Distribution Box
The distribution box is an electrical equipment featuring a compact size, easy installation, special technical performance, fixed location, unique configuration functions, unrestricted by the site, wide application, stable and reliable operation, high space utilization, minimal land occupation, and environmental benefits.
A vast array of parameters on the distribution box data typically involves the construction of low-voltage electrical wiring. It requires assembling switchgear, measuring instruments, protective devices, and auxiliary equipment within enclosed or semi-enclosed metal cabinets or panels to form a low-voltage distribution box. During normal operation, circuits can be connected or disconnected using manual or automatic switches.
Usage
The distribution box features a compact design, easy installation, unique technical performance, fixed position, distinctive configuration functions, unrestricted by location, wide application, stable operation, high space utilization, minimal footprint, and environmental benefits. It serves as the control center for the rational distribution of electrical energy among various components in the command power supply lines. It is the reliable control point for accepting upstream power sources and accurately feeding out the load power. It is also the key to determining whether users are satisfied with the quality of power supply. Enhancing the operational reliability of the power distribution box is the goal of good engineering.
Purpose of Distribution Box: Reasonably distributes electrical power, facilitates the operation of opening and closing circuits. Features a high level of safety protection and can directly display the conduction status of the circuit.
Categories
Common distribution boxes come in wooden and metal varieties. Due to the higher protection class of metal distribution boxes, they are more commonly used.
Categorized by structural characteristics and application:
(1) Fixed panel-type switch cabinets, commonly known as switchboards or distribution boards. These are open-type switch cabinets with a panel shield, offering protection on the front but still allowing access to live parts on the back and sides, with a low protection rating. They are suitable for industrial and mining enterprises with lower requirements for power supply continuity and reliability, used for centralized power supply in transformer rooms.
(2) Enclosed (i.e., closed) switch cabinets refer to low-voltage switch cabinets where all sides except the installation face are sealed. The electrical components such as switches, protection, and monitoring controls are installed within a closed housing made of steel or insulating material, which can be mounted against or away from the wall. Isolation measures between each circuit within the cabinet may or may not be required, and can be achieved using grounded metal plates or insulating boards. Typically, the door is mechanically interlocked with the main switch operation. There are also enclosed desk-type switch cabinets (i.e., control desks) with control, measurement, and signaling electrical equipment mounted on the panel. Enclosed switch cabinets are primarily used as distribution devices in process sites.
(3) Drawer-type switch cabinets. These cabinets are constructed with steel plates to form a sealed housing, with electrical components of the incoming and outgoing line circuits installed in removable drawers, forming functional units capable of completing a specific power supply task. Functional units are separated from busbars or cables by grounded metal plates or functional boards made of plastic, creating three areas: busbars, functional units, and cables. There are also isolation measures between each functional unit. Drawer-type switch cabinets offer high reliability, safety, and interchangeability, making them a more advanced type of switch cabinet. Most switch cabinets refer to drawer-type switch cabinets. They are suitable for industrial and mining enterprises, high-rise buildings, and serve as centralized control power distribution centers where high power supply reliability is required.
(4) Power and lighting distribution control cabinets. Typically installed vertically in an enclosed design. The degree of protection for the housing varies depending on the application. They primarily serve as power distribution units for industrial and mining production sites.
Structure
The structure of distribution boxes is divided into two types.
1. Welded Structures [3]: Simply cut, bend, punch holes, and weld sheet metal parts together.
2. Assembly Structure: Sheet metal parts are processed separately, then assembled after each component is finished. Secured with screws and tees for a tight fit, offering a nice appearance and simple operation, which can significantly reduce transportation costs.
Power distribution boxes and cabinets, panels, switchboards, electrical cabinets, etc., are complete sets of equipment that centrally install switches, meters, and other devices.
Principle
A distribution box is an assembly of switchgear, measuring instruments, protective devices, and auxiliary equipment, mounted in a closed or semi-closed metal cabinet or panel, forming a low-voltage distribution box. It can connect or disconnect the circuit manually or automatically during normal operation. In case of a fault or abnormal operation, the protective devices are used to cut off the circuit or trigger an alarm. The measuring instruments can display various parameters during operation and can also adjust certain electrical parameters, providing alerts or signals for deviations from normal operating conditions.
Reason
1.1 Faults caused by environmental temperature affecting low-voltage electrical appliances
Low-voltage electrical appliances in the distribution box include fuses, AC contactors, residual current protective devices, capacitors, and meters. The maximum ambient air temperature should not exceed 40℃; the average 24-hour ambient air temperature should not exceed 35℃; and the minimum ambient air temperature should not be lower than -5℃ or -25℃.
The distribution box for rural network renovation operates outdoors, exposed to direct sunlight which generates high temperatures. Additionally, it generates heat during operation. Therefore, during the peak summer heat season, the internal temperature of the box can reach above 60℃, which is significantly higher than the environmental temperature specified for these electrical appliances.
