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Fiber Optic Technology - Part Three - Fiber Installation

In part three of our series, we cover the general guidelines used for installing fiber optic cable. 

In general, fiber optic cable can be installed with many of the same techniques used with conventional copper cables - we list the basic guidelines that can be applied to any type of cable installation below.

  • Conduct a thorough site survey prior to cable placement
  • Develop a cable pulling plan
  • Follow proper procedures
  • Do not exceed cable minimum bend radius
  • Do not exceed cable maximum recommended load
  • Leave extra cable
  • Document the installation

Conduct a thorough site survey prior to cable placement

The purpose of a site survey is to recognize circumstances or locations in need of special attention. For example, physical hazards such as high temperatures or operating machinery should be noted and the cable route planned accordingly. If the fiber optic cable has metallic components, it should be kept clear of power cables. Additionally, local building code regulations must be considered. If there are questions regarding local building codes or regulations, they should be addressed to the authority having jurisdiction, such as the appropriate building inspector.

The site survey should focus on determining the following factors:
​The most appropriate route for each cable. This could be with regard to the existing cable runs or in respect of the proposed newly installed cable housings. It will generally be more cost effective to use existing infrastructure, but the decision depends on the amount of space in the existing housing and on the condition of the housing. Also, an existing cabling route may take a longer path and the extra cabling costs associated with this may exceed the costs of installing a new route. ​The condition of existing cable housings. (Whether costly maintenance is required before new cables are installed.)

Is there any potential danger that could cause damage to the cable because of the poor condition of housings? 

For example, are the housings prone to be affected by flooding?
​Whether there are any locations that need special attention. Should tradesmen with special skills be deployed to carry out the job?

Are there any locations that could subject cables to extreme temperatures?

If so, provision is necessary to use fire or explosion-proof cables.
​The need to run the cables in cable trays, underground, in roof tops or as aerial cables. ​Are there locations that could subject cables to possible physical damage? If so, provision is necessary to provide appropriate steel armored cables. ​Would the cable route run near high power cables? If so, ensure that the fiber optic cable does not contain any metal (strengthening member or sheath).
​Would the cable route run near areas of high transient voltages (for example, lightning)? If so, ensure the fiber optic cable does not contain any metal.​Ensure that the installation adheres to all existing electrical and fire codes of the country to which the installation is planned.​Obtain all required council and government permits before commencing any civil works on public land.
​Will there be sufficient room to use the cable pulling equipment?

If not, what equipment needs to be moved to carry out the installation without hindrance.
​Will cars or trucks be driving over the cable, people walking over it or heavy objects laid across it? If so, plan to take the necessary precautions to protect the cable (for example, conduits) and/or to use the correct sheathed cables.​Locate all the intermediate points from where the cable is to be pulled and where junction boxes are to be located.
​Identify appropriate locations for installing termination cabinets and splicing trays.Determine the exact locations for each data equipment hub.​Talk to local employees to determine if there are any foreseeable problems that may arise during the installation that can be averted now by careful planning.
All these particulars should be carefully noted during the site survey and then officially and completely documented after the survey is done. These findings would be useful while designing the cable system.

Develop a cable pulling plan

​A cable pulling plan should communicate the considerations noted during the site survey to the installation team. This includes the logistics of cable let-off/pulling equipment, the location of intermediate access points, splice locations and the specific responsibilities of each member of the installation team.

Follow proper procedures

​Because fibers are sensitive to moisture, the cable end should be covered with an end cap, heavy tape or equivalent at all times. The let-off reel must never be left unattended during a pull because excess or difficult pulls, center-pull or back feeding techniques may be employed.

Do not exceed cable minimum bend radius

​Every fiber cable has an installation minimum bend radius value. During cable placement it is important that the cable not be bent to a smaller radius. After the cable has been installed, and the pulling tension removed, the cable may be bent to a radius no smaller than the long term application bend radius specification. The minimum bend radii values still apply if the cable is bent more than 90 degrees. It is permissible for fiber optic cable to be wrapped or coiled as long as the minimum bend radius constraints are not violated.

