The equipment within these boxes varies: primary distribution cabinets usually contain isolating switches, circuit breakers, and residual current devices (RCDs); secondary cabinets contain large three-phase circuit breakers; tertiary cabinets contain single-phase circuit. The equipment within these boxes varies: primary distribution cabinets usually contain isolating switches, circuit breakers, and residual current devices (RCDs); secondary cabinets contain large three-phase circuit breakers; tertiary cabinets contain single-phase circuit. Primary distribution systems consist of feeders that deliver power from distribution substations to distribution transformers. A feeder usually begins with a feeder breaker at the distribution substation. Many feeders leave substation in a concrete ducts and are routed to a nearby pole. Secondary distribution boxes, also known as sub-distribution boxes, generally serve specific power supply areas. The following electrical ratings are typical: As a result of locating power transformers and their close-coupled. 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.