A secondary distribution box (SDB) is an intermediate electrical panel that receives power from the main distribution box and distributes it to specific areas, buildings, or floors, typically handling three-phase power for equipment.

Function and Role

A secondary distribution box serves as a sub-distribution point in a multi-level electrical system. It receives power from the primary distribution cabinet (connected to the transformer) and distributes it to specific zones, such as individual buildings, floors, or large equipment modules . Unlike tertiary distribution boxes, which supply single-phase power to individual devices or households, secondary boxes typically handle three-phase power and are designed for higher load capacities .

Design and Features

Secondary distribution boxes are designed for both safety and durability, often featuring:

• Inner and outer doors for secure access and protection .
• Powder-coated or sprayed exteriors for corrosion resistance and aesthetic appeal .
• Rainproof tops for outdoor installation .
• Copper busbars for main connections to ensure reliable conductivity .
• Terminal blocks for custom wiring and flexible circuit connections .
• Front-opening doors and bottom entry/exit for cables, facilitating easy installation and maintenance .

Components

The internal components of a secondary distribution box typically include:

• Large three-phase circuit breakers to protect equipment and circuits .
• Optional residual current devices (RCDs) for additional safety, forming part of a multi-level protection system .
• Metering systems for monitoring power usage in specific areas .

Applications

Secondary distribution boxes are widely used in:

• Construction sites, where temporary power distribution is required .
• Residential or commercial buildings, distributing power from the main transformer to each building or floor .
• Industrial facilities, feeding power to machinery or equipment modules .

Safety and Standards

Secondary distribution boxes are part of a three-tier protection system:

1. Primary distribution box – main power intake from the transformer.
2. Secondary distribution box – intermediate distribution to buildings or floors.
3. Tertiary distribution box – final connection to individual devices or households. This system ensures “one machine, one switch, one protection”, preventing multiple devices from being controlled by a single switch and enhancing electrical safety . In summary, a secondary distribution box is a critical component in electrical distribution networks, bridging the main power supply and localized circuits, providing both safety and efficient power management for medium-to-large electrical loads.