) Drawer type switch cabinet. This kind of switch cabinet is made of closed shell made of steel plate, and the electrical components of incoming and outgoing circuit are installed in withdrawable drawers to form a functional unit that can complete a certain type of power supply task. The functional unit is separated from the bus or cable by a grounded metal plate or a functional plate made of plastic to form three areas of bus, functional unit and cable. Isolation measures are also taken between each functional unit. Drawer type switchgear has high reliability, safety and interchangeability. It is a cutting-edge switchgear in technology. At present, most of the switchgear produced are drawer type switchgear. They are suitable for industrial and mining enterprises and high-rise buildings that require high power supply reliability as centralized control distribution centers.

(4) Power and lighting distribution control box. Mostly closed vertical installation. Due to different use occasions, the enclosure protection grade is also different. They are mainly used as power distribution devices at the production site of industrial and mining enterprises.

3、 Fault improvement of distribution box

（1） Cause of failure

1. Faults caused by the influence of ambient temperature on low voltage apparatus

The low-voltage apparatus in the distribution box is composed of fuse, AC contactor, residual current action protector, capacitor and meter. These low-voltage electrical appliances are designed and manufactured according to gb1497 basic standard for low voltage electrical appliances, and their normal working conditions are specified accordingly: the upper limit of ambient air temperature shall not exceed 40 ℃; The average value of ambient air temperature in 24h shall not exceed 35 ℃; The lower limit of ambient air temperature shall not be lower than - 5 ℃ or - 25 ℃.

The distribution box of rural power grid transformation operates outdoors. It not only generates high temperature under the direct irradiation of the sun, but also generates heat during operation. Therefore, in the high-temperature season in midsummer, the temperature in the box will reach more than 60 ℃. At this time, the temperature greatly exceeds the specified ambient temperature of these electrical appliances, so the fault caused by overheating of electrical components in the distribution box will occur.

2. Faults caused by product quality

In the transformation of rural power network, due to the large number of distribution boxes required and the short construction period, the distribution box factory needed the supply time and quantity of low-voltage appliances, resulting in the phenomenon of lax requirements for product quality, resulting in some products failing soon after they were put into operation. For example, some models of AC contactors cannot operate because the closing coil of the contactor is burned out shortly after the distribution box is put into operation.

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

Due to improper selection of AC contactor capacity during manufacturing, AC contactors with the same capacity are installed for different outgoing circuits, and the imbalance of three-phase load is not considered, so the current level of some outgoing contactors cannot be increased by one current level based on the normal selection model, Therefore, the AC contactor is burnt out during operation in high temperature season in summer.

（2） Improvement scheme

1. For the distribution box with the capacity of distribution transformer of 100kV · A and above, the temperature control relay (ju-3 or ju-4 ultra small temperature relay) and axial flow fan shall be installed on the box on the left side above the control electrical board at the side wall of the cooling window in the box, so that the exhaust fan can be started automatically when the temperature in the box reaches a certain value (such as 40 ℃), Forcibly expel heat to dissipate heat from the box.

2. Adopt protection circuit to prevent the failure of external circuit powered by distribution box. Select a small intelligent phase loss protector. For example, da88cm-ii motor phase loss protection module (Shanghai Product) can be installed in the distribution box to prevent the motor from burning out due to low-voltage phase loss operation.

3. Improve the wiring mode 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 between the low-voltage incoming line of the distribution box and the meter. Prevent incorrect metering of the metering device due to phase failure of the capacitor circuit or capacitor damage during operation. In addition, the selected model of capacitor shall be bsmj series products to ensure reliable component quality and safe operation.

4. If a column mounted distribution stand is added, 2 mm thick stainless steel plate shall be selected when making the distribution box shell, and the size of the distribution box shall be appropriately enlarged in proportion (on the basis of jp4-100 / 3W used in the agricultural reform project, the size in the width direction of the original box shall be increased by about 100 mm, that is, from 680 mm to 780 mm. The overall size of the improved distribution box is 1300 mm × 780 mm × 500 mm), so as to increase the electrical safety distance between each branch outgoing line and between the outgoing line and the box shell, which is conducive to the operation and maintenance of agricultural electricians and the replacement of melting parts, as well as heat dissipation.

5. Select energy-saving AC contactor (similar to cj20si), and pay attention to the connection between the coil voltage of AC contactor and the corresponding terminal selected as current action protector, and pay attention to correct load matching. When selecting AC contactor, products with insulation grade of class A or above shall be selected, and the rated current of its main circuit contact must be greater than or equal to the load current of the controlled line. The rated voltage of the electromagnetic coil of the contactor is 380V or 220V, and the coil is allowed to be used within 80% ~ 105% of the rated voltage.

6. Selection of residual current action protector. Products conforming to GB 6829 General requirements for residual current operated protectors and certified by China Electrotechnical Product Certification Committee must be selected. Time delay protector with low sensitivity similar to LJM (J) series can be selected. The mode of protector device shall comply with the industrial standard gb13955-2005 installation and operation of residual current operated protection device. The breaking time of leakage protector, when the leakage current is rated leakage current, its action time shall not be greater than 0.2S.

7. The incoming and outgoing lines of the distribution box shall be low-voltage cables, and the selection of cables shall meet the technical requirements. For example, vv22-35 is used for the incoming line of the distribution box of 30KVA and 50KVA transformers × 4. Vlv22-35 of the same specification shall be used for shunt outgoing line × 4 cable; The incoming line of distribution box of 80KVA and 100kVA transformer shall use vv22-50 respectively × 4、VV22-70 × 4. Vlv22-50 shall be used for cable and shunt outgoing line respectively × 4、VLV22-70 × 4. After the cable is crimped with the copper aluminum wiring nose, it is connected with the wiring pile head in the distribution box with bolts.

8. Selection of fuse (RT, NT type). The rated current of the total overcurrent protection fuse at the low-voltage side of the distribution transformer shall be greater than the rated current at the low-voltage side of the distribution transformer, generally 1.5 times of the rated current. The rated current of the melt shall be determined according to the allowable overload multiple of the transformer and the characteristics of the fuse. The rated current of the melt of the overcurrent protection fuse of the outgoing circuit shall not be greater than the rated current of the total overcurrent protection fuse. The rated current of the melt shall be selected according to the normal load current of the circuit and shall avoid the normal peak current. The rated current of shunt capacitor bank fuse can generally be selected as 1.5 ~ 2.5 times of the rated current of capacitor.