Methods of laying optical cables in ducts

Optical cables in ducts can be installed using pulling, blowing (jetting), or floating techniques, each suited to specific duct conditions and cable types.

1. Pulling Method

The pulling method is the traditional technique for installing optical fiber cables in ducts. It involves manually or mechanically pulling the cable through the duct using a rope or winch. Key considerations include:

• Tensile load limits: Excessive pulling can damage fibers; maximum tension should follow the cable datasheet specifications, typically under 600 lbF (2,700 N) for standard loose tube or ribbon cables .
• Bend radius: Maintain the minimum bend radius during installation to prevent microbending or fiber breakage. For example, a cable with 0.46 in diameter requires a minimum working bend radius of 6.9 in (17.7 cm) during pulling .
• Coiling and handling: Use figure-eight coiling for lengths over 30 m to prevent twisting and kinking .
• Route planning: Windings in the duct trajectory contribute more to tension than bends, so careful planning of duct layout is essential .

2. Blowing (Jetting) Method

The blowing method uses compressed air to propel the cable through the duct, often combined with a short initial push using a mechanical device:

• Mechanism: Compressed air is injected at the duct inlet, creating a drag force that reduces friction between the cable and duct wall, allowing long-distance installation with minimal stress .
• Cable types: Suitable for standard optical cables, microcables, and microducts. Microcables can be installed over continuous lengths exceeding 1,000 meters depending on duct conditions .
• Advantages: Reduces manpower, installation time, and mechanical stress on the cable. Flexible cables perform better in complex routes, while stiffer cables are preferred for straight ducts with undulations .
• Equipment: Requires a blowing device with traction rollers and a compressed air source. Additional pushing may be needed for maximum performance in long or curved ducts .

3. Floating Method

The floating method is less common and involves using a fluid (air or water) to support the cable inside the duct, reducing friction and mechanical stress. It is often integrated with blowing techniques for microduct installations .

4. Duct Infrastructure Considerations

• Duct sizing: Ensure ducts are appropriately sized for the cable and future network expansion. Oversizing increases cost, while undersizing may require costly upgrades .
• Sub-ducts: Small diameter sub-ducts can be used for individual cable installations, allowing easier upgrades and maintenance .
• Route obstacles: Bridges, railways, and major road crossings may require oversized ducts to accommodate multiple cables or future growth .
• Material: Use high-quality HDPE or similar materials to minimize friction and protect the cable during installation .

Summary

• Pulling: Simple, traditional, suitable for short or straightforward routes; requires careful tension and bend management.
• Blowing/Jetting: Efficient for long distances and complex routes; reduces cable stress and installation time.
• Floating: Supports cable with fluid to minimize friction; often combined with blowing for microducts.
• Duct planning: Proper sizing, sub-ducts, and route design are critical for successful installation and future network expansion. By selecting the appropriate method based on duct conditions, cable type, and route complexity, optical fiber installations can be performed efficiently while minimizing mechanical stress and ensuring long-term reliability.