Essential Rules for 3-Level Electrical Distribution

Essential Rules for Three-Level Power Distribution System Design

1.Hierarchical and Branch Circuit Distribution

(1) Power distribution from the primary main distribution board (distribution cabinet) to secondary distribution boards can be branched; that is, one main distribution board may supply power via multiple branch circuits to several secondary distribution boards.

(2) Similarly, power distribution from a secondary distribution board to tertiary switch boxes can also be branched; that is, one distribution board may supply multiple branch circuits to several switch boxes.

(3) Power distribution from tertiary switch boxes to electrical equipment must follow the “one machine, one switch” principle, with no branching allowed. Each switch box shall connect to and control only one associated piece of electrical equipment (including sockets).

According to the hierarchical and branch circuit principle, in a three-level distribution system, no electrical equipment shall be connected by bypassing levels. Neither the main distribution board nor the distribution boards shall be directly connected to any other equipment; otherwise, the structural form and hierarchical branching principle of the three-level distribution system will be compromised.

2.Separate Power and Lighting Circuits

Power distribution boards and lighting distribution boards should preferably be installed separately. When power and lighting are combined in the same distribution board, they must be distributed via separate branch circuits. Moreover, power and lighting switch boxes must be set up separately—there shall be no shared enclosure with separate branch circuits.

3. Minimize Distribution Distances

The principle of minimizing distribution distances means that the distances between distribution boards and switch boxes should be kept as short as possible. The main distribution board should be located near the power source. Distribution boards should be placed in areas where electrical equipment or loads are relatively concentrated. The distance between a distribution board and a switch box shall not exceed 30 meters. The horizontal distance between a switch box and its controlled fixed electrical equipment should preferably not exceed 3 meters.

4. Environmental Safety

Environmental safety refers to the safety requirements for the installation and operational environment of the distribution system, including three aspects: operational environment, protective environment, and maintenance environment. The requirements are as follows:

(1) Protective Environment: Distribution boards and switch boxes shall be installed in dry, well-ventilated, and normal-temperature locations. They shall not be installed in environments containing harmful gases, fumes, excessive moisture, or other hazardous substances that could cause severe damage. They also shall not be installed in locations subject to external mechanical impact, strong vibration, liquid splashing, or heat radiation. If such conditions exist, the hazards must be eliminated or appropriate protective measures implemented.

(2) Maintenance Environment: Sufficient space and access for two persons to work simultaneously shall be provided around distribution boards and switch boxes. No items that obstruct operation or maintenance shall be stored nearby, and no shrubs or weeds shall be present.

(3) Operational Environment: Must comply with the principle of minimizing distribution distances.