1. Product quality-related defects
During the rural grid transformation, due to the large quantity of distribution boxes required and the short construction period, the distribution box factory urgently needed a significant supply of low-voltage electrical equipment. This led to a relaxed standard for product quality, resulting in some products failing shortly after operation. For instance, certain models of AC contactors failed to operate shortly after being installed in the distribution box due to the burnout of the contactor's closing coil.
1. Fault caused by improper selection of electrical appliances inside the distribution box
Due to an inappropriate selection of the contactor capacity during manufacturing, the same capacity AC contactors were installed on different outgoing circuit loops, and the imbalance of the three-phase load was not considered, resulting in the failure to increase the current rating of some outgoing contactors by one level above the normal selected model.
Enhanced
2.1 For distribution boxes with power transformer capacities of 100kV·A or more, temperature control relays (JU-3 type or JU-4 ultra-miniature temperature relays) and axial flow fans should be installed on the left side of the box above the control electrical panel, near the side wall of the internal heat dissipation window. This ensures that when the internal temperature reaches a certain level (e.g., 40℃), the exhaust fan will automatically activate to forcibly expel heat and dissipate the box.
2.2 Employ protective circuits to prevent external circuit failures from occurring in the power distribution box's supply. Select a compact intelligent phase loss protector, such as the DA88CM-II motor phase loss protection module (Shanghai product), to be installed inside the power distribution box to prevent motor burnout due to low voltage phase loss operation.
2.3 Improve the wiring method of the low-voltage capacitor bank in the original distribution box, changing its installation position from the AC contactor's upper terminal to between the low-voltage incoming line of the distribution box and the meter. This prevents inaccuracies in the measurement devices due to phase failure in the capacitor circuit or capacitor damage during operation. Additionally, the capacitor should be selected from the BSMJ series to ensure reliable component quality and safe operation.
2.4 If a new pole-mounted distribution equipment rack is added, when manufacturing the distribution box casing, you may opt for 2mm thick stainless steel sheets, and appropriately increase the size of the distribution box in proportion (based on the JP4-100/3W type used in rural electrification projects, increase the width dimension of the original casing by approximately 100mm, changing from the original 680mm to 780mm. The improved distribution box dimensions are: 1300mm x 780mm x 500mm), in order to enhance the electrical safety distance between each branch circuit outlet, the outlet and the box casing. This is beneficial for the operation and maintenance of rural electricians and for replacing fuses, as well as for heat dissipation.
2.5 Select energy-saving AC contactors (similar to CJ20SI type) and ensure the AC contactor coil voltage corresponds to the connecting terminals of the selected residual current action protector. Pay attention to correct load matching. When selecting an AC contactor, choose a product with an insulation grade of A or higher, and the rated current of its main circuit contacts must be greater than or equal to the load current of the controlled line. The rated voltage of the contactor's electromagnetic coil is 380V or 220V.
2.6 Selection of residual current circuit breakers. It is mandatory to choose products that comply with the GB 6829 "General Requirements for Residual Current Circuit Breakers" standard and have been certified as qualified by the China Electrotechnical Product Certification Committee. Products similar to the LJM (J) series energy-saving models, which are low-sensitivity time-delayed protectors, can be selected. The installation method of the protector must comply with the national GB13955-2005 "Installation and Operation of Residual Current Circuit Breaker Devices" standard. The breaking time of the leakage protector should not exceed 0.1 seconds when the leakage current is equal to the rated leakage current.
2.7 The incoming and outgoing lines of the distribution box should use low-voltage cables, and the selection of cables must meet the technical requirements. For example, the incoming line of a distribution box for a 30kVA or 50kVA transformer uses VV22-35×4 cables, while the branch outgoing lines use the same specification of VLV22-35×4 cables; the incoming lines of a distribution box for an 80kVA or 100kVA transformer use VV22-50×4 or VV22-70×4 cables respectively, and the branch outgoing lines use VLV22-50×4 or VLV22-70×4 cables respectively. After the cables are crimped with copper-aluminum terminals, they are then connected to the distribution box's terminal blocks with bolts.
2.8 Selection of Circuit Breakers (RT, NT types). 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 transformer, typically 1.5 times the rated current. The rated current of the fuse element should be determined based on the transformer's allowable overload multiples and the fuse characteristics. The rated current of the fuse element for the outgoing circuit overcurrent protection should not exceed the rated current of the total overcurrent protection fuse. The rated current of the fuse element should be selected based on the normal high load current of the circuit, and it should also avoid normal peak currents.
2.9 To analyze reactive power in rural low-voltage power grids, install a DTS(X) series two-in-one multifunctional electric energy meter (mounted on the metering board side of the cabinet) to replace the original three single-phase electric energy meters (DD862 series meters) for online monitoring of load operation.
Product Selection
There are numerous types of distribution cabinets in domestic power supply and distribution systems, with varying cabinet structures and technical specifications. Under the influence of the following factors, well-designed plans often need to be modified, or even redesigned, which not only impacts the construction schedule of the power supply and distribution system but also poses certain difficulties for manufacturers in completing the production of distribution cabinets on time and with quality.
Factors affecting changes to pre-designed drawings include:
Manufacturers of electrical control panels may recommend their own products to customers, even if they may not be suitable.
2. The design institute is not very familiar with some newly launched cabinet models; it designs based on customer requirements only.
3. Users have a limited understanding of their own needs and are unable to select the correct cabinet type based on their requirements.
To address the aforementioned issues and to meet users' need for a more accurate selection of specific distribution box models, the following analysis is provided on the characteristics of low-voltage distribution boxes commonly used in domestic power supply and distribution systems.
The following analyzes the cabinet structure and technical parameters of the aforementioned imported distribution cabinet.
Key features of domestically produced distribution boxes include the inconsistency of specific models among different domestic manufacturers, variations in the specific structure, and the main electrical components installed can be either imported renowned brands or domestic ones.
The following explains the relevant technical parameters.
1. Main Busbar Rated High Current: The rated current capacity of the main busbar.
2. Rated Short-Time Current: The root mean square (RMS) value of the short-time current that a specific circuit in the complete set of equipment can safely carry under the test conditions specified in section 8.2.3 of the national standard GB7251.1-2005, as provided by the manufacturer.
3. Peak Short-Time Current Tolerance: The peak current that the manufacturer specifies this circuit can safely withstand under the specified test conditions.
4. Enclosure Protection Class: The level of protection provided by complete equipment against access to live parts, as well as against the intrusion of foreign solid objects and the ingress of liquids, in accordance with IEC60529-1989 standard. Specific classification levels are detailed in the IEC60529 standard.
5. Internal partitioning methods: Partition the switchgear into several compartments in different ways according to the IEC60529-1989 standard for the safety of personnel.
Technical specifications among different types of distribution boxes vary greatly, and imported distribution boxes generally have superior technical parameters compared to domestic ones, but it cannot be assumed that imported distribution boxes are always better than domestic ones.
Below, based on actual work experience, I discuss the following points:
1. Imported electrical distribution panels are developed and manufactured overseas, generally marketed for the global power distribution market. Due to the varying requirements and customs of power distribution systems across different countries, imported electrical distribution panels may not be entirely suitable for the domestic market.
2. The main electrical components of imported distribution cabinets are all imported brand products, and some cabinets or auxiliary parts must be imported from abroad, which usually results in the price of imported distribution cabinets being significantly higher than that of domestically produced ones.
3. Although the technical specifications of imported distribution cabinets are high, they are often only utilized partially, and sometimes not at all. For instance, an imported distribution cabinet can accommodate more circuits than domestic ones, but this is only achieved by reducing the circuit capacity, which usually does not meet the users' needs.
4. Although the technical specifications of domestically-manufactured distribution panels are lower than those of imported ones, they are already able to meet user needs in the majority of domestic power supply and distribution systems.
5. When it comes to the quality of distribution panels, domestic ones are not necessarily inferior to imported panels as long as manufacturers strictly adhere to the 3C production and inspection standards.
In summary, when selecting the model of a distribution box, the following points should be considered:
Understand the customer's needs and select an appropriate container type based on the actual situation.
2. Prefer using domestically renowned manufacturers' cabinets, as blindly opting for high-technical parameter imported distribution panels can easily lead to resource wastage.
3. Since the main components in imported distribution cabinets share the same brand as the cabinet itself, when selecting an imported distribution cabinet, attention should be given to the parameters of the main components, which must meet the user's requirements.
NotePlease note
(1) The construction power distribution system should be equipped with a main distribution box, sub-distribution box, and switch box, and arranged in a hierarchical sequence following the "main-sub-switch" order, forming a "three-level distribution" pattern.
(2) The installation locations of each distribution box and switch box in the construction power distribution system must be reasonable. The main distribution box should be as close as possible to the transformer or external power source for easy power supply. The distribution boxes should be installed as centrally as possible near the electrical equipment or load concentration to ensure balanced three-phase load. The switch boxes should be placed as close as feasible to the controlled electrical equipment based on the on-site conditions and working environment.
(3) Ensure the three-phase load balance of the temporary power distribution system. The power and lighting power at the construction site should form two separate power loops, with the power distribution box and lighting distribution box to be set up individually.
(4) All electrical equipment on the construction site must have its own switch box.
(5) All distribution box bodies and internal setups must comply with safety regulations. Switchgear should be clearly marked for its purpose, and the boxes should be uniformly numbered. Distribution boxes that are no longer in use should have the power supply disconnected and the doors locked. Fixed distribution boxes should be equipped with fences and have rain and impact protection measures.
(6) The Difference Between Distribution Boxes and Distribution Cabinets. According to GB/T20641-2006 "General Requirements for Enclosures of Low-Voltage Switchgear and Controlgear"
Distribution boxes are commonly used for residential purposes, whereas distribution panels are more frequently employed in centralized power supply scenarios, such as industrial and construction electricity. Both distribution boxes and panels are part of complete sets of equipment; distribution boxes fall under low-voltage complete sets, and distribution panels come in both high and low voltage options.
