Detailed structural diagram of a three-level distribution box

A three-level distribution box system consists of a main distribution box, secondary distribution boxes, and final-level switch boxes, forming a hierarchical tree-like power distribution network.

Overview of the Three Levels

First-Level Distribution Box (Main Distribution Box): This is the primary hub installed near the power source. It receives incoming electrical energy and distributes it to the second-level boxes. Key components include the main circuit breaker, main leakage protector, voltmeter, and ammeter. It provides centralized control and monitoring for the entire site and determines the main switch rating based on the number of downstream boxes and total load . Second-Level Distribution Box (Sub-Distribution Box): Installed near large electrical equipment or in specific power consumption areas, it receives power from the main box and redistributes it to third-level boxes. It typically contains branch circuit breakers and branch leakage protectors to protect and manage each branch circuit . Third-Level Distribution Box (Switch Box): This is the final junction box directly connected to electrical equipment. It provides power to individual devices and includes the last layer of protection, such as leakage protection switches. It can be fixed or movable depending on the equipment layout .

Internal Structure and Components

A typical distribution box includes:

• Phase Busbar: Distributes live current to outgoing circuits.
• Neutral Bar: Collects return current from circuits.
• Earth Bar: Provides grounding for safety.
• MCBs (Miniature Circuit Breakers): Protect individual circuits.
• RCCBs/RCBOs: Provide leakage protection.
• SPD (Surge Protective Device): Protects against voltage spikes.
• DIN Rails and Cable Entries: Organize components and wiring .

Functional Layout

The system forms a tree-like hierarchy:

1. Main Distribution Box → 2. Sub-Distribution Boxes → 3. Switch Boxes → End Equipment. Each level ensures proper load distribution, protection, and monitoring. The boxes are typically located within 30 meters of the equipment they serve, and fixed equipment like tower cranes may have dedicated boxes, while temporary facilities may share a box .

Safety and Protection Principles

• Two-Level Protection: Leakage protectors are installed at both the main and final-level boxes.
• One Machine, One Switch, One Leakage, One Box, One Lock: Each switchgear controls only one device to prevent overloads and hazards .
• Load Capacity: Each box usually handles 50–150 kW, with ratings determined by downstream load . This hierarchical design ensures efficient, safe, and manageable power distribution across construction sites or industrial facilities, minimizing risks and allowing centralized monitoring.