A comparison of conventional fiber and blown cable

Evaluating installation characteristics, costs and maintenance shows blown fiber has advantages in every category.

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From the August, 2014 Issue of Cabling Installation & Maintenance Magazine

Evaluating installation characteristics, costs and maintenance shows blown fiber has advantages in every category.

By Julie Paulson and Philip D. Klingensmith

Traditional papers about the pros and cons of fiber versus blown cable installation almost always say conventional fiber is more reliable and far less expensive to install. In this paper, we will first describe the background of both types of fiber, then address each type; finally we will show the most-efficient solution for a straight-line application is blown cable. (Editor's note: The terms "blown fiber" and "blown cable," as well as the terms "air-blown fiber" and "air-blown cable" will be used in this paper. The term "air-blown fiber" is a registered trademark of Sumitomo Electric Lightwave. Information from Sumitomo Electric Lightwave was used in the analysis that is being reported in this paper; where the phrase "air-blown" is used, particularly in the cost table, it relates to Sumitomo Electric Lightwave's air-blown fiber products and systems.)

For this case, we have chosen a typical application: the phased installation of 96 fibers along a pipeline, in the Southwest United States, completely underground. The first phase will be the installation of 48 fibers, followed by 24 fibers in the second phase and a final 24 fibers in the third phase. We selected a 5-mile length for the comparison.


Blown fiber originated with British Telecom in 1982, developed to permit switching between fiber types as they evolved. British Telecom planned to accommodate unforeseen applications by laying cable with extra space for blowing in new types of fiber.

Years passed, and fiber evolved. No individual-application fiber was developed. Instead, singlemode and multimode fiber were developed. Unlike British Telecom, installers became as concerned with running fiber within buildings as they were with running it between them.

Singlemode fiber has a small light-carrying core, 8 to 10 microns in diameter. It is normally used for long-distance (greater than 10 miles) transmission with laser-diode-based fiber-optic transmission equipment. Multimode fiber has a relatively large light-carrying core, usually 50 or 62.5 microns in diameter. It is normally used for shorter distances, up to 10 miles.

Singlemode and multimode fiber cables were designed as conventional cables, complying with existing cable standards for quality and performance. Existing conduits between and within buildings do not have much interior space, having been designed for earlier technology. In general, offices do not want to have work disrupted. Therefore, the goals for conventional fiber design were small-size and ease-of-installation, in addition to code compliance.

Both singlemode and multimode conventional fibers deliver high speeds-10 gigabits or more. However, just like a new highway built in the middle of nowhere that instantly fills with automobiles once it is completed, the new fiber became challenged by large-bandwidth software applications that could now be developed because the speed of the fiber was far better than copper cable. In response, more fiber was installed. Installation techniques became standardized, and installation could be performed by uncertified technicians.

Blown fiber was still in its infancy while conventional fiber was becoming accepted. Comparisons were made between the two, and blown fiber was found to be not as small, less-easily installed, more disruptive in an office, and more expensive to install. Further criticism of blown fiber was that it needed special cabinetry, room for the blower and gas tank, and space for the fanning out of the individual fiber strands. The weightiest criticism was that blown fiber does not comply with existing cable codes because it is "manufactured in the field." Furthermore, at the time, there was no waterproof material lining the microduct tube.

Nevertheless, manufacturers of blown fiber continued developing their products, and answered all the criticisms aimed at them by conventional fiber companies. Blown cable is currently installed in a smaller microduct, with more capacity, is less expensive to install, and is not necessarily more disruptive than any other installation type. Waterproof material now lines the inside of the microduct between the cover and the tubes.

When high functionality is required, the diameter of the conventional fiber innerduct would exceed the size of the blown-fiber microduct.

Getting back the specifics of the project we are using for comparison purposes, the requirement is for 96 strands, installed in two or three time periods. If conventional fiber is used, three cables in a three-section innerduct (like fabric mesh) will occupy a 3-inch diameter space inside a 4-inch PVC pipe. The diameter of the blown-cable microduct with 3 tubes for phased installation of 96 strands, plus 4 extra tubes for future needs, would be at most 1.7 inches-almost half the size of conventional fiber.

