Requirements for Lightning Protection and Grounding of Optical Cables

Optical cables require both intermediate and terminal grounding to safely conduct lightning currents to the earth, with specific grounding methods, cable cross-sections, and installation distances depending on cable type and environment.

Lightning Protection Principles

Although fiber optic signals are optical, most outdoor optical cables contain metal components (armored cores or reinforced layers) that can conduct lightning currents, making them vulnerable to damage or fire during lightning strikes . The main goal of lightning protection is to safely divert high-current discharges into the ground, preventing damage to equipment and maintaining network integrity.

Grounding Solutions

1. Intermediate Grounding

Intermediate grounding is applied along the optical cable route, particularly for direct burial and aerial cables:

• Direct burial cables: Lightning protection wires are installed according to soil resistivity. Wet or conductive soils require careful grounding to prevent potential rise during strikes .
• Aerial cables: Grounding is achieved via pole grounding and suspension wire grounding:
  • Pole grounding: Lightning protection wires are installed every 250 meters. For poles taller than 12 meters or with angular deviations over 1 meter, additional grounding wires are required. When near high-voltage lines (>10 kV), wires should maintain a 50 mm gap from the pole .
  • Suspension wire grounding: Suspension wires are grounded through pole grounding every 300–500 meters, with electrical insulators added approximately every 1 km to ensure electrical disconnection .
• Underground extension: Lightning protection wires should be buried at least 700 mm below ground, with resistance and length following standard requirements .

2. Terminal Grounding

Terminal grounding ensures that all optical cable terminal equipment is properly grounded:

• Equipment includes optical distribution frames (ODFs), fiber cabinets, distribution boxes, and terminal boxes .
• Grounding connections:
  • Metal components of the optical cable are connected to a high-voltage protective grounding device using a copper wire ≥6 mm².
  • The grounding device is connected to the existing grounding system in the machine room using a multi-strand copper cable ≥35 mm².
  • For connections entering fiber distribution boxes, the copper cable cross-section can be ≥16 mm² instead of 35 mm² .

Additional Considerations

• Material and routing: Ground cables should be routed through PVC conduits along walls or poles and buried underground; overhead routing is not recommended .
• Connection to earthing network: Grounding devices should connect to a reliable earthing electrode or copper bar, ensuring low-resistance paths for lightning currents .
• Compliance with standards: Follow domestic and international standards for lightning protection, such as BS EN 62305, and ensure proper cross-sectional areas and grounding distances are maintained .

Summary

To protect optical cables from lightning:

1. Use intermediate grounding along the cable route for both aerial and buried cables.
2. Implement terminal grounding for all optical cable equipment.
3. Ensure adequate copper wire cross-sections (6 mm² to 35 mm²) and proper burial depth.
4. Maintain safe distances from high-voltage lines and follow standard grounding intervals.
5. Connect all grounding points to a reliable earthing network to safely dissipate lightning currents. These measures collectively ensure network safety, equipment protection, and compliance with lightning protection standards.