Fiber optic cable is usually (but not always) installed in an innerduct that provides mechanical protection for the fiber optic cable. Generally, the duct is available in plastic, concrete, steel, iron and so on. As fiber optic cable is sensitive to excessive pulling, bending and crush forces, much care shall be taken to avoid cable damage during its duct installation. Methods of duct installation, installation preparation and procedures would be covered in this article.
Installation Methods of Fiber Optic Cable into Duct
Generally, there are two approaches for optical cable installation into a duct, pulling method and air blowing method.
“Pulling Method” refers to cable installation into a pre-installed underground ducts by manual pulling or by puller machine. In this method, cable is pulled through duct with the help of pre-installed rope inside the duct. This pulling method is generally preferred in access networks where underground ducts are not continuous for more than 200 to 300 meters. Under this condition, the fiber cable needs to be stored in coil form at each manhole/handhole available at every 200 to 300 meters. Hence, fiber optic cable installation into a duct using a pulling method is suitable to apply within short distances.
Limitations: Optical fiber cables must be handled in compliance with their stated design ratings. Handling crews must be familiar with the cable’s design ratings and the critical events during installation where design limits may be approached.
Air Blowing Method
Cable installation by using high speed air flow combined with additional mechanical pushing force is called as “air blowing”. Air blowing method includes high air speed blowing and push/pull (piston) blowing. The basic principle is injecting the compressed air into the duct and then they flow at high speed through the duct and along the cable. Standard optical fiber cables (like uni-tube, multi-tube, unarmored & armored), microduct cables, and micro-ducts can be installed by using this method. This method is applicable to continuous lengths of more than 1000 meters.
Limitations: Special air blowing equipment are required for air blowing method. Air blowing of fiber cable into duct systems requires continuous run of inner-duct and pressure testing for integrity. Quality of inner-duct joints is critical to the success of air blowing techniques.
Pulling Method VS Air Blowing Method: How to Choose?
Site conditions, availability of machinery and resources are crucial factors for choosing cable installation method. The differences between pulling method and air blowing method for optical fiber cable installation are shown in the table below.
|Factors||Pulling Method||Air Blowing Method|
|Rope or not||Pulling rope pre-installed||No pulling rope to install|
|Power side||Equipment and manpower at two sides||Equipment and manpower at one side|
|Possibility of cable damage||High sidewall forces on cables and ducts may lead to cable damage.||Cable and duct can be monitored and controlled to minimize the cable damage.|
|Main forces||Manual pulling||Large compressor and/or hydraulic power pack|
|Main forces||Straight duct route||Duct route with multiple bends and undulations|
|Distance||Suitable for short distance (few 100 meters) installation||Preferred for long distance (over 2 km) installation|
As a rule of thumb, air blowing method for fiber optic cable installation is more favored than pulling method due to improved installation efficiency, particularly in longer ducts with multiple bends and undulations and savings in manpower and installation time.
Preparations for Duct Installation Using an Air Blowing Method
General precautions should always be applied when handing optical cable, such as not exceed the cable’s stated maximum pulling tension and stated bending radius, as well as not exceed the cable’s maximum crush load, and etc. In addition, other general safety precautions should be paid much attention as well. Before installation begins, construction planning also should be initialed. Last but not least, necessary equipment and materials are required for every installation, including inner-duct, cable pulling lubricants, a pulling eye or grip, a swivel, the cable blower equipment, and etc. For detailed information about preparation for duct installation, you should read installation manual carefully.
Duct Installation Procedure by Using Air Blowing Method
Step 1: Inspection for safety. a.Prior to installation, all traffic safety zones, barricades, and flagmen must be in place. b.Observe all local safety ordinances and practices. All manholes in the system must be checked for harmful gases and ventilation established. c.Water should be pumped and any obstacles cleared. d.Inspect manholes for general safety conditions such as ladders, existing cables, and rack conditions. Step 2: Place blower unit(s), hydraulic and air supply equipment to establish connection between blower and receiving end, and all intermediate locations. Step 3: Set up and align the reel. a. Visually inspect each cable reel for physical damage. b. Ensure the cable reel inner flanges are smooth and without nails to allow free payoff of the cable. c. Set up the reel and reel trailer at the predetermined manhole location. Pay off the cable from the top of the reel. d. Align the reel so that the cable makes a smooth transition through the blower and into the duct system. Step 4: Attach the pulling grip to the cable or attach the swivel to the pulling grip’s flexible eye to prevent the cable from twisting during the evolution. The swivel will later attach to the air carrier unit.
Step 5 to Step 11 are about setting up inner-duct and cable blower unit(s). Please refer to detailed equipment procedures on manual. Step 12: Make adjustments at all the intermediate manholes for slack storage. At optical fiber cable ends and intermediate splice point locations, making sure to leave enough cable slack to easily reach an above-ground splicing vehicle plus 30 feet (9 meters) minimum. Step 13: Slack coils may be assembled in a continuous direction loop configuration or a “figure eight”. “Figure eight” configuration is best way to minimize torsion and stress in the cable fibers. “Figure eight” coil sizing should be approximately 15 feet (5 meters) end to end with each half loop about 5–8 feet (1.5–2.5 meters) in diameter.
Step 14: After the pulling is complete, cut 10 feet (3 meters) from the end of the fiber cable. Place a protective cap over the exposed cable end and tape it in place to prevent water intrusion. Coil any cable free ends and fix in place securely with cable ties to prevent damage. Step 15: At all appropriate manhole locations, putting inner-duct in place. It begins from the center manhole and then the work towards the system ends. It is preferred that all exposed cable coils be mounted in a rack mounted closure box designed for holding the cable coil and the fiber splice closure.
Fiber optic cable installation into the duct provides both extra protection for optical fiber cable and an opportunity for future cable expansion. Optical fiber cable installation into the duct has widely been applied for telecommunication application so far.