Grounding Standard Diagram for Secondary Distribution Box

Secondary distribution boxes should be grounded using an equipotential busbar network connected to the main grounding grid, ensuring safe dissipation of fault currents and minimizing touch and step voltages.

Key Components of the Grounding System

1. Grounding Busbar: A copper busbar with a minimum cross-sectional area of 100 mm² is installed at the bottom of the distribution box or relay panel. This busbar serves as the central point for all grounding connections within the panel ( ).
2. Device Grounding Connections: All devices mounted on the panel, including meters, relays, and control equipment, are connected to the grounding busbar using multi-strand copper wires with a minimum cross-sectional area of 4 mm² ( ).
3. Connection to Main Grounding Network: The busbar is connected to the main equipotential grounding network of the facility using a copper conductor of at least 50 mm². Multiple connections (at least four) are recommended to ensure redundancy and low impedance ( ).
4. Equipotential Grid in Panel Room: In the cable compartment beneath the control or protection room, a copper bar or cable of at least 100 mm² is laid along the panel arrangement. The ends are interconnected to form a mesh or grid, creating an equipotential network that minimizes potential differences within the room ( ).
5. Connection to Outdoor Grounding Grid: The indoor equipotential network is welded or securely connected to the outdoor grounding grid using a copper bar or cable of at least 100 mm². This ensures that fault currents are safely directed to earth ( ).

Design Considerations

• Fault Current Capacity: All grounding conductors and busbars must be capable of safely carrying the expected ground fault currents for the duration of fault clearing times ( ).
• Soil Resistivity: The grounding system design should account for soil resistivity at the installation site to achieve low resistance to earth ( ).
• Step and Touch Voltage: The layout must limit step and touch potentials to safe levels to protect personnel ( ).
• Material Durability: Copper is preferred for its conductivity and corrosion resistance. Mechanical protection should be considered to prevent damage ( ).

Diagram Description

A typical secondary distribution box grounding diagram includes:

• A main copper busbar at the bottom of the panel.
• Individual device grounding wires connecting each device to the busbar.
• Interconnected copper bars forming a mesh in the cable compartment.
• Multiple connections from the indoor busbar to the main outdoor grounding grid.
• Ground rods or plates at the outdoor grid to dissipate fault currents into the earth. This configuration ensures an equipotential bonding network, minimizes voltage differences, and provides a safe path for fault currents, complying with IEC and GB/T standards ( ). By following this standard, secondary distribution boxes achieve safe operation, personnel protection, and reliable fault current dissipation.