Repeated grounding of the three-level mobile distribution box

Repeated grounding ensures safety and system stability by connecting the neutral and metal parts of a distribution box to multiple grounding points along the system.

Purpose of Repeated Grounding

Repeated grounding, also known as multigrounding, is used in systems where the neutral point is directly grounded. Its main purposes are to:

• Provide a low-impedance path for fault currents, ensuring rapid operation of protective devices.
• Maintain voltage stability and reduce overvoltage risks during transient conditions.
• Ensure personnel safety by connecting all exposed conductive parts to earth, preventing dangerous touch voltages .

Implementation in a Three-Level Distribution Box

For a low-voltage three-phase four-wire system, repeated grounding involves:

1. Neutral Line Grounding: The neutral main line and branch line terminals are grounded during installation. For lines exceeding 50 meters, the neutral connected to the user should also be grounded repeatedly, with grounding resistance not exceeding 10 ohms .
2. Enclosure Grounding: The distribution box enclosure, mounting boards, doors, and secondary doors are all connected to the protective earth (PE) conductor .
3. Grounding Materials: Galvanized steel pipes or angle irons are driven at least 2 meters into the ground, with anti-corrosion treatment at joints. Multiple grounding points can be connected using galvanized flat iron to ensure continuity .

System Considerations

• Multigrounded vs. Ungrounded Systems: In multigrounded systems, the neutral is grounded at the substation and at each transformer along the circuit. This allows fault currents to divide between the neutral and earth, improving safety and reliability .
• Ground Fault Protection: High-resistance ground faults are more difficult to detect in multigrounded systems because the fault current is combined with unbalanced load currents. Protective relays must account for this to ensure proper fault detection .
• IEC Standards: Modern IEC standards have moved away from multiple earthing in some contexts due to potential step and touch voltage hazards, emphasizing controlled single-point grounding for safety and electromagnetic compatibility .

Practical Tips

• Ensure all metal parts inside the distribution box are bonded to the PE.
• Verify grounding resistance is within recommended limits (≤10 ohms).
• Use corrosion-resistant materials for long-term reliability.
• Consider system design and local regulations to determine the number and location of grounding points. Repeated grounding in a three-level mobile distribution box is essential for safe operation, fault protection, and voltage stability, but it must be implemented carefully to comply with standards and avoid unintended hazards.