Electrical equipment in a three-level distribution box

In a three-level distribution system, electrical appliances are connected to the third-level distribution box, which provides direct control and protection for each appliance.

Structure of the Three-Level Distribution System

1. First-Level Distribution Box (Main Distribution Box)
   • Installed near the power source, it receives electrical energy and distributes it to second-level boxes.
   • Contains main circuit breakers, main leakage protectors, voltmeters, and ammeters for centralized monitoring and control of the entire site ( ).
2. Second-Level Distribution Box (Branch Distribution Box)
   • Located near large electrical equipment or power consumption areas.
   • Receives power from the first-level box and distributes it to third-level boxes.
   • Equipped with branch circuit breakers and branch leakage protectors to protect individual circuits ( ).
3. Third-Level Distribution Box (Switch Box)
   • Positioned closest to the electrical appliance, it directly powers and protects the device.
   • Typically, one switch box controls one appliance or a group of similar appliances.
   • Contains isolation switches, circuit breakers, and leakage protectors to ensure safe operation ( ).

Connection Rules for Electrical Appliances

• One machine, one switch, one leakage, one box: Each appliance must have its dedicated switch and leakage protection in the third-level box ( ).
• No bypassing levels: Appliances should not be connected directly to the first- or second-level boxes, maintaining the hierarchical structure ( ).
• Separation of circuits: Power and lighting circuits should be separated, and their switch boxes should not share enclosures ( ).
• Distance limitations: The horizontal distance between a switch box and its controlled appliance should ideally not exceed 3 meters, and the distance between distribution boxes should be minimized, typically under 30 meters ( ).

Safety and Reliability Considerations

• Leakage protection: The last protection device is installed in the third-level box to prevent electric shock and equipment damage ( ).
• Environmental safety: Distribution boxes should be installed in dry, ventilated areas free from harmful gases, moisture, or extreme temperatures ( ).
• Reliability: The three-level system ensures that a failure in one box affects only its downstream appliances, not the entire site ( ).

Practical Implications

Electrical appliances such as fans, lights, TVs, and industrial equipment are connected to the third-level distribution box. This setup allows direct control, localized protection, and easier maintenance, while the hierarchical system ensures efficient power distribution and safety compliance ( ).