Underground (Direct-buried) Fiber optic 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：

1. Planning and Design: a. Conduct a thorough survey of the installation area to determine the route, including any existing utility lines or obstacles. b. Create a detailed design plan that includes the cable path, splice points, and any necessary equipment or tools.

2. Obtain Permissions and Permits: a. Check with local authorities and obtain any necessary permits or permissions required for underground cable installations. b. Coordinate with relevant utility companies to avoid interference or conflicts with existing infrastructure.

3. Safety Measures: a. Prioritize safety by ensuring that all personnel involved in the installation are properly trained and equipped with personal protective equipment (PPE). b. Establish safety protocols, such as using warning signs, barriers, or other means to prevent accidental damage or injury.

4. Cable Protection: a. Choose appropriate cable types for underground installations, such as armored or direct-buried cables, which are designed to withstand the conditions and potential hazards of underground environments. b. Implement proper cable protection measures, such as using conduit, ducts, or trenches, to shield the cables from external damage.

5. Grounding and Bonding: a. Ensure proper grounding and bonding techniques are used to protect the cable and equipment from electrical surges or lightning strikes. b. Follow industry standards and guidelines for grounding and bonding practices.

6. Cable Placement: a. Carefully dig trenches or lay conduit according to the planned cable path, taking care to avoid existing utilities or sensitive areas. b. Avoid sharp bends or excessive tension when laying the cables to prevent signal loss or cable damage.

7. Cable Splicing and Testing: a. Properly splice the fiber optic cables using appropriate techniques and tools. b. Conduct thorough testing, such as optical time-domain reflectometer (OTDR) testing, to ensure proper cable continuity and signal integrity.

8. Documentation and Mapping: a. Maintain accurate documentation of the installation process, including cable paths, splice points, and any deviations from the original plan. b. Create detailed maps or diagrams of the installed cables to assist with future maintenance or repair activities.

By following these preparations and precautions, you can help ensure a successful underground fiber optic cable installation with minimized risks of damage or signal loss. It is also recommended to consult with industry experts or professionals for specific guidance and to adhere to local regulations and guidelines.

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.

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.

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.

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.