Construction Methods for Micro Fiber Cable Networks

Overview:

Micro fiber cable network installation is characterized by a multi-step process. First, a circular mother duct is buried underground. The inner wall of the mother duct is designed with an antenna-like profile or is enhanced with multiple layers of lubrication. Throughout the entire length of the buried duct, the width must remain consistent. Next, a bundle of sub-ducts is inserted and blown into the inner wall of the mother duct. Following this, micro fiber cables are connected and blown into the sub-duct bundle’s inner wall. In the access network, the sub-ducts are initially connected in a simple manner; then, based on customer requirements, micro cables with outdoor cable performance are blown into the sub-ducts, allowing branch connections without splicing. With this method, the capacity of the access network can expand as demand grows. This approach overcomes the limitations of conventional outdoor fiber cable laying techniques by inserting micro sub-ducts into an HDPE mother duct, which is installed by direct burial. The micro cables can be blown directly into these specially designed sub-ducts without splicing; the bundled sub-ducts are blown into the mother duct while leaving ample space inside the mother duct. When a branch is required, one simply cuts the mother duct and the desired sub-duct, then connects the cut sub-duct to the branch sub-duct using fixed or push-pull connectors. Finally, a disassemblable branch connector is used to join the mother duct to the branch mother duct, enabling branching at any location.

When the interior of the mother duct has sufficient space, it provides the necessary mechanical protection. The combined cross-sectional area (calculated based on the outer diameter of the sub-duct) typically occupies only half of the inner space of the mother duct. This ensures that the sub-duct bundle can be successfully blown into the mother duct while also facilitating later connections. The mother duct can be installed in conduit holes or directly buried underground. In densely populated network areas, it is possible to complete terminations within distribution boxes without cutting the fiber cable. Therefore, in principle, junction boxes are installed only where splicing is required or when the already activated fiber cable is to be connected to another branch. In such cases, the distribution box must be disassemblable, avoiding the need to cut the sub-duct or fiber cable; however, all the components (mother duct, sub-duct, sub-duct connectors, fiber cable, cable port, junction box, and other required sealing components) must be sealed.

Key Components Explanation:

1. Sub-Duct Bundle:

The micro sub-duct bundle is blown into the mother duct. The mother duct must leave enough space—about half of the total head area—to ensure mechanical protection of the sub-ducts and facilitate smooth blowing and subsequent selection of a sub-duct at any location.

2. Mother Duct:

The primary function of the mother duct is to guide and protect the micro sub-ducts that contain the fiber cables. To ensure that the sub-duct bundle is successfully blown into the mother duct, the mother duct must withstand the necessary pressure differential. Typically made of HDPE, the mother duct is buried directly underground. It must be circular and maintain consistent dimensions throughout its length. Its inner and outer walls must be free of cracks, pinholes, joints, water stains, mold marks, repairs, or any other defects. Reducing the reinforcement ratio between the sub-duct bundle and the mother duct is crucial; the inner wall of the mother duct should ideally feature a fire-fighting interface design (though not mandatory) or be enhanced with an additional lubrication layer. Before blowing the sub-ducts, the inner wall of the mother duct should be coated with a layer of spray lubricant to further reduce friction (this may be applied regardless of whether the mother duct already has a lubrication layer).

3. Installation Considerations:

Although it is not necessary to straighten the mother duct at the end of the trench, it should be laid as straight as possible. During the cable laying process, the mother duct reel should be rotated rather than pulling the duct from a flat reel to avoid twisting. The curved radius on the side walls should be at least 1 meter; however, near the branch (for a φ25mm duct), the turning radius can be smaller (e.g., 0.5 meters). A smaller turning radius can affect the fiber blowing efficiency for both the sub-duct and the fiber cable; similarly, excessive bends will also impair the blowing process. The sub-duct reel should be fixed on a specially designed multi-head cable dispensing rack, whose specific role in the network is to introduce the fiber cable while avoiding splicing. To ensure the fiber cable and sub-duct bundle are blown in smoothly, the sub-duct must withstand the necessary external pressure (with the sub-duct bundle pressurized during blowing to avoid implosion). For reduced friction when blowing the sub-duct into the sub-duct, the inner wall of the sub-duct should ideally have a corrugated pattern. It is important not to pull the sub-duct out of a flat reel to prevent twisting.

