Summary of Three-Tier Power Distribution System: Primary: The main distribution panel, supplies power from the transformer. In a newly constructed residential area, a 10kV power line is introduced into the substation. This. Utilities may have some control over and access to the energy stored in electric vehicles attached to the grid. Power distribution hierarchy in building. detailed explanation of DB, SDB, MDB, RMU, and Switchgear along with any commonly related equipment you might have missed, including their purpose, application, and hierarchy in an electrical distribution system. Distribution Overview In a typical. What do the primary, secondary, and tertiary boxes of a distribution box mean? This is a relative issue.