Secondary distribution box switches are designed according to IEC standards, typically for medium-voltage applications from 1 kV to 52 kV, with air-insulated (AIS) or gas-insulated (GIS) configurations depending on space, environment, and reliability requirements.

Voltage and Insulation Standards

Secondary distribution switchgear connects the primary medium-voltage network to low-voltage end users. Typical voltage ratings for secondary distribution boxes are up to 24 kV for indoor applications and up to 40.5 kV for outdoor or high-altitude installations, fully compliant with IEC standards . Insulation can be:

• Air-insulated (AIS): Uses dry air as the insulation medium, suitable for indoor or less harsh environments.
• Gas-insulated (GIS): Uses SF6 or alternative gases, compact and suitable for limited space, high pollution, or extreme weather conditions .

Design and Configuration

Secondary distribution boxes are often modular and compact, allowing integration into urban substations or industrial facilities. Key design features include:

• Metal-clad or metal-enclosed construction for safety and durability.
• Busbar arrangements: Single or double busbars, sometimes with tie breakers for redundancy.
• Circuit breakers: Vacuum or SF6 type, often with built-in fuses for overload and short-circuit protection .
• Close-coupled transformers: Secondary switchboards are located near transformers to minimize cable lengths and voltage drop .

Protection and Reliability

Standards require that secondary distribution boxes provide:

• Overcurrent and short-circuit protection using breakers or fuses.
• Automatic or manual switching to maintain service continuity during faults or maintenance.
• Coordination with primary protection to ensure selectivity and minimize outage impact .

Compliance and Certification

Secondary distribution switchgear must comply with:

• IEC 62271 series for medium-voltage switchgear.
• NBR 6979 for air-insulated cubicles (in some regions like Brazil).
• IAC AF and LSC2A-PM classifications for internal arc and mechanical performance .
• Environmental standards such as EU F-gas regulation for gas-insulated equipment .

Application Considerations

• Indoor vs outdoor installation: GIS is preferred where space is limited or environmental conditions are harsh.
• Urban substations: Compact, modular designs with corrosion-resistant materials.
• High-reliability networks: Ring main units (RMU) or spot networks allow multiple feeders to maintain service during component failures . In summary, secondary distribution box switches are standardized to ensure safety, reliability, and compliance with IEC and local regulations, with design choices influenced by voltage level, environmental conditions, and operational requirements. They integrate protection, switching, and modularity to support efficient secondary distribution in urban and industrial networks.