A Main Distribution Board (MDB) is the primary power distribution point in large buildings or industrial systems. It receives electricity from a transformer or main source and sends it to sub-panels or circuits. In IEC 61439 terms, an MDB is typically designed and. Main distribution board (MDB): a distribution board that fulfills all the functions of a main electrical distribution for the supplied area assigned to it and where the voltage is measured for operating the electric supply system for safety services [defined in the IEC 60364-7-710-2021]. 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.