This three-tier distribution system structure — with the main distribution board acting as the primary delivery point, secondary distribution boards serving as intermediate power hubs, and tertiary distribution boards directly supplying end-use equipment — ensures. This three-tier distribution system structure — with the main distribution board acting as the primary delivery point, secondary distribution boards serving as intermediate power hubs, and tertiary distribution boards directly supplying end-use equipment — ensures. A distribution box is installed under the main distribution box, and a switch box is installed under the distribution box. Electrical equipment is installed under the switch box, forming a three-level distribution. "Two level protection" mainly refers to the use of leakage protection measures. In. Primary Distribution Box: Serves as the main distribution box for a construction site or project (usually only one). From there, it is routed to individual building distribution boxes (secondary distribution boxes), which subsequently supply power to unit-level distribution boxes. Class I distribution box: the construction power distribution cabinet is used for construction power on the construction site.