Three level distribution box: a distribution box is set under the main distribution box, a switch box is set under the distribution box, and electrical equipment is set under the switch box to form a three-level distribution box. Rules And Regulations Of Distribution Box Grade How is the grade planning. What do the primary, secondary, and tertiary boxes of a distribution box mean? This is a relative issue. The outgoing line from the low-voltage end of the transformer is 0. "Two-level protection" mainly refers to the use of leakage protection. The third-level distribution box is the distribution box to which the cables drawn from the second-level distribution box are connected.