Published: April 2007
Electrical contractors engaged in power line construction are experienced in handling and pulling cable with the cable trailers, pullers, puller/tensioners, line winders, stringing blocks and related hardware required for making aerial and underground cable installations. Construction of long-haul fiber networks during the 1990s set the stage for the Internet revolution, and today, providers of communication services are rushing to extend fiber networks directly to or near to new residential and business customers to bring enhanced broadband services in a highly competitive market.
For electrical contractors with data communications construction capabilities, fiber-to-the premises (FTTP) offers significant opportunities.
However, handling and installation procedures for fiber optic cable are not the same as those for power conductors and copper communication cables, and electrical contractors with divisions specializing in telecommunications work are taking advantage of the “new” technology of blowing cable into underground and overhead ducts.
Actually blowing fiber is not really new, and the process doesn’t actually blow the cable; it uses a combination of low-strain pushing force and high-speed flow of air to cushion cable and move it through a duct.
“The technology was widely used in the construction of long-haul networks in the mid-to-late ’90s,” said Robert Orr, vice president of marketing for telecom-CATV at Sherman & Reilly Inc., a leading manufacturer and supplier of cable installation equipment. “The twist on this story today is the further evolution of this process and the benefits it offers to network providers and the contractors that serve them.”
Orr said standard outside plant (OSP) fiber optic cables have fiber counts typically in the 12 to 864 fiber categories and that cable manufacturers are rapidly developing new types of fiber that increase bandwidth and speed capacities well beyond what was previously available.
“This range of fiber cables may be pulled into ducts,” Orr said, “but they are increasingly blown because the procedure brings significant increase in production with speeds over twice that of pulled cable, and virtual stress-free installation of the fiber avoids damage and extends cable life.”
Air-blown cable terminology includes high-air-speed-blown cable (HASB); cable-air systems (CAS); and on the smallest scales, microtechnology (MJT), or air-blown fiber (ABF).
Cable jetting air systems are used on long-haul projects of thousands of miles of network backbone, for city rings, local area networks (LANs), communication and trunk lines and on FTTP projects.
“The only times in which this technology might not be cost-effective,” Orr said, “would be when cable runs are so short that capstan or hand-pulling fiber a few hundred feet is justified. If installs are a few thousand feet or more, the increased production of jetting or CAS alone is justified. Another situation in which CAS would not be beneficial would be if the duct systems into which a cable were to be installed were cracked or collapsed and were not capable of being made air tight.”
How it works
A cable fiber pusher moves the cable into an air chamber to which an air compressor supplies pressures between 100 PSI and 175 PSI with the air flowing down the duct path and exiting the duct end. Although the pusher plays an important part in moving the cable, it is the friction of the air flowing over the cable’s outer surface that places a drag force on the cable, moving it through the duct. The pusher becomes the controller by which the cable may be started, stopped or moved faster or slower at any time.
Why blowing is better than pulling
Winching cables puts tension on the cable during installation, Orr said. The pull line or rope is attached to the cable end with a grip or “sock,” and load concentration increases with the amount of cable installed due to weight and friction buildup during the pull.
“At bends,” he said, “the pulling tensions increase exponentially, making it almost impossible to pull a cable through more than three 90-degree bends. Direct-buried ducts are even worse, and a trenched-in duct in which the duct helixes and is not straightened can yield virtually no pulling distances due to friction between the cable and duct.”
With jetting cables with no air- capturing device on the cable end, there is no pulling force on the cable—only an equal distribution of drag forces along the length of the entire cable, which floats it through the duct at speeds between 150 to 300 fpm.
“This is two to three times the production speed of a pulled cable, and the cable is installed virtually stress free,” Orr said.
“Typical underground OSP fiber optic cables have pull ratings of 600 pounds, which is the reason capstan winches with load limiters were the first evolution of fiber-installation equipment. Capstans with load-limiting devices may be preset so when pulling tensions reach near the 600-pound limit, the pulling force stops to prevent damaging the cable. With jetting, HASB or air-assisted cable installations, there is no pulling tension, eliminating the possibility of damaging the cable from over stressing it in addition to doubling or tripling production,” he said.
Orr said equipment used for a typical fiber-blowing job includes the air unit—an air compressor rated at 375 cfm at 150 psi. Air pressures for jetting are between 100 and 175 psi, but compressure volume requirements vary with inner diameters of ducts. Standard hand tools needed include duct cutters, duct slitters and splitable, airtight couplers for ducts.
Many types of ancillary items are available to the contractor for specific situations or requirements, such as air after coolers and hydraulic oil coolers for working in high ambient temperatures.
Orr said that many city build or long-haul OSP installations today use 11/4-inch SDR 11 (HDPE) duct with 144 fibers having an outer diameter of approximately 7/10 inch. A fill of two-to-one (outside diameter of duct to outside cable diameter) is ideal to achieve maximum distances. However, higher fill ratios do not mean installations cannot be completed; expectations for maximum distances simply must be reduced. Average distances for a single jet setup usually are from 4,000 to 6,000 feet.
Careful preparation is an important element of the fiber-blowing process.
“A site survey should be made comparing the work prints of the job to the actual survey, as many times there can be discrepancies between the print and the ‘real world,’” he said. “This is important to verify cable installation lengths, distances between manholes, splice points for slack coil locations and elevation changes. On the job, each section of duct will be calibrated and checked for obstructions by blowing a foam plug through the duct to identify potential problems in the duct, such as water, mud, leaking couplers, etc. Once this process is complete, the duct is lubricated with a special lubricant, which is air and heat compatible, and the cable is installed.”
For long runs of 20,000 to 30,000 feet, a contractor may use multiple jets to reduce or eliminate cable handling or make figure-eight cables by hand or with mechanical devices for intermediate cable storage until the job is complete. It is not unusual for contractors to install between 20,000 and 50,000 feet per day when properly equipped and set up. Orr reportedly blew 108,000 feet of fiber in one day on a long-haul project.
Terrain, weather, duct construction, integrity of the duct system and other factors play into the production numbers. Proper training is essential before personnel attempt to use fiber-blowing techniques.
“Manufacturer personnel conduct training, which typically lasts two to three days,” Orr said. “Training is required, no exceptions.”
Jetting cables or cable-air systems have become the method of choice for installing fiber optic cables in ducts when projects are properly engineered for this technology, Orr said.
“Network owners want their systems built at the best possible costs,” he said. “Cable air systems or HASB systems provide this potential, and air-blown cables of all sizes and configurations are here to stay, and growth is positive.” EC
GRIFFIN, a construction and tools writer from Oklahoma City, can be reached at 405.748.5256 or firstname.lastname@example.org.