4. Micro Sub-Duct:

The function of the sub-duct in the network is to guide the fiber cable while avoiding splicing. To ensure the fiber cable and the sub-duct bundle are blown in smoothly, the sub-duct must withstand the required internal and external pressure (with the sub-duct bundle being pressurized during blowing to prevent implosion). The sub-duct is made from HDPE and must be circular, maintaining uniform dimensions throughout its length. Its inner and outer walls must be free from cracks, pinholes, joints, water stains, mold marks, decorative elements, or other defects. The outer wall of the sub-duct should not have any lubricant or other contaminants. To reduce friction when blowing the sub-duct into the sub-duct, the inner wall should ideally have a circular cross-sectional area.

The number of sub-ducts that can be accommodated within a mother duct (primarily determined by mechanical protection requirements) is such that the total area of the sub-ducts (calculated based on their outer diameters) should not exceed half of the area of the mother duct.

5. Blowing Process:

During the installation of the sub-duct bundle and micro fiber cable, a cable blowing machine is used. This machine features symmetric tracks that stack and clamp the sub-duct bundle; each end of the track has a guide mold and drive wheel. Once the sub-duct bundle passes through the guide mold, it forms the desired shape. The sub-duct bundle is then inserted while being blown into the inner wall of the mother duct, and simultaneously, the micro fiber cable is inserted and blown into the sub-duct bundle. The blowing air pressure is maintained at 5 to 10 atmospheres. In other words, during the blowing process (insertion plus blowing) of the sub-duct bundle, the cable blowing machine precisely stacks and clamps the sub-duct bundle with guide molds on both ends. The sub-duct bundle is then fixed on a specially designed multi-head cable dispensing rack. Before blowing, the mother duct is lubricated using a spray lubricant (as per the manual). The sub-duct bundle must be pressurized to the level required for fiber blowing, which helps prevent implosion. After installing air valves on both ends of the sub-duct, the pressure will automatically increase. To prevent the lubricant from seeping into the sub-duct, the sub-duct can only be blown into the mother duct after it is fully pressurized (after the connection to the air compressor has been completed). Alternatively, other methods can be used to pressurize the sub-duct. A specialized pneumatic device is provided for this purpose, and a pressure gauge can be used on the other end of the sub-duct to monitor the pressure. Once the sub-duct is installed, both ends are sealed with waterproof caps.

6. Installation Case Examples:

In one instance, 10 sub-duct bundles with diameters of 7/5.5mm and 5 bundles of 10/8mm were installed within mother ducts of 40/33mm. The mother duct had additional space equal to half its width remaining.

l In a 32/26mm mother duct, 7 sub-ducts of 7/5.5mm or 3 sub-ducts of 10/8mm can be accommodated.

l In a 50/40mm mother duct, 14 sub-ducts of 7/5.5mm or 7 sub-ducts of 10/8mm can be accommodated.

l In a 63/50mm mother duct, 20 sub-ducts of 7/5.5mm or 10 sub-ducts of 10/8mm can be accommodated.

l In a 25/19mm mother duct, 2 sub-ducts of 7/5.5mm or 1 sub-duct of 10/8mm can be accommodated.

l A Y-shaped branch connector can branch two sub-ducts simultaneously.

7. Common Techniques for Branching and Connecting:

In a network composed of sub-ducts and fiber cables, branches, joints, and terminations need to be established. Various components are required. The fiber cable can branch out from the sub-duct bundle without splicing. Simply cut the mother duct and the required sub-duct, and connect the cut sub-duct to the branch sub-duct using fixed or push-pull connectors, then use a disassemblable branch connector to join the mother duct to the branch mother duct. This enables branching at any location.

