Underground Cable Installation and Conduit Design

Technical Terminology

Conduit: In the context of cable installation, a conduit refers to a tube or pipe used to protect and route electrical wires or cables. It shields the cables from external elements and provides a safe passage for them, commonly placed underground or within buildings. 

Coefficient of Friction: This measures the force resisting the motion between two surfaces in contact. A lower coefficient of friction between cables and conduits reduces tension and sidewall pressure during pulls.

Friction Reduction and Lubricants: Lubricants, such as Polywater, are specially designed substances that minimize friction between cables and the walls of the conduit during installation. Lowering friction allows for smoother cable pulls, reducing tension, and enabling longer and safer cable installations. Polywater is a type of lubricant used to reduce friction during cable installation. It’s notable for minimizing residue within the conduit, aiding in cable removal, and ensuring smoother pulls over longer distances. 

Vaults: Underground chambers or structures used for accessing and maintaining cables or electrical equipment in an underground system. 

Pulling: The process of drawing cables through conduits during installation, which necessitates reducing friction to prevent damage to cables and conduit walls. 

Splices: Joints or connections made between two cable segments to facilitate continuous cable runs. Minimizing splices enhances the reliability of the cable system. 

Polywater for Longer Cable Pulls  

The drive towards undergrounding cables is tied to the pursuit of reliability, safety, and aesthetic appeal. Underground installations offer protection against weather elements, reducing outages caused by storms and other environmental factors. Yet, the challenges lie in the complexities of installing and maintaining these underground systems. The technology that enables longer cable pulls plays a pivotal role in overcoming these challenges. 

Traditionally, shorter cable pulls necessitated frequent splices and the installation of vaults or manholes at regular intervals. These splices and infrastructure additions not only increase the cost but also present potential points of failure in the system. However, by harnessing advancements in lubrication, materials, and installation techniques, engineers can now extend cable pulling lengths significantly. This means fewer splices and reduced reliance on frequent manholes or vaults, streamlining the undergrounding process.

The ability to pull cables over longer distances effectively eliminates the need for frequent interruptions, reducing construction disruptions and costs in densely populated areas. Moreover, longer cable pulls minimize the need for additional infrastructure, alleviating the complexities associated with obtaining permits and navigating congested urban spaces. This not only enhances the reliability and durability of the system but also contributes to cost efficiency and a more seamless integration of underground infrastructure within urban landscapes. 

The process of installing underground cables involves a complex interplay of materials, forces, and design considerations. At the heart of this operation lies the significance of reducing frictional forces between the cables and the conduit walls. This reduction in friction is paramount for ensuring longer, safer cable pulls. Lubricants are the unsung heroes in this scenario, engineered specifically to mitigate frictional forces. They facilitate smoother cable pulls by minimizing tension, thereby elongating the cable pull length and reducing the need for splicing.   

Traditional lubricants, such as Bentonite clays mixed with water and salt were commonly employed in cable installation processes. These lubricants often led to shorter cable pull lengths due to their higher frictional characteristics. The increased friction necessitates more frequent splices, thereby limiting the distance over which cables can be successfully pulled. Additionally, these older lubricants tend to leave a substantial residue within the conduit systems—often exceeding 20% of the lubricant volume. This residue, when left behind, can solidify and cement the cables within the conduits, posing significant challenges during maintenance, repairs, or future cable upgrades. Removal of such high-residue lubricants becomes a laborious and time-consuming task, requiring extensive effort and specialized equipment to free the cables for any necessary alterations or replacements. 

In contrast, modern lubricants like Polywater have revolutionized the installation process by leaving significantly less residue, usually less than 3% of the lubricant volume within the conduit system. This reduced residue minimizes the risk of cementing the cables in place, facilitating easier cable removal or reinstallation in the future. Polywater’s lower residue not only simplifies maintenance tasks but also reduces downtime and labor costs associated with any necessary modifications or upgrades to the cable system. Therefore, choosing lubricants with minimal residue, such as Polywater, not only ensures smoother cable pulls during installation but also mitigates potential hurdles in future maintenance and system upgrade 

Practical Tips and Tools for Underground Cable Installation 

Cable Pushers/Feeders: These tools, equipped with rubberized rollers, serve a dual purpose in underground cable installation. By effectively guiding cables through conduits, they not only reduce back tension but also prevent potential damage to both the cables and the conduit walls. The rubberized rollers ensure a smoother feeding process, minimizing friction and abrasion, which is crucial for maintaining the integrity of the cables and the conduit surfaces. Their design specifically caters to reducing stress on the cables, ensuring a more secure and damage-free installation. 

Pulling Tape vs. Polylines: The choice between pulling tape and polylines significantly impacts the cable installation process. Pulling tape’s flat and flexible nature reduces rope burn through, preserving the cable’s integrity during pulls. Moreover, when measuring the ending tension on cable pulls, selecting a tugger that utilizes at least twice the predicted tension proves more effective. This selection not only prevents maxing out the equipment but also ensures a safer margin, allowing the cable pull to operate well within its limits, reducing the risk of damage or overexertion. 

Pulling Eyes vs. Grips: Opting for pulling eyes over grips, especially when lengthening cable pulls, is a strategic choice. Pulling eyes are engineered to handle higher ending tensions, effectively reducing friction and potential rub within the conduit. This choice contributes significantly to a smoother installation process by minimizing stress on the cables and ensuring a more controlled and damage-free pull. Their ability to manage increased tension makes them an ideal choice for longer cable pulls, promoting a more efficient and reliable installation process.  

Coefficient of Friction Testing Methods 

The efficiency of cable installation heavily relies on accurate assessments of friction between cables and conduit surfaces. Several rigorous testing methodologies have been developed to measure these frictional forces, ensuring optimal cable pulls and system longevity. 

Friction Tables: Cable lubricant manufacturers utilize comprehensive friction tables to analyze tensions between various cable and conduit combinations. These tables are generated through extensive testing, measuring pulling force against constant conduit movement speed. They provide kinetic coefficients of friction (μk) that guide engineers in selecting suitable lubricants. Lubricants typically reduce friction by at least 50%, enhancing cable pulls and minimizing wear. The friction table equation involves the ratio of pulling force to normal force.

Telcordia Reel Test: This method involves wrapping a continuous duct around a drum to create a bend of approximately 420 degrees. As the cable moves through this simulated configuration, dynamic tension values are recorded. The test measures the cable’s response to bending, providing critical data, especially for lighter-weight communication cables. The formula used to calculate kinetic coefficients of friction (μk) in this test involves analyzing the logarithmic ratio of measured pulling tension (Tout) to measured back tension (Tin) relative to the total angle of the duct (Θ): μk = (ln(Tout/Tin) / Θ). 

Multi-Bend Device Test: This test subjects cables to a series of bends, simulating real-world installation conditions and rapidly increasing tension. Pneumatic pressure breaks and pulling winches are employed to measure back tension, providing essential insights into cable behavior. The formula used to calculate the kinetic coefficient of friction (μk) in this scenario involves factors such as the number of bends (N), measured pulling tension (Tout), and measured back tension (Tin): μk = (2 / nπ) ln(Tout/Tin).