The grounding wire in a secondary distribution box ensures safety and system stability, typically requiring copper conductors sized according to local standards and connected to a low-resistance earth path.

Wire Sizing and Material

For proper grounding, the wire size depends on the market and system requirements. In the US, a 10 AWG (5.26 mm²) copper wire is commonly used, while in other regions, a 6 mm² copper wire is standard . For connections between mounting plates and equipment like spindle plates, larger wires such as 16 mm² (or 6 AWG in the US) may be required to maintain low resistance and ensure safety under fault conditions . Copper is preferred for buried or exposed runs due to its high conductivity and corrosion resistance .

Connection Points

The grounding wire should be connected from a threaded stud at the bottom of the distribution box housing to the mounting plate, and then from the mounting plate to the central grounding point of the facility . This ensures that all metallic parts of the system are at the same potential, reducing the risk of electric shock and equipment damage.

Resistance Requirements

A properly installed grounding system should maintain a ground resistance of less than 0.1 ohm between all system parts . This low resistance ensures that fault currents are safely directed to the earth, protecting personnel and equipment . In some applications, additional grounding wires may be omitted if the cable lengths are short and multiple devices are grounded together, but the total resistance must still meet safety standards .

Safety and System Considerations

Grounding serves multiple purposes:

• Personnel Safety: Reduces the risk of electric shock by minimizing potential differences between conductive parts and the earth .
• Equipment Protection: Protects distribution equipment from lightning strikes, transient overvoltages, and fault currents .
• System Stability: Stabilizes voltage levels and ensures proper operation of protective devices like circuit breakers and relays .
• Fault Detection: Provides a clear path for fault currents, enabling rapid isolation of faults .

Installation Best Practices

• Use certified copper conductors and avoid aluminum for buried runs, as aluminum corrodes over time .
• Ensure mechanical protection for grounding conductors to prevent damage and maintain continuity .
• Test the grounding system after installation to verify resistance values and proper connections .
• Consider soil resistivity and environmental factors; in high-resistivity areas, deeper or chemically enhanced grounding electrodes may be necessary . By following these guidelines, the secondary distribution box will have a reliable grounding system that protects both personnel and equipment while maintaining system performance.