Topic: Installing Fiber Optic Cable Table of Contents: The FOA Reference Guide To Fiber Optics

General Guidelines For Installing Fiber Optic Cable

Fiber optic cable may be installed indoors or outdoors using several different installation processes.  Outdoor cable may be direct buried, pulled or blown into conduit or innerduct, or installed aerially between poles. Indoor cables can be installed in raceways, cable trays above ceilings or under floors, placed in hangers, pulled into conduit or innerduct or blown though special ducts with compressed gas. The installation process will depend on the nature of the installation and the type of cable being used.

Installation methods for both wire and optical fiber communications cables are similar. Fiber cable is designed to be pulled with much greater force than copper wire if pulled correctly, but excess stress on the cable may harm the fibers, potentially causing eventual failure. Particular care should be taken during installation to prevent kinking the cable which can harm the fibers.

Since there are so many types of fiber optic cable and so many different applications, it is hard to cover each application in detail. However there are some general rules that should be followed:

Installation Guidelines
Follow the cable manufacturer's recommendations. Fiber optic cable is often custom-designed for the installation and the manufacturer may have specific instructions on its installation.

Check the cable length to make sure the cable being pulled is long enough for the run to prevent having to splice fiber and provide special protection for the splices.

Try to complete the installation in one pull.  Prior to any installation, assess the route carefully to determine the methods of installation and obstacles likely to be encountered.

All fiber optic cables have specifications that must not be exceeded during installation to prevent irreparable damage to the cable. This includes pulling tension, minimum bend radius and crush loads. Installers must understand these specifications and know how to pull cables without damaging them.

Pulling tension
Cable manufacturers  install special strength members, usually aramid yarn (DuPont Kevlar), 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. 

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 5 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.

capstan pulling cable

When pulling long lengths of cable in conduit or innerduct (up to approximately 3 miles or 5 kilometers in the outside plant, hundreds of meters in premises cabling), use proper lubricants and make sure they are compatible with the cable jacket.

If possible, use an automated puller with tension control and/or a breakaway pulling eye.  On very long OSP runs (farther than approximately 2.5 miles or 4 kilometers), pull from the middle out to both ends or use an automated fiber puller at intermediate point(s) for a continuous pull.

When laying loops of fiber on a surface during a pull, use figure-8 loops to prevent twisting the cable.

Bend radius
Bending of a fiber optic cable can damage the cable if the radius of the bend is too small. The normal recommendation for fiber optic cable bend radius is the minimum bend radius under tension during pulling is 20 times the diameter of the cable. When not under tension (after installation), the minimum recommended long term bend radius is 10 times the cable diameter.

Fiber Optic Cable Bend Radius

Under tension (top) and after installation (bottom)

Note: Always check the cable specifications for cables you are installing as some cables such as the high fiber count cables have different bend radius specifications from regular cables!

Bend radius example: A cable 13mm (0.5") diameter would have a minimum bend radius under tension of 20 X 13mm = 260mm (20 x 0.5" = 10") That means if you are pulling this cable over a pulley, that pulley should have a minimum radius of 260mm/10" or a diameter of 520mm/20" - don't get radius and diameter mixed up!

Why is bend radius important? Not following bend radius guidelines can lead to cable damage. If the cable is damaged in installation, the manufacturer's warranty is voided. Here is what one manufacturer's warranty says: "This warranty does not apply to normal wear and tear or damage caused by negligence, lack
of maintenance, accident, abnormal operation, improper installation or service, unauthorized repair, fire, floods, and acts of God.
" And their specifications call our the minimum bend radius as "20 X OD-Installation, 10 X OD-In-Service."
Do not exceed the cable bend radius. Fiber optic cable can be broken when kinked or  bent too tightly, especially during pulling.

Twisting cable
Do not twist the cable. Twisting the cable 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.
swivel pulling eye
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.
despooling cable
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.
figure 8

Vertical cable runs (Premises)
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.
Use service loops can to 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.

More on Outside Plant Construction and Installation

Table of Contents: The FOA Reference Guide To Fiber Optics


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