(3) Power distribution from tertiary switch boxes to electrical equipment must follow the “one machine, one switch” principle, with no branching allowed. (1) Power distribution from the primary main distribution board (distribution cabinet) to secondary distribution boards can be branched; that is, one main distribution board may supply power via multiple branch circuits to several secondary distribution boards. Three-phase distribution boards are used in large factories, buildings, manufacturing units. Before installation, it's important to know what makes up a distribution box. Let's break it down into two main parts: the outer shell and the electrical parts inside.