Blown-fiber and blown-cable installation requires pairs of skilled workers, trained and certified in the use of the blower, cutting of the tube cable and testing the tubing. The skill and certification of the installers is critical to compliance with the warranty. All blown-cable companies provide a warranty for their product through the time of delivery. Extended warranties, up to 25 years, are available at no extra cost to the purchaser, provided the purchaser returns to the manufacturer documentation of proper installation and function.

The expense of training the installers ranges from as much as $2,650 per person, to as little as $200 per person if there is a minimum of 12 people from the same company in one class.

Both conventional and blown fiber can be buried directly, or installed in a conduit. The cost for digging and laying the conduit is the same for either type of installation. The cost to dig and place the conduit is not a differentiating factor in the comparison between conventional fiber and blown fiber.

Nor is it a differentiating factor to lay the conventional fiber innerduct or the blown-fiber microduct. According to the National Electrical Contractors Association (NECA) manual of labor units, it takes three hours to place 1,000 feet of innerduct. There will be an extra installation step for the conventional fiber because the innerduct must be divided to accommodate the phased installation. Commercially available 3-inch fabric-innerduct cells (available in lengths from 500 to 1,000 feet) would be installed in 4-inch PVC, which in turn would be placed inside the 12-inch conduit. It would then be ready for the conventional fiber to be pulled.

Blown cable comes in lengths of 5,000 feet. After the innerduct is placed, it will take additional time to test the tubes before the cable is blown.

For both cable types, when there is transitioning from an outdoor run to a riser or plenum in an indoor run, the National Electrical Code, published by the National Fire Protection Association (NFPA), requires the infrastructure to meet flame-retardant specifications. Either the cable is indoor/outdoor rated, or there is a transition requiring additional hardware.

For both cable types, freezing and thawing would cause the cables to expand and contract, and would lead to shattered fiber. Therefore, either type must be buried below the frost line.

Both cable types are part of a continuum of technological advances, which can lead to new types or features of cables every 5 to 10 years.

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Specifics of blown cable

Blown cable has four components: 1) microduct, 2) the blowing apparatus, 3) the optical-fiber bundles, and 4) the connecting/terminating hardware. The microduct consists of multiple individual tubes, bundled into a single sheath. The fiber packages or units are designed to be blown into preplaced tubes using a compressed-gas system. Tubes are connected by splices and connectors that route the fibers from one continuous pipe to the next. On the ends of each run, the tubes require fittings.

Blown fiber is not designed for preconnectorized cabling. Consequently, installers connectorize the fibers on both ends once the fibers are in place. Installers also fit the blown fibers with fanouts to finish the connection, or they splice pigtails into place in the blown-fiber cabinet. In a long continuous run, there are fewer splices for blown cable than there would be for conventional cable.

For this application, the 96 strands are divided into an initial installation of 48, followed by 24, then a final 24. We recommend a 7-tube microduct, of which 3 will be used as intended and 4 will remain for future use. It will be quick and easy to blow in new fiber in the future. Fiber can be blown up to 4,000 feet between points at a rate of 200 feet per minute.

If the blown cable is cut, repairing it is a matter of replacing the damaged section with new microduct, coupling it to the old undamaged microduct, testing the new tubing, and blowing new fiber in. This need not be a lengthy process to restore functionality.

Because blown cable is "manufactured in the field," there are no standards governing it. Therefore, the warranty for it is very important. We recommend taking the extra steps for completing the training and the field documentation to get the 25-year warranty.

Recent advances have meant problems of the past are gone. For example, some blown cable is riser- and plenum-rated, which means there is no expense involved with the transition from outside to inside.

Specifics of conventional cable

Conventional cable is governed by standards, including the ANSI/TIA-568-C.3 Commercial Building Telecommunications Cabling Standard, the ICEA-S-83-596 Standard for Fiber-Optic Premises Distribution Cable, and the ICEA-S-87-640 Fiber-Optic Outside-Plant Cable Standard.

Manufacturers of fiber-optic cable design, manufacture and test their cables to validate that they meet the requirements of established standards. Conventional cables are developed to withstand environmental extremes and permit upgrade to higher-speed networks when required. Conventional cables are designed to protect fibers from potential mechanical hazards, including crushing, lightning, smoke, fire, and ultraviolet light.

Inside a structure, conventional fiber rearrangement is simple, but adding more bandwidth is not necessarily possible without adding more cables.

