Construction site electrical distribution is classified into three levels: primary (main), secondary (intermediate), and tertiary (final switch) distribution boxes, each with distinct functions, equipment, and safety measures.

Primary Distribution Box (Level 1)

The primary distribution box serves as the main hub of the construction site power system. It is typically installed near the power source or transformer and handles the incoming three-phase power, ground, and neutral lines. Its main functions include receiving, controlling, and distributing electrical energy to secondary distribution boxes. Key equipment includes the main circuit breaker, main leakage protector, voltmeter, and ammeter, which allow centralized monitoring and management of the entire site's electricity. The box is usually rainproof and designed for outdoor use, with copper busbars ensuring reliable connections and a metering system for safety and monitoring .

Secondary Distribution Box (Level 2)

The secondary distribution box acts as an intermediate panel, receiving power from the primary box and distributing it to specific areas or large equipment on the site. It is often installed near high-consumption zones or machinery. Typical equipment includes branch circuit breakers and branch leakage protectors, providing protection and control for each branch circuit. The box often features an inner and outer door design, plastic-sprayed exterior, and a rainproof top for outdoor conditions. It ensures two-level protection by combining leakage protection at both the primary and secondary levels .

Tertiary Distribution Box (Level 3)

The tertiary distribution box, also called the switch box, is the final distribution point directly connected to electrical equipment. It can be fixed or movable and is designed to supply power to a single device or a group of similar devices. Equipment inside includes isolation switches, circuit breakers, and leakage protectors, ensuring direct control and protection of the connected equipment. The tertiary box follows the principle of “one machine, one switch, one leakage, one box, one lock”, preventing a single switch from controlling multiple devices and enhancing safety .

Installation and Safety Considerations

• Placement: Primary boxes should be installed in high, dry, and ventilated areas. Secondary and tertiary boxes should be close to the equipment they serve to minimize voltage drop and facilitate maintenance .
• Protection: The system typically uses a TN-S grounding system, separating the protective earth (PE) and neutral (N) lines. All metal enclosures must be reliably grounded to ensure leakage currents are safely directed to the earth .
• Equipment Selection: Choose circuit breakers, leakage protectors, and isolation switches based on the power load, rated current, voltage, and short-circuit capacity to match actual site requirements .
• Maintenance: The three-level system allows targeted inspections, troubleshooting, and maintenance, improving efficiency and reducing management costs . This three-level classification ensures effective power management, safety, and reliability on construction sites, particularly for large-scale projects with multiple electrical loads.