Do not exceed cable maximum recommended load

​While fiber optic cables are typically stronger than copper cables, it is still important that the cable maximum pulling tension not be exceeded during any phase of cable installation. In general, most cables designed for outdoor use have a strength rating of at least 600 lbs. Most fiber optic cables also have a maximum recommended load value for long term application. After cable placement is complete, the residual tension on the cable should be less than this value. For vertical installations, it is recommended that the cable be clamped at frequent intervals to prevent the cable weight from exceeding the maximum recommended long term load. The clamping intervals should be sufficient to prevent cable movement as well as to provide weight support.

Leave extra cable

​A common practice is to leave extra cable at the beginning and at the end of the cable run. Also, extra cable should be placed at strategic points such as junction boxes, splice cases and cable vaults. Extra cable is useful should cable repair or mid-span entry be required.

Document the installation

​Good record keeping is essential. This will help to ensure that the cable plant is installed correctly and that future trouble shooting and upgrading will be simplified. DINTEK Fiber Optic Cables have a unique lot number shown on the shipping spool. It is important that this number be recorded. Cable pre and post-installation test data should be recorded in an orderly and logical fashion.

​In order to effectively pull cable without damaging the fiber, the following guidelines should always be adhered to:

  • Properly assess cable for pulling first

  • Do not attach connector prior to pulling

  • Prevent moisture getting in

  • Keep spacing in duct to below 53%
Properly assess cable for pulling first 

​Fiber optic cable has special strength members - usually aramid yarn, for pulling. Fiber optic cable should only be pulled by these strength members unless the cable design allows pulling by the jacket. Any other method may put stress on the fibers and harm them.

Do not attach connector prior to pulling

​As a general rule, it is best to install cable prior to connector attachment. After connectors have been attached, it becomes more difficult to protect the fiber from inadvertent stress. If a pull is to be made entirely in one direction, connectors may be pre-installed on one end, leaving the other end for pulling. If the cable must be installed with connectors attached, every practical means must be taken to protect the connectorized end from damage or stress. Cushioned enclosures should be used to protect connectors during pulling.

Prevent moisture getting in

The leading end of the cable should be sealed to prevent intrusion of water or other foreign material while pulling.

Keep spacing in duct to below 53%

​For ease of cable installation, the area of the cable divided by the area of the duct or conduit should be less than 53% per a single cable. Permissible area to be occupied for two cables is 31%, for three or more cables it is 40%.

​Underground Conduits: We recommend that underground conduits, if newly installed, should be a minimum size of between 40mm to 50mm or 1.5" to 2". If the run is long, or if you anticipate the possibility of additional future pulls, then you may want to install a conduit of up to 100mm (or greater). Once the trench is dug, the installation of the conduit is comparably easy and inexpensive. You may also want to consider installing additional conduits for future use.

Conduit Installations General Information:

Try to design the conduit run with as few bends as possible. If there are too many bends in the run then you may consider installing junction boxes in lieu of bends.
Note: There is some controversy on installing junction boxes in lieu of bends.
Use plastic bushings on conduit ends to avoid damage to the cable during the pull.

​In Buildings: It is not necessary to pull fiber optic cable in innerduct. Innerduct is strictly a personal preference. The following instruction takes into consideration that you will pull the fiber optic cable directly into the ceiling or other space. Plan for straight pulls only (point A to point B). Diagonal pulling across an area will be OK but it would make for a neater and more professional installation to install the cable at an angle that is parallel to a wall. Do not attempt to pull the cable around a corner, even if you plan on having a person feeding it from hand to hand around the corner. Instead, pull all of the cable out (at the turn) and lay it on the floor in a figure 8 pattern.

