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 breakers. With secondary selective service, each distribution transformer must be able to supply the entire load for maximum reliability benefits. 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. 4kV), power is distributed to a main distribution panel. The secondary box adopts an inner and outer door design, with a spray painted exterior. It is safe and aesthetically pleasing, and the rainproof top of the box is suitable for outdoor work.