How to read a three-level electrical distribution box diagram

A three-level electrical distribution box system consists of a primary (main) distribution board, secondary (sub) distribution boards, and tertiary (final) distribution boxes, each progressively distributing power safely to end-use equipment.

Structure and Function

1. Primary Distribution Box (Main Distribution Board)

• Acts as the central hub, receiving low-voltage power (typically 0.4kV) from the transformer .
• Does not supply power directly to devices but distributes it to secondary boards.
• Components include isolating switches, circuit breakers, residual current devices (RCDs), and main busbars connected with copper strips for reliable contact .
• Often features bottom entry/exit cables, front-opening doors, and rainproof tops for outdoor installations . 2. Secondary Distribution Box (Sub-Distribution Board)
• Serves specific areas, buildings, or floors, distributing three-phase power to heavy loads like motors .
• Equipped with larger capacity three-phase circuit breakers and protective devices.
• Designed with dual-door protection, powder-coated exteriors, and rainproof tops for safety and durability .
• Connects the primary board to tertiary distribution boxes, forming an intermediate power hub. 3. Tertiary Distribution Box (Final or Switch Box)
• Supplies power directly to end-use equipment or household circuits, typically 220V single-phase .
• Implements strict safety measures: "one device, one circuit breaker, one RCD, one box" to ensure independent protection.
• Can be fixed or portable, forming the last junction point before electrical appliances.
• Supports a two-layer protection strategy with RCDs at both tertiary and secondary levels .

Conceptual Diagram

Transformer (10kV) | vPrimary Distribution Box (Main Board) |-- Isolating Switches, Circuit Breakers, RCDs | vSecondary Distribution Boxes (Sub-Boards) |-- Three-phase breakers, protective devices | vTertiary Distribution Boxes (Final/Switch Boxes) |-- Single-phase breakers, RCDs | vEnd-Use Equipment / Household Circuits

Key Safety Principles

• One machine, one switch, one RCD, one box, one lock: ensures no single switch controls multiple devices .
• Two-layer protection: RCDs installed at both secondary and tertiary levels to prevent leakage and electrical hazards .
• Outdoor suitability: rainproof tops and durable coatings for boxes installed outside or on construction sites . This three-level system ensures efficient power management, high safety, and reliability, making it suitable for residential complexes, construction sites, and large-scale projects .