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 space, widespread application, stable and reliable operation, high space utilization, minimal footprint, and environmental benefits.

Ample parameters on the power distribution box typically involve the assembly of switchgear, measuring instruments, protective devices, and auxiliary equipment within enclosed or semi-enclosed metal cabinets or panels to form a low-voltage distribution box. Normal operation allows for circuit connection or disconnection using either manual or automatic switches.

Application

The distribution box features a compact size, easy installation, unique technical performance, fixed location, distinctive configuration functions, unrestricted by site conditions, 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 control segment responsible for reliably receiving power from the upper source and correctly 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 a good engineering project.

The purpose of the distribution box: Reasonably distributes electrical power, facilitating the operation of circuit opening and closing. It features a high level of safety protection and can directly display the circuit's conduction status.

Categories

Common distribution boxes come in wooden and metallic varieties. Due to the higher protection grade of metallic distribution boxes, they are more widely 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 grade. 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) switchgear cabinets refer to low-voltage switchgear 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, and can be mounted against or away from the wall. Isolation measures are not required between each circuit within the cabinet, but can be achieved using grounded metal plates or insulating plates. Typically, there is a mechanical interlock between the door and the main switch operation. Additionally, there are enclosed desk-type switchgear cabinets (i.e., control desks) with control, measurement, and signaling electrical appliances mounted on the panel. Enclosed switchgear cabinets are mainly used as distribution equipment in process sites.

(3) Drawer-type switch cabinets. These cabinets are made of steel plates with a sealed housing, where electrical components of the incoming and outgoing line circuits are 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 distinct 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 an 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 mounted vertically in an enclosed design. The protective grade of the casing varies depending on the application. They are mainly used as power distribution units in industrial and mining production sites.

Structure

There are two types of electrical distribution box structures.

1. Welded Structures [3]: Simply cut, bend, drill, and weld sheet metal parts together.

2. Assembly Structure: The sheet metal parts are processed separately, then assembled after each component is finished. Secured with screws and tees for a tight fit, offering a sleek appearance and simple operation, which can significantly reduce transportation costs.

Distribution boxes and cabinets, distribution panels, switchboards, and electrical cabinets, etc., are complete sets of installations that centrally house switches, meters, and other equipment.

Principle

A distribution box is a unit that assembles switchgear, measuring instruments, protective devices, and auxiliary equipment according to electrical wiring requirements, either in a sealed or semi-enclosed metal cabinet or panel. It forms a low-voltage distribution box. During normal operation, circuits can be connected or disconnected using manual or automatic switches. 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 equipment

Low-voltage electrical appliances in the distribution box, including fuses, AC contactors, residual current protective devices, capacitors, and meters, etc. The maximum ambient air temperature should not exceed 40℃; the average ambient air temperature over 24 hours should not exceed 35℃; the minimum ambient air temperature should not be lower than -5℃ or -25℃.

The distribution box for rural grid renovation operates outdoors, exposed to direct sunlight which generates high temperatures. Additionally, it generates heat during operation. Therefore, during the peak summer heat, the internal temperature of the box can exceed 60℃. This temperature significantly surpasses the environmental temperature specifications for these electrical appliances.

2. Faults caused by product quality issues

During the rural network transformation, due to the large quantity of distribution boxes required and the short construction period, the distribution box factory urgently needed a large supply of low-voltage electrical appliances. This led to a relaxed quality requirement, resulting in some products failing soon 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.

Improper selection of electrical appliances in the 3-way distribution box caused a fault

Due to an inadequate selection of the contactor capacity during manufacturing, the same capacity AC contactors were installed in different outgoing circuit loops, and the imbalance of the three-phase load was not considered. As a result, the current rating of some outgoing contactors was not increased by one current 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 near the side wall of the internal heat dissipation windows. They should be mounted on the left side of the control panel above the box, to automatically activate exhaust fans and forcibly expel heat when the internal temperature reaches a certain level (e.g., 40℃), facilitating the box's heat dissipation.

2.2 Employ protective circuits to prevent external circuit failures from affecting the power supply to the distribution box. Select a compact intelligent phase-loss protector, such as the DA88CM-II motor phase-loss protection module (Shanghai product), to be installed inside the distribution box to prevent motor burnout due to low-voltage phase loss operation.

2.3 Improved the wiring method of the low-voltage capacitor bank in the original distribution box, changing its installation position from the AC contactor's terminal to between the low-voltage incoming line of the distribution box and the meter. This prevents inaccuracies in the metering device 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 additional pole-mounted distribution equipment racks are to be installed, when manufacturing the distribution box housing, 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 model used in rural electrification projects, increase the width dimension of the original box by approximately 100mm, changing from the original 680mm to 780mm. The improved distribution box external dimensions are: 1300mm x 780mm x 500mm), to enhance the electrical safety distance between different branch outputs, between branch outputs and the box housing, which 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 that the AC contactor coil voltage corresponds to the wiring 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 the main circuit contacts must be greater than or equal to the load current of the controlled circuit. 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 standard GB 6829 "General Requirements for Residual Current Circuit Breakers" and are certified as qualified by the China Electrotechnical Product Certification Committee. Options include similar LJM (J) series energy-saving models with low sensitivity and time-delayed protection. The installation method of the protector must conform to the national standard GB13955-2005 "Installation and Operation of Residual Current Circuit Breaker Devices." 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 technical requirements. For example, the incoming line of a distribution box for 30kVA and 50kVA transformers uses VV22-35×4 cables, and the branch outgoing lines use the same specification VLV22-35×4 cables; the incoming lines of distribution boxes for 80kVA and 100kVA transformers use VV22-50×4 and VV22-70×4 cables, respectively, and the branch outgoing lines use VLV22-50×4 and VLV22-70×4 cables, respectively. The cables are then connected to the distribution box terminal blocks with copper-aluminum terminals and 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 multiplier and the fuse characteristics. The rated current of the fuse element for the outgoing line 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 should avoid normal peak currents.

