Primary Distribution Box: Serves as the main distribution box for a construction site or project (usually only one). At this. secondary unit substation is a close-coupled assembly consisting of enclosed primary high voltage equipment, three-phase power transformers, and enclosed secondary low-voltage equipment. 1 This document is one of a suite of documents intended for designing and installing substations for adoption, and/or for use, by Scottish and Southern Electricity Networks (SSEN) Designers and Installers, covering the following situations. Let's make an example for clarity: A newly constructed residential area introduces a 10kV power line to a substation. From the transformer's low-voltage side (0.