Telecom Ducts: Long-haul Vision

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A telecom duct is a long-term investment. Network builders, pipelines, other right-of-way owners who typically buy telecom ducts are making a decision for the long haul. They are people who expect to be around for a long long time. Like the city planners of a century ago, they have to be visionaries. They have to think far into the future as they lay their networks. The networks willl be used to provide service to consumers and their complete satisfaction is of paramount importance. The networks necessarily have to be future proof.

The city planners who envisioned and built the London Underground or the Paris Metro a hundred years ago knew they were building a transport system for Londoners and Parisians generations into the future. They built these systems when these cities were still growing and the urban centers had not yet seen today’s mushrooming growth of population. They were thus able to build stations and networks of lines in the bowels of earth which as people-movers more than rival the urban sprawl of flyovers and highways which have sprung up with the invention and subsequent proliferation of the motor car. Despite the advent of the motor car even today the city dweller or any visitor to these two metros would find it more convenient and faster to travel by the UG or the Metro. The reason they continue to be the preferred mode is because of the durability and flexibility that the visionaries