For direct connections through the mother duct, a threaded connector is sufficient. Cable distribution boxes and junction boxes are used to complete terminations or fiber splitting. After the sub-duct and fiber cable are introduced into the distribution box, they are sealed with airtight and waterproof seals.

8. Sub-Duct Connectors:

The purpose of sub-duct connectors is to extend or repair sub-ducts to facilitate fiber blowing. To ensure that fiber blowing does not cause blockages, sub-duct connectors must be airtight and pressure-resistant, with smooth and uniform inner diameters. There are two types: fixed and push-pull connectors.

9. Mother Duct Connectors:

The purpose of mother duct connectors is to extend or repair the mother duct to facilitate the blowing of the sub-duct bundle. To prevent the sub-duct bundle from being caught during blowing, the mother duct connectors must also be airtight and pressure-resistant, with smooth and consistent inner diameters. The connector must maintain the correct cut angle at the mother duct joint to ensure the sub-duct does not get stuck.

10. Disassemblable Mother Duct Repair Devices:

In some cases, both the mother duct and the sub-duct need to be connected. For example, to repair a section of a mother duct that has already been installed with sub-duct bundles and fiber cables, a large disassemblable mother duct repair device is used. This device includes two half-duct clamps and a disassemblable connector, providing ample space to accommodate sub-duct connectors.

11. Y-Shaped Branch Connectors:

The role of Y-shaped branch connectors is to restore the mother duct after branching and provide mechanical protection. The increased branching angle allows for a sufficient bending radius for the inserted sub-duct, ensuring that fiber blowing is not compromised. The cut end of the sub-duct must be sealed with a cap. Since the branch connector does not undergo further fiber blowing, its requirements for smoothness and pressure resistance are less stringent, but it must prevent the ingress of mud. A rubber pad at the end of the connector is often used to prevent water ingress.

12. Airtight and Waterproof Seals:

Seals for fiber cables, sub-ducts, and mother ducts are critical to prevent water ingress at manholes, local nodes, and user endpoints. If gas pipelines are nearby, airtight seals must be used to minimize risks in case of gas leaks. In such cases, both the sub-duct (regardless of whether it contains fiber) and the mother duct (regardless of whether it contains a sub-duct bundle) must be sealed with these seals. These seals must be both airtight and waterproof.

13. Terminal Boxes:

Terminal boxes are usually located at the local node and user end (user fiber cable box) and are primarily used for introducing fiber cables from the outdoors into indoor environments, while preventing water or gas ingress. They may also serve to house junction boxes or fiber cable connectors. Sub-ducts can be connected with indoor sub-ducts (such as lighting sub-ducts). Typically, at the point where the fiber cable enters indoors, an airtight seal is applied, and the fiber cable is wound around a tension release device to prevent breakage. Tension release devices can also provide grounding for fiber cables, especially for steel-pipe structures where a grounding loop is necessary at both ends (preferably at the local node) to avoid ground loops. If there are junctions in the network, grounding must be implemented at both the local and user ends.

14. Distribution Holes:

Distribution holes are used to house and protect produced fiber cables and junction boxes, facilitating operations by construction personnel. Distribution holes can accommodate a certain number of mother ducts, and they must be designed to prevent mud ingress. For ease of future access without interrupting already activated fibers, the distribution hole must support repeated access, eliminating the need to cut the sub-duct or fiber cable.

