The power distribution system at construction sites shall implement hierarchical distribution. A main distribution box (or distribution room) shall be installed, followed by distribution boxes under the main distribution box, and switch boxes under the distribution boxes. Electrical equipment is supplied under the switch boxes.
The main distribution box shall be located close to the power source; distribution boxes shall be installed in areas where electrical equipment or loads are relatively concentrated. The distance between distribution boxes and switch boxes shall not exceed 30 meters. Switch boxes shall be powered by the final-level distribution box, and the horizontal distance between a switch box and the fixed electrical equipment it controls should not exceed 3 meters.
Distribution boxes and switch boxes shall be installed in well-ventilated, dry places at normal temperature. Installation is strictly prohibited in environments containing gases, smoke, steam, liquids, or other harmful media that may cause serious damage. They shall not be installed in locations prone to impact, vibration, liquid splashing, or heat exposure. Sufficient space for two people to work simultaneously and clear access shall be provided around them. No items that hinder operation or maintenance, nor weeds or shrubs, are allowed nearby.
Distribution boxes and switch boxes shall be made of steel plates or high-quality insulating materials. The thickness of the steel plate shall be greater than 1.5 mm. Electrical components inside the box shall first be mounted on a metal or non-wood insulating mounting plate, and then securely fixed inside the enclosure. The metal plate shall be electrically connected to the enclosure.
The internal wiring of distribution boxes and switch boxes shall use insulated conductors with secure connections and no exposed live parts. The working neutral conductor shall be connected via a terminal block and separated from the protective neutral terminal block. The metal enclosure, metal mounting plate, and non-current-carrying metal bases and housings of electrical components must be connected to protective earthing (neutral protection), which shall also be connected through a terminal block.
Power distribution boxes and lighting distribution boxes should be installed separately. If combined in the same box, power and lighting circuits shall be arranged in separate circuits.
The cable inlet and outlet of distribution boxes and switch boxes shall be located at the bottom of the enclosure. Installation at the top, sides, rear, or door of the box is strictly prohibited. Incoming and outgoing cables shall be sleeved, bundled by circuit, and provided with drip loops for waterproofing. Cable bundles shall not be in direct contact with the enclosure openings. Power supply cables entering switch boxes must not be connected using plugs. For mobile distribution and switch boxes, the inlet and outlet cables must use rubber-insulated cables.
Distribution boxes and switch boxes shall be installed firmly and upright. Mobile boxes shall be mounted on sturdy supports. For fixed boxes, the vertical distance between the bottom of the box and the ground shall be greater than 1.3 m and less than 1.5 m. For mobile distribution boxes and switch boxes, the vertical distance should be greater than 0.6 m and less than 1.5 m. All distribution boxes and switch boxes must be rainproof and dustproof.
