What are primary, secondary, and tertiary distribution boxes? 

Many electrical professionals are well aware of their definitions. However, when it comes to their meanings and functions, some might be quite unclear. The primary distribution box refers to the main distribution box, typically located in the distribution room. These boxes feature bottom entry and exit cables, front-opening doors, and main busbars connected with copper strips for optimal contact. They also include metering systems, ensuring safety, aesthetics, and suitability for outdoor use due to their rainproof tops.

Secondary distribution boxes, also known as sub-distribution boxes, generally serve specific power supply areas. These boxes have inner and outer doors, powder-coated exteriors, and are designed for safety and aesthetic appeal, with rainproof tops for outdoor work. Tertiary distribution boxes, or switch boxes, are designated for individual equipment.

What are the specific meanings and functions of primary, secondary, and tertiary distribution boxes? 

The following introduction aims to deepen the understanding of these concepts among electrical professionals.

Meanings of Primary, Secondary, and Tertiary Distribution Boxes

The primary, secondary, and tertiary distribution boxes are relative concepts. For example, in a newly built residential area with a 10kV incoming line and a distribution room, power is distributed from the low-voltage end of the transformer at 0.4kV to the primary distribution cabinet. From there, power is further distributed to the secondary distribution boxes located in each building, and then to tertiary distribution boxes in each unit, ultimately reaching individual households.

Typically, primary distribution does not directly supply power to devices, secondary distribution handles power equipment with three-phase electricity, and tertiary distribution refers to household electricity (220V). 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. The equipment selection depends on the specific power load requirements.

Primary Distribution Box: Serves as the main distribution box for a construction site or project (usually only one).

Secondary Distribution Box: Serves each floor or building as needed.

Tertiary Distribution Box: The final connection box for each electrical appliance, which can be movable or fixed. Residual current devices must be the final protective device, with each appliance connected to a separate RCD.

Differences Between Primary, Secondary, and Tertiary Distribution Boxes

Primary Distribution Box:

Designed specifically for construction sites, conforming to relevant electrical codes.

Forms a complete three-level protection system to achieve one machine, one switch, and one protection.

Features bottom entry and exit cables, front-opening doors, copper busbars for main connections, metering systems, and rainproof tops for outdoor work.

Secondary Distribution Box:

Also designed for construction sites, meeting specific on-site electrical standards.

Forms part of the three-level protection system.

Features inner and outer doors, powder-coated exteriors, and rainproof tops for outdoor use.

Tertiary Distribution Box:

The system includes a main distribution box, sub-distribution boxes, and switch boxes connected to electrical equipment, forming a three-tiered distribution.

Employs a two-tiered protection approach with residual current devices in both the final switch boxes and the preceding sub-distribution or main distribution boxes.

Follows the principle of "one machine, one switch, one RCD, one box, one lock," ensuring no single switch controls multiple devices.

This explanation aims to clarify the roles and functions of primary, secondary, and tertiary distribution boxes, enhancing the understanding of these critical components in electrical distribution systems.