15. Junction Boxes:

Junction boxes serve to house fiber cable spools and protect the inserted fiber cable (adapter sub-duct) from water ingress. Sometimes, junction boxes also contain fiber splitters. Both the junction box and the fiber cable entry must be airtight and waterproof. Junction boxes are typically installed in pre-designed network nodes or at fiber cable joint points. When repairing fiber cables (replacing an entire segment), the fiber cable connectors can be fixed to the junction box at any time. Additionally, if fiber cable connectors are routed through a junction box, they should be sealed to prevent water ingress. If the fiber cable structure is a steel-pipe type, a protective cover (such as a showerhead) must be used on the cut fiber connector to protect the fiber. The fiber spool remaining on the spool should be left intact.

16. Fiber Spools:

The purpose of fiber spools is to secure the connectors and leave spare or tail fibers. According to their function in the network, fiber cables must be managed as either a single loop or a single unit. Fiber spools are placed in the terminal box at the local node or user end, or in distribution holes with junction boxes.

17. Fiber Cables:

Fiber cables are a critical component in the access network, responsible for transmitting information. They must have a minimum service life of 20 years during which their performance must remain undamaged. As fiber cables are installed and used outdoors, they must be resistant to external environmental influences. In a 7/5.5mm sub-duct, between 4 to 24 strands of fiber cable can be blown in; in a 10/8mm sub-duct, fiber cables of 48, 60, or 72 strands (or other lower strand counts) can be blown in. For steel-pipe structured fiber cables, the center is a seamless-welded waterproof steel pipe with fiber cables filled with a water electrolyte, while a layered HDPE jacket is applied externally. Seamless welded steel pipes prevent water or other substances from entering the fiber. All-dielectric structured fiber cables, which are non-metallic, prevent dielectric bridging; the water electrolyte in the center serves as a waterproofing measure. Non-metallic fiber cables are made from polyester, with an inner tube stronger than ordinary loose-tube cables for fiber protection. The inner tube is jacketed with HDPE and reinforced. As the sub-duct is made of HDPE, the HDPE jacket of the fiber cable is fully compatible with the micro-duct and experiences minimal friction during the blowing installation process.

Fiber Cable Installation:

During the process of fiber cable blowing (insertion plus blowing), a cable blowing machine is used to adjust the lateral pressure of the drive wheel to an appropriate level to eliminate the minimum sliding distance. Since the sub-duct already has a lubrication layer, no additional lubrication is needed during blowing. The cable joint must be tightened with a compact, smooth copper or steel nut to prevent the fiber cable from getting blocked in the sub-duct. A straightening rack is used ahead of the cable blowing machine to improve the blowing effect.

One clear method to extend the intermittent blowing distance without splicing is as follows:

1. Second Cascading Blowing Method:

At the termination of the first segment of the sub-duct and the beginning of the next segment, a second cable blowing machine is installed. This machine can be placed in a distribution hole or a trench. The drive wheel, made of a special metal material with teeth to prevent slippage (with the first segment of the cable coated with lubricant), is used. The cable blowing machine employs a disassemblable sealing ring; the section of the cable wound around it is non-disassemblable and will be cut off after installation. The cable blowing machine is then connected, and the sub-duct is fitted with a fixed connector. Once the cable installation is complete, the cable blowing machine is removed, the cable’s outer winding is cut off, and the sub-duct is connected. In the cascaded blowing method, multiple cable blowing machines can be used, and the above procedure repeated. In this way, the overall blowing distance can be extended indefinitely without splicing. In fact, the final axial cable blowing distance is the maximum reel length of the cable, estimated at 6 kilometers.

2. Intermediate Point Cascade Blowing Method:

At an intermediate point of the sub-duct, a reel of fiber cable is blown into the sub-duct as a relay. The cable blowing machine is then removed from the sub-duct, and the cable already blown is wound onto an “O-type reel” device. Using the cable blowing machine, a suitable length of cable is rewound from the cable reel onto the O-type reel, then the cable is cut, the machine’s direction is reversed, and the cable wound on the O-type reel is blown into the sub-duct on the other side. Note that the cable blowing machine must use a disassemblable sealing ring, and the sub-duct must be fitted with a fixed connector. This method is similar to the first method. Under non-splicing conditions, the above operation can only be repeated once, effectively doubling the overall cable blowing distance.