Conventional cable technology has advanced to the point at which some cables meet the environmental requirements of outdoor applications as well as the flame- and smoke-retardancy requirements of indoor cable. With these riser-rated cables, transition splice or connector points and their associated costs can be eliminated.

Installing conventional fiber-optic cables has become easier over the last several years because cable, connector, and splicing products have been improved, and standardized installation procedures have been established.

The first step in installation is laying the innerduct, through which the conventional fiber-optic cable is pulled. This fiber can be terminated after it is pulled, or left dark for future use. Termination requires a few tools and less space than the blown-fiber installation equipment. However, the process of pulling does carry with it the increased risk of damage to the fiber. The maximum distance recommended for pulling conventional fiber is 600 feet. Connectors or splices are installed, and then the next 600 feet of cable is pulled. The pulling is at a rate of 100 feet per minute, or 6 minutes per segment.

However, manholes (maintenance holes) are a consideration. They must exist every 500 to 600 feet for access to the underground conduit. Manholes are priced by type; Type A costs approximately $5,000 to $7,000 and Type J3 costs approximately $11,000. Although it is best industry practice to install a manhole, a handhole may be installed at a cost of $1,784.

In order to pull the cable, two capstans are required. A capstan is a wheel, mounted on a truck or other type of pulling rig. One capstan feeds the cable while the other pulls the cable. These units cost $3,265 each, exclusive of the cost of the truck/trailer they sit on. The two capstans are moved for each 500- to 600-foot segment.

Conventional fiber-optic cabling can be installed with connectors factory-terminated on one or both ends of the cable. This solution can simplify installations when the cable length is known. Once the preconnectorized cables are pulled in place, the installer plugs the connectors into the hardware and the system is operational. If the ends are not preconnectorized, the installer strips the buffer from the fiber using a conventional stripping tool, installs the connector, and places the termination in a fiber panel.

Should a conventional cable be cut, new cable must be pulled, which is a more-time-consuming process than blowing fiber.

If the conventional fiber solution is selected, the downside would be for unanticipated needs. To expand to meet unanticipated needs when there is a three-cell innerduct, higher-strand-count cables could be installed in place of the planned-for 24-strand cables. Another choice would be to install an entirely new innerduct for the length of the project, 600 feet at a time, 6 minutes per segment.

There are huge differences in the length of time both types of installation require. Conventional fiber can be pulled a maximum of 600 feet between pulling points, at the rate of 100 feet per minute. Fiber can be blown up to 4,000 feet between points, at the rate of 200 feet per minute. Over the same 4,000-foot distance, pulling the cable would take 40 minutes-twice the amount of time to blow the cable, not taking into account the time to complete the connectors.

Because the conduit may be attractive to rodents, we recommend using armored microduct for a truly maintenance-free installation.

The cost differential for material and labor between conventional fiber and blown cable over 5 miles is $5,370,494. Blown cable costs less, and saves time, resulting in being the most-efficient choice.

To more-easily see a comparison of the costs and labor estimates for conventional fiber versus blown cable, a table formatted as an Excel spreadsheet is contained within this paper. In it, Sumitomo Electric Lightwave air-blown fiber products are shown.

This comparison does not include delivery costs, which are going to be roughly the same whether conventional fiber or blown cable is selected.

Julie Paulson, MBA is a consultant with Practical Business. Philip D. Klingensmith, RCDD, OSP, RTPM, ITS Technician is owner of Compass Consulting . Both authors are available to answer questions about this article and the analysis contained in it. Julie Paulson can be reached at 520-906-2055. Philip D. Klingensmith can be reached at 740-704-0457.

Extrapolating to a longer-distance installation

Using the data developed for the 5-mile case in this article, and simply extrapolating to a bigger case-such as one found in the real world-following are installation-time estimates for 155 miles of blown cable.

First, the project would include a little more than 300 days to place microduct. It is then ready for the blown cable. Blown cable can come in lengths up to 27,000 feet, and microduct in 5,000-foot lengths. Fifteen-thousand feet of cable can be blown in one day, based on a straight and level application. When there are hills and bends, the installation time increases. Sixty-thousand feet of cable can be blown in one week, so once the microduct is laid, it will take approximately 14 weeks to complete 155 miles.

For laid cable, it will take the same amount of time to lay the innerduct-300 days. It will take twice as long, or 28 weeks, to install laid cable.

-J.P, P.K.

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