Underground Fiber Optic Cable Installation

Posted on Oct 13, 2014 by

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Underground Fiber Optic Cable Installation

The practice of handling fiber optic cables has become much more common in recent years. Fiber optic cables are designed to withstand all typical installation and environmental stresses expected in the specific application. But it can still be damaged if not handled properly during the installation process. The two most common outdoor fiber optic cable installations are pole line aerial installation and underground cable installation. And the latter is what we are going to talk about in this tutorial.

Underground cable installation can be buried directly underground or placed into a buried duct. The cables are plowed in or buried in a trench when buried directly and the installation process can be very quick. The most common cables used for direct burial are steel armored outdoor fiber cables. While underground duct installation can protect the cables from harsh environment and provides opportunity for future expansion without the need to dig. And this is the most common practice in many areas nowadays. Another benefit is that fiber cables without armor can be used which makes the installation even easier.

Preparations and Precautions Before Underground Cable Installation

To ensure a successful job, some preparation steps are needed before the installation process.

Obtain proper right-of-way permits. Identify existing underground utilities such as buried cables, pipes. Investigate the soil condition in order to determine the installation depth, the type of fiber cable should be used, and the plowing equipment needed.

Except for the above preparation work, all personnel must be thoroughly familiar with the following general precautions when handing fiber optic cable.

DO NOT exceed the cable’s stated maximum pulling tension. DO NOT exceed the cable’s stated minimum bending radius. DO NOT exceed the cable’s maximum crush load. DO NOT use detergent or petroleum based compounds as a cable lubricant. NEVER set a cable reel on a flange side (to prevent cable crossings during payoff).

general precautions

Steps for Cable Placement

Methods used for placing fiber optic cables in ducts are essentially the same as those used for placing copper cables. However, fiber optic cable is a high capacity transmission medium which can have its transmission characteristics degraded when subjected to excessive pulling force, sharp bends, and crushing forces. These losses may not be revealed until long after installation is complete. For these reasons extra care must be taken during the entire installation procedure.

First, identify the innerduct in which the fiber optic cable is to be placed. After the correct innerduct has been identified, it should be tied off to keep it from "creeping" as the cable is being pulled into it. Any spare ducts should be capped off so that they do not interfere with the cable pulling operation.

cable tie-off manhole

Prepare the pull-through manholes. This includes un-racking the innerduct and removing slack caused by the racking, placing lubricant where appropriate, preparing the pulling line and usually re-coupling the innerduct to provide a continuous path for the cable to follow. The amount of lubricant used in intermediate manholes will depend on the length between manholes, type of innerduct, etc.

Position the pulling equipment (winch or capstan) at the pulling manhole. The pulling equipment should be fitted with a tension monitor and is to be operated at the manufacturer’s recommendations. Never exceed the 600 pound pulling limit of the cable.

pull manhole setup

At the pull end manhole, install the proper guides specified by your company's practices. These guides are to ensure that the pull line and fiber optic cable enter and exit the innerduct in a straight path. And intermediate manholes should be prepared for the cable pull by having any problems that were observed during the pre-pull survey already sorted out.

Position the cable reel adjacent to the feed manhole so that the cable can be hand-fed in the manhole. The cable should be pulled off the reel by hand and manually fed into the manhole to reduce pulling tensions. Then Connect the pulling line to the pulling eye/grip installed on the fiber optic cable with a swivel connector.

reel and setup

Use an approved cable lubricant to lubricate the entire duct run in order to reduce the pulling tension. Apply the lubricant to the cable before it is fed into the innerduct according to standard company practices. The method of lubricant application will vary according to company practice. And before the pulling operations begin, a communications link must be established between the feed and pull manholes (and any intermediate manholes the cable may pass through).

Start the pull by engaging the winch/capstan at a slow speed. Hand turn the reel as the pull begins to decrease start-up tension. After the pulling eye/grip has entered the duct at the feed manhole, the speed of pull may be increased. The speed should be slowly built up to a maximum speed of approximately 100 feet per minute (30 meters per minute).

The winch/capstan operator at the pull manhole controls the speed of the cable pull. He must be kept informed of the cable’s progress as it passes through each intermediate manhole. A constant pull rate is the desired method of placing cable in innerduct. Variations in pulling speeds, starts and stops are to be avoided. If it is necessary to stop the pull at any point, the winch/capstan operator should stop the pull but not release the tension on the cable. Pulls are more easily resumed if tension is maintained on the pull-line and cable.

Once the cable appears in the pull manhole, it may be pulled over a sheave or quadrant block as long as the diameter of the sheave or block meets the cable’s minimum bend radius under tension. No attempt should be made to inch the cable to its final manhole length. This may cause undesirable surges to the end portion of the cable.

Post-installation Inspection

The final thing in completing a underground cable installation is a thorough inspection of the entire route from start to finish. Engineering personnel and involved parties should inspect the construction area above ground to ensure the following:

Restoration has been accomplished. Permanent markers have been installed immediately beside the cables. Road bores, if used, are properly completed and will not collapse a portion of the road. Debris and trash have been removed from the site. Other instructions specific to the installation have been completed to the drawing’s specifications.

Summary

In fact, the process of cable installation is the most aggressive event that the cable will most likely ever be exposed to, and only those specially trained people can do this job. Even for those professionals, they cannot guarantee the whole process is smooth without any problem. But adhering to the above steps and precautions can help to maximize the chance that the cable will perform properly throughout its full design lifetime.