Primary distribution box: three-phase power supply, ground wire and zero wire are introduced from the transformer. 4kV), power distribution is achieved through three levels of distribution boxes: the main distribution board, secondary distribution boards, and tertiary distribution boards. Main Distribution Board Serves as the primary. Power distribution hierarchy in building. Let's make an example for clarity: A newly constructed residential area introduces a 10kV power line to a substation. From the transformer's low-voltage side (0. Features bottom entry and exit cables, front-opening doors, copper busbars for main connections, metering systems, and rainproof tops for outdoor work.