According to the hierarchical and branch circuit principle, in a three-level distribution system, no electrical equipment shall be connected by bypassing levels. A distribution box is installed under the main distribution box, and a switch box is installed under the distribution box. "Two level protection" mainly refers to the use of leakage protection measures. Neither the main. An air switch can be used to turn devices on or off. Momentary air switches allow. Numerous load break switches use an interrupter head of some type in series with an air switch, such that when opening the sequence is: interrupter opens, air switch opens, interrupter closes. Residual current devices (RCDs) at both the tertiary (equipment-level) and secondary (zone-level) stages.