Rope Size: It is important to use a rope size that gives minimal stretching during the pull. Stretching of the rope is undesirable for several reasons including that it makes for a very unstable pull, and takes away control from those doing the pulling. This is why we do not recommend using 200 lb. pull string (aka: jetline) to pull the cable. We recommend a heavier rope that is anywhere from a 1/4" to 1/2" in thickness. The thickness of the rope should increase with greater pull lengths. Have the pull rope installed before the cable pulling crew arrives for maximum efficiency.

​Communication between the person feeding and the person pulling the cable is absolutely essential. If the person feeding runs into a snag then the puller must stop immediately to avoid damage to the cable. Walkie-talkies would come in handy for this purpose. Use of great pulling force is not recommended and will likely damage the terminations and/or the cable.

​Use a generous amount of cable pulling lubricant on the entire run, especially on the leader (pulling eye & mesh). The person may stop the cable pull from time to time to prepare and apply more lubricant. Use only lubricant that is expressly designed for cable pulling. When working in freezing temperatures, use a lubricant that is designed not to freeze.

​Do make every effort to pull cables from a conduit in as straight an angle as possible. Pulling on an angle can cause damage to the cable.

How to Handle Slack: When pulling cable out from a conduit or ceiling, it will become necessary to coil up the cable in preparation to pull it in another direction. Fiber optic cable has a "memory". That is to say that it holds its contour that it formed while being on a reel. Do not try to re-coil the cable in a circular pattern; instead use a figure 8 pattern as shown in the illustration. When you are finished pulling out all of the slack, turn the figure 8 cable 360° (upside down) and proceed to setup to pull it in the other direction.

Removing the Pull Eye: Use extreme caution when removing the pull eye. Do not use a blade to slit the mesh. Instead, use a pair of electricians scissors or diagonal cutters. Work your way from the back of the cable toward the pull eye, while lifting the mesh upward, away from the cable.

Follow Building Codes: Always obey all local, and national, fire and building codes. Be sure to "firestop" all cables that penetrate a firewall. Use plenum rated cable where it is mandated, etc.

Pre-Terminated Fiber Optic Cable Assemblies: These factory pre-terminated fiber-optic cable assemblies (aka: "Ready Pull") have received rave reviews from our customers. The assemblies can be ordered in either indoor (plenum) or outdoor versions, and in 4, 6, 12 or 24 strand fiber counts, and in multimode or singlemode. A pulling eye can be factory installed on either end or on both ends of the cable. The pulling eye (and associated cable netting) will protect the pre-terminated ends during the pull. This product is a great time and money saver for those who occasionally install fiber optic cabling.

Pulling Procedures 
Direct Attachment 

With direct attachment, cable strength material is tied directly to the pulling fixture. Conventional cable tools may be used. Loose fiberglass threads are not suitable for direct attachment because they may break if knotted.

​Fiberglass epoxy rods are too rigid to tie, but may be secured to the pulling fixture by using tight clamping plates or screws. Swivel pulling eyes should be used to attach the pulling rope or tape to the cable to prevent cable twisting during the pull. Cables should not be pulled by the jacket unless it is specifically approved by the cable manufacturers and an approved cable grip is used. These grips are usually tied to the strength members also. Tight buffer cable can be pulled by the jacket in premises applications if a large (~40 cm, 8 in.) spool is used as a pulling mandrel. Wrap the cable around the spool five times and hold gently when pulling.

Do not exceed the maximum pulling tension rating. Consult the cable manufacturer and suppliers of conduit, innerduct, and cable lubricants for guidelines on tension ratings and lubricant use.

Indirect Attachment 

With indirect attachment, pulling forces are distributed over the outer portion of the cable structure. If cable strength materials are located directly beneath the jacket, this method will produce the least amount of stress on the fiber.

A popular type of pulling fixture for indirect attachment is the "Kellems Grip" or Cable Sock. The Kellems Grip is usually reliable for cables of 1/4" diameter or more. Large pulling forces are possible with a Kellems Grip if the grip's diameter and length are properly matched to cable characteristics. It should spread pulling forces over a 400mm to 900mm length of cable. For small cables, pre-stretching and taping the Kellems Grip to the cable helps to assure even pulling.

