Secondary Distribution Boards (SDBs) are typically installed as a single intermediate layer between the Main Distribution Board (MDB) and final Distribution Boards (DBs), with one SDB per floor or zone being standard practice.

Hierarchical Placement

In a typical low-voltage electrical distribution system:

1. Utility/Grid → RMU → Transformer → Switchgear → MDB → SDB → DB → Loads SDBs serve as an intermediate distribution layer, receiving power from the MDB and distributing it to multiple DBs or load points within a floor, section, or zone of a building .
2. Voltage Level: SDBs operate at low voltage (typically 415V) and are positioned after the MDB but before the final DBs .
3. Typical Deployment:
   • One SDB per floor or section is common in commercial buildings, warehouses, or multi-story structures.
   • Each SDB can feed multiple DBs, which then supply individual rooms, offices, or equipment .

Practical Considerations

• Layering Limit: While there is no strict numeric limit codified in general standards, practical design usually restricts to a single SDB layer per floor or zone to maintain manageable cable runs, minimize voltage drop, and simplify protection coordination .
• Substation Integration: In larger facilities, secondary unit substations may include close-coupled secondary switchboards, effectively acting as SDBs, but still maintain a single intermediate layer between the main transformer/MDB and DBs .
• Reliability and Maintenance: Limiting SDB layers to one per floor or zone ensures easier maintenance, fault isolation, and operational clarity.

Summary

• Permissible layers: Typically one SDB layer per floor or zone.
• Function: Intermediate distribution between MDB and DBs.
• Voltage: Low voltage (415V).
• Design rationale: Simplifies distribution, reduces cable lengths, and ensures reliable operation . This approach aligns with standard electrical distribution hierarchy practices and ensures both safety and operational efficiency in commercial and industrial installations.