2.9 To analyze reactive power in rural low-voltage power grids, a DTS(X) series two-in-one multifunctional electric energy meter (mounted on the metering board side) is installed inside the box. This replaces the original three single-phase electric energy meters (DD862 series) to facilitate online monitoring of load operation.

Product Selection

There are numerous types of distribution boxes in China's power supply and distribution systems, each with different cabinet structures and technical specifications. Due to the influence of various factors, the originally designed drawings often require modifications, sometimes even complete redesigns. This not only affects the construction schedule of the power supply and distribution system but also poses certain challenges for distribution box manufacturers to complete production on time and meet quality standards.

Factors affecting changes to pre-designed drawings include:

Manufacturers of electrical distribution panels may recommend their products to customers, even if they may not be the most suitable.

2. The design institute isn't 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 of 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 electrical distribution cabinets.

The main features of domestically produced distribution cabinets include inconsistent specific models among domestic manufacturers, varying specific structures, and the main electrical components installed can be either import brands or domestic brands.

The following details the relevant technical specifications.

1. Main Busbar Rated High Current Capacity: The rated current value that the main busbar can withstand.

2. Rated Short-Time Current: The root mean square (RMS) value of the short-time耐受 current that a specific circuit in a 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 maximum peak current that the manufacturer specifies this circuit can safely withstand under specified test conditions.

4. Enclosure Protection Class: The level of protection against access to live parts, as well as the intrusion of foreign solid objects and the entry of liquids, provided by the complete equipment according to IEC60529-1989 standard. Specific classification details are available in the IEC60529 standard.

5. Internal Partitioning Methods: Dividing the switchgear into several compartments in various ways, in accordance with the IEC60529-1989 standard, for the protection of personal safety.

Technical specifications vary greatly between different models of distribution boxes, and imported distribution boxes generally have superior specifications 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 distribution panels are developed and manufactured overseas, typically for global power distribution markets. Due to the varying requirements and habits of power distribution systems across different countries, imported distribution panels may not be fully suitable for the domestic market.

2. The main electrical components used in imported distribution cabinets are all imported brand products. Some cabinets or auxiliary components of certain cabinets must be imported from abroad, which usually results in the price of imported distribution cabinets being significantly higher than that of domestically produced cabinets.

3. While the technical specifications of imported distribution cabinets are high, they are often only utilized to a fraction, or even not at all. For instance, an imported distribution cabinet can accommodate more circuits than domestic ones, but this is only possible by reducing the circuit capacity, which typically does not meet user requirements in most cases.

4. Although the technical specifications of domestically produced distribution panels are lower than those of imported ones, they have already met the needs of most domestic power supply and distribution systems.

5. As for the quality of distribution panels, as long as manufacturers strictly adhere to the 3C production and inspection standards, the quality of domestic distribution panels is not necessarily inferior to that of imported ones.

In summary, when selecting the model of a distribution box, the following points should be considered:

Understand the user's needs and select an appropriate container type based on the actual situation.

2. Prefer domestic models from well-known domestic manufacturers; avoid盲目 selecting imported distribution panels with high technical parameters, as this can lead to resource waste.

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.

NoteMatters to Note

(1) The construction power distribution system should be equipped with a main distribution box, sub-distribution box, and switch box, arranged in a hierarchical manner following the "main-sub-switch" sequence, forming a "three-level distribution" pattern.

(2) The installation locations of all distribution boxes and switch boxes in the construction power distribution system should be reasonable. The main distribution box should be as close as possible to the transformer or external power source for easy power intake. The distribution boxes should be installed as centrally as possible around the electrical equipment or load concentration areas to maintain balanced three-phase loads. The switch boxes should be installed as close as possible to the controlled electrical equipment, considering the on-site conditions and working environment.

(3) Ensure that the temporary electrical distribution system maintains a balanced three-phase load. The power and lighting electricity at the construction site should form two separate electrical circuits, with the power distribution box and lighting distribution box to be set up separately.

(4) All electrical equipment on the construction site must have its own switch box.

(5) All distribution box bodies and internal arrangements must comply with safety regulations. Switching electrical equipment should be clearly marked for use, and the boxes should be uniformly numbered. Distribution boxes that are no longer in use should have their power supply disconnected and the doors locked. Fixed distribution boxes should be equipped with fences and have measures to prevent rain and impact damage.

(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 typically used for residential purposes, while distribution panels are more commonly employed in centralized power supply, such as industrial and construction power needs. Both distribution boxes and panels are part of complete sets of equipment; distribution boxes are for low-voltage systems, and distribution panels come in both high and low voltage.