Tensile Loading & Bend Radius

Tensile Loading

For a proper cable installation, it is important to understand the cable specification. The two most important specifications are the tensile loading and bend radius specifications. It is very important to adhere to these limits.

Although there are two different types of tension in fiber optic cables, the important tension for the installation is the maximum load the cable can be subjected to without causing permanent damage. We call it the "maximum load installation" and it is measured in Newton or pounds. The "maximum load installation" can also be known as "short term tension", "dynamic load", "installation load" or "pulling tension".

The maximum allowable pulling tension on fiber cable can vary from as low as 50 lbs. force to as much as 800 lbs., depending on the cable construction. The maximum tension for a particular cable should be available from the cable manufacturer, and is often found in the cable specs. This maximum recommended pulling tension should be noted on any drawings, installation instructions, etc. If the field installers do not know the maximum pulling tension, they cannot be expected to respect it.

  • Whenever possible, the tension of the installation should be monitored.
  • If pulling a cable in an outside plant conduit, the use of approved lubricants can help minimize friction.
  • If a run is too long, or if several bends are in the conduit, intermediate pull boxes should be used to separate one pull into two or more shorter pulls.
  • A cable should not be pulled through more than two 90° bends at one time.
  • When running cable vertically, take note of the cable weight. Install cables in a sequence that applies the least amount of strain on the cable.

Bend Radius

​There are two different types of bend radius: Dynamic Bend Radius and Static Bend Radius.

The short term minimum bend radius, or dynamic bend radius, is the tightest recommended bend while installing cable at the maximum rated tension. It is the larger of the two specified bend radii. Throughout the pull, the minimum bend radius must be strictly followed. 

If a location exists in the middle of a run where a relatively tight bend is unavoidable, the cable should be hand-fed around the bend or a pulley can be used.

​The long term bend radius, or static bend radius, is the tightest recommended bend while the cable is under a minimum tension. It is the smaller of the two specified bend radii. After the pull is complete, the cable can be bent more tightly to fit into existing space, but not to exceed the long term minimum bend radius.

​Always follow the manufacturer's guidelines for minimum bend radius and tension. Failure to do so may result in high attenuation (macrobends) and possible damage to the cable and fiber. Guidelines are normally supplied with the cable manufacturer specification sheets. If the bend radius specifications are unknown, the industry de facto standard is to maintain a minimum radius of 20X the diameter of the cable.

​The minimum bend radius must also be adhered to when using service loops. Fiber optic splice trays and patch panels are designed to accommodate the bend radii of the individual fibers, but outside of the hardware, extra care must be taken.

Twisting Cable 

Do not twist the cable as this can stress the fibers. Tension on the cable and pulling ropes can cause twisting. Use a swivel pulling eye to connect the pull rope to the cable to prevent pulling tension causing twisting forces on the cable.

Roll the cable off the spool instead of spinning it off the spool end to prevent putting a twist in the cable for every turn on the spool. When laying cable out for a long pull, use a "figure 8" on the ground to prevent twisting. The figure 8 puts a half twist in on one side of the 8 and takes it out on the other, preventing twists.

Vertical Cable Runs

Drop vertical cables down rather than pulling them up whenever possible. Support cables at frequent intervals to prevent excess stress on the jacket. Support can be provided by cable ties (tightened snugly, not tightly enough to deform the cable jacket) or Kellems grips. Using service loops can assist in gripping the cable for support and provide cable for future repairs or rerouting.

Use of Cable Ties

Fiber optic cables, like all communications cables, are sensitive to compressive or crushing loads. Cable ties used with many cables, especially when tightened with an installation tool, are harmful to fiber optic cables, causing attenuation and potential fiber breakage.

When used, cable ties should be hand tightened to be snug but loose enough to be moved along the cable by hand. Then the excess length of the tie should be cut off to prevent future tightening. Hook-and-loop fastener ties are preferred for fiber optic cables, as they cannot apply crush loads sufficient to harm the cable.

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