three-level distribution system

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The three-level distribution system refers to a system that distributes electric power through three levels of distribution devices from the incoming power line at the construction site to the electrical equipment. The following is a detailed introduction about it:

### System Composition
- **First-level Distribution Box**: Also known as the main distribution box, it is the main hub of the power distribution system at the construction site and is generally installed near the power source. Its main function is to receive the electrical energy from the power source, distribute and control it, and supply power to the second-level distribution boxes. Usually, electrical equipment such as the main circuit breaker, main leakage protector, voltmeter, and ammeter are installed in the main distribution box, which is used for centralized management and monitoring of the power at the entire construction site.
- **Second-level Distribution Box**: It belongs to the distribution box and is usually installed in various power consumption areas at the construction site or near large electrical equipment. It receives the electrical energy from the first-level distribution box and further distributes it to the third-level distribution boxes. Generally, electrical equipment such as branch circuit breakers and branch leakage protectors are installed in the second-level distribution box, which is used for distributing and protecting the power of each branch circuit.
- **Third-level Distribution Box**: That is, the switch box, which is at the end of the power distribution system and directly provides power for electrical equipment. It is closely connected to the electrical equipment. Generally, one switch box only controls one electrical equipment or a group of the same kind of electrical equipment. Electrical equipment such as isolation switches, circuit breakers, and leakage protectors are installed in the box, which are used for the direct control and protection of electrical equipment.

### Main Functions
- **Improve the Reliability of Power Supply**: Through the three-level distribution, the electric energy is gradually distributed to various power consumption areas and equipment, avoiding the situation that the entire construction site is powered off due to a single failure. Even if a certain level of distribution box or circuit fails, it will only affect some areas or equipment at its lower level, and the power supply in other areas can still remain normal, thus improving the reliability and stability of the power supply at the entire construction site.
- **Ensure the Safety of Electricity Use**: Corresponding protection devices such as leakage protectors and circuit breakers are equipped in distribution boxes at all levels. When faults such as leakage, overload, and short circuit occur in the circuit, these protection devices can act quickly to cut off the power supply, avoiding safety accidents such as electric shock to personnel and damage to equipment.
- **Facilitate Management and Maintenance**: The three-level distribution system clearly grades and divides the power distribution at the entire construction site, making power management and maintenance more convenient. Electricians can conduct patrol inspections, maintenance, and troubleshooting work targeted at different areas and equipment, improving work efficiency and reducing management costs.

### Design and Installation Requirements
- **Reasonable Site Selection**: The installation positions of distribution boxes at all levels should be convenient for operation, maintenance, and inspection. Meanwhile, they should avoid being affected by mechanical damage, water immersion, and flammable and explosive substances. Generally, the first-level distribution box is required to be installed in a place with a relatively high terrain, dry and ventilated, and the second-level and third-level distribution boxes should be as close as possible to the electrical equipment.
- **Selection of Electrical Equipment**: According to the power consumption load and equipment types at the construction site, reasonably select the electrical equipment in the distribution boxes at all levels, such as circuit breakers, leakage protectors, and isolation switches. The parameters such as the rated current, rated voltage, and short-circuit breaking capacity of the electrical equipment should meet the actual use requirements and have good quality and reliability.
- **Grounding and Zero-connection Protection**: The three-level distribution system should adopt the TN-S system, that is, a zero-connection protection system in which the working zero line (N line) and the protective zero line (PE line) are set separately. The metal shells of distribution boxes at all levels and electrical equipment must be reliably connected to the protective zero line to ensure that in the event of a leakage accident, the current can be quickly introduced into the earth through the protective zero line to ensure the safety of personnel and equipment.
- **Cable Laying**: The cable lines between distribution boxes and between distribution boxes and electrical equipment should be laid in accordance with the specified laying methods, such as overhead laying, buried laying, or pipe laying. The selection of cables should be determined according to factors such as load size and laying environment. Meanwhile, it should be ensured that the insulation performance of the cables is good and there is no damage, aging, and other phenomena.

### Safe Use and Maintenance
- **Special Personnel in Charge**: The three-level distribution system at the construction site should be operated and managed by professional electricians. Non-electrician personnel are not allowed to tamper with the electrical equipment in the distribution boxes without permission. Electricians should regularly conduct patrol inspections on distribution boxes and lines to discover and handle potential safety hazards in a timely manner.
- **Regular Inspection**: Regularly inspect and maintain the electrical equipment in distribution boxes at all levels, including checking the operating status of the equipment, whether the wiring is firm, and whether the protection devices are sensitive and reliable. Meanwhile, the insulation resistance test of cable lines should be carried out regularly to ensure that the insulation performance of the lines is good.
- **Strict Operating Procedures**: When operating the electrical equipment in the distribution boxes, it should be carried out strictly in accordance with the operating procedures, such as power outage, electricity inspection, and grounding wire connection. It is strictly prohibited to carry out maintenance and repair operations under live conditions to ensure the personal safety of operators.