3. First Buffer Cascading Blowing Method:

At the termination of the first segment of the sub-duct and the beginning of the second segment, an empty cable reel is placed. A suitable length of fiber cable is blown into the sub-duct from the start of the segment, and the blown cable is wound on the empty reel. Then, the portion of the cable between the already blown cable and the second cable reel is wound on an O-type reel. The fiber cable on the second reel is then completely rewound onto the O-type reel using the cable blowing machine. The machine is then rotated to blow the cable into the second sub-duct. Again, be sure not to forget the disassemblable sealing ring and sub-duct connector. This operation can be repeated along the entire route without splicing, thereby extending the overall cable blowing distance indefinitely. Compared to the first method, this method’s advantage is that it requires only one cable blowing machine and one O-type reel, with the key benefit of blowing a long distance for each segment, especially the first segment.

Implementation Details:

1. Sub-Ducts:

The micro sub-ducts used in the process have diameters ranging from 7 to 10 millimeters. Their inner walls have a corrugated texture and are made of high-density polyethylene (HDPE). The sub-ducts must be circular and maintain consistent dimensions along their entire length. Their inner and outer walls must be free of cracks, pinholes, joints, water stains, mold marks, repairs, or any decorative elements. The outer wall of the sub-duct should not have any residual lubricant or contaminants. To reduce friction when blowing the sub-duct into the sub-duct, the inner wall should ideally be uniformly circular.

2. Fiber Cables:

The micro fiber cables have diameters ranging from 3.9 to 5.9 millimeters. Their inner walls are made of stainless steel or non-polyester materials, and the outer jacket is made of HDPE. For micro fiber cables with approximately 72 cores, when installed in a 7/5.5mm sub-duct, 4 to 24 strands can be blown in. In a 10/8mm sub-duct, fiber cables with 48, 60, or 72 cores (or other lower core counts) can be blown in. In steel-pipe fiber cables, the center is a seamless-welded waterproof steel pipe with the fiber and water electrolyte filling, and an external layered HDPE jacket is applied. Seamless welded steel pipes prevent water or other substances from entering the fiber. All-dielectric fiber cables, which are non-metallic, prevent dielectric bridging, and the water electrolyte in the center provides waterproofing. Non-metallic fiber cables are made from polyester, with an inner tube that is stronger than ordinary loose-tube cables and provides protection for the fiber. The inner tube is jacketed with HDPE and reinforced. Since the sub-ducts are made of HDPE, the HDPE jacket of the fiber cable is fully compatible with the micro-duct, resulting in minimal friction during the blowing installation process.

3. Cable Installation Process:

During the fiber blowing (insertion plus blowing) process, the cable blowing machine adjusts the lateral pressure of the drive wheel to an appropriate level to eliminate any minimum sliding distance. Because the sub-duct already has a lubrication layer, additional lubrication is not required during blowing. The cable joints must be tightly fastened with a compact, smooth copper or steel nut to prevent the fiber cable from getting stuck in the sub-duct. A straightening rack is used ahead of the cable blowing machine to enhance the blowing effect.

4. Extending Blowing Distance:

One effective method to extend the intermittent blowing distance without splicing involves using a series of cable blowing machines in cascade (second cascading method) or employing a buffer system (first buffer cascading method) to double the overall cable blowing distance. These methods ensure the fiber cable can be installed continuously over long distances—up to a maximum calculated length of 6 kilometers.

Conclusion:

The described methods enable a highly efficient, continuous installation of fiber cables in microduct networks. By using cable blowing technology, the micro sub-duct bundles and fiber cables can be installed without splicing, reducing installation time, lowering costs, and minimizing network disruptions. The process leverages advanced pneumatic devices, precise alignment techniques, and robust materials to ensure reliable network performance over extensive distances.

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