Three-level power supply to the distribution box

A three-level distribution box system distributes power hierarchically from the main source to end-use equipment, ensuring safety, control, and efficient load management.

Overview of the Three Levels

Primary Distribution Box (First-Level): This is the main hub of the system, typically installed near the power source or transformer. It receives electrical energy and distributes it to secondary distribution boxes. Key components include the main circuit breaker, main leakage protector, voltmeter, and ammeter, which allow centralized monitoring and control of the entire site's power supply . The main switch rating is determined based on the number of downstream boxes and the total power demand . Secondary Distribution Box (Second-Level): Installed near large electrical equipment or in concentrated load areas, it receives power from the primary box and redistributes it to tertiary boxes. It usually contains branch circuit breakers and branch leakage protectors to protect individual circuits and equipment. This level ensures that power is safely and efficiently allocated to different zones or buildings . Tertiary Distribution Box (Third-Level): Also called the switch box, it is the final distribution point, directly supplying power to individual electrical devices or groups of similar equipment. Each tertiary box follows the “one machine, one switch” principle, meaning it controls only one piece of equipment or a dedicated set of devices. Components include isolation switches, circuit breakers, and leakage protectors, providing direct control and protection .

Key Principles of Power Supply

1. Hierarchical Distribution: Power flows sequentially from primary to secondary to tertiary boxes, with no bypassing of levels to maintain safety and structural integrity .
2. Branching Rules: Primary and secondary boxes can supply multiple downstream boxes via branch circuits, but tertiary boxes must connect directly to their designated equipment without further branching .
3. Load Capacity and Distance: Typical load capacity for a single distribution box ranges from 50–150 kW. The distance between distribution boxes and switch boxes should be minimized—ideally, no more than 30 meters between boxes and 3 meters from a switch box to its controlled equipment .
4. Safety and Environment: Distribution boxes should be installed in dry, ventilated areas, away from harmful gases, moisture, or mechanical hazards. Adequate space must be provided for maintenance and operation .

Cable and Connection Considerations

Cables connect the secondary and tertiary boxes, allowing flexible power transmission and expansion. Proper cable sizing ensures stable power supply and prevents overloads. Copper busbars are often used in primary and secondary boxes for optimal conductivity .

Summary

The three-level distribution box system ensures safe, hierarchical, and efficient power distribution from the main source to end-use equipment. By adhering to principles such as hierarchical branching, one machine-one switch, proper load management, and environmental safety, the system provides reliable power for construction sites, industrial facilities, or large-scale projects .