A secondary distribution box must be properly grounded using dedicated conductors to a central grounding point, ensuring a low-resistance path (<0.1–25 ohms) for fault currents.

Key Grounding Requirements

1. Grounding Conductors and Connections

• Each distribution box must be grounded with a dedicated ground wire connected to a threaded stud or ground bar on the box housing .
• The ground wire should then connect to the mounting plate and from there to the facility's central grounding point .
• In the US, a 10 AWG (5.26 mm²) ground wire is standard, while in other markets, a 6 mm² wire is typically used .
• For larger setups or longer cable runs, additional ground wires may be required to maintain low resistance, e.g., 6 AWG (13.29 mm²) for extended tool cable connections . 2. Resistance Requirements
• The total ground resistance between all system parts should be less than 0.1 ohm for industrial applications .
• For general installations, a maximum of 25 ohms is recommended, with multiple rods or conductors used if soil resistivity is high . 3. Grounding Methods
• Ground rods or copper-bonded steel rods are commonly used, with lengths typically around 1.2–2.4 meters and connected with bare or tinned copper conductors .
• The grounding system should provide a continuous low-resistance path for fault currents, protecting personnel and equipment from electric shock and transient overvoltages .
• Metal parts of the distribution box and associated equipment should be bonded to the grounding system to prevent potential differences and ensure system stability . 4. System Considerations
• The secondary side of transformers feeding the distribution box is usually solidly grounded, often connected to the neutral and bonded at multiple points .
• Grounding must account for expected fault currents, soil resistivity, and corrosion potential to ensure long-term reliability .
• For multi-spindle or long cable applications, parallel grounding paths may be used to maintain low resistance and meet safety standards . 5. Professional Installation
• Grounding must be installed by a qualified electrician to ensure compliance with local electrical codes and safety standards .
• Testing of the grounding system is essential to verify resistance values and proper connections before energizing the distribution box . By following these guidelines, a secondary distribution box will have a safe, low-resistance grounding system that protects both personnel and equipment while ensuring reliable operation of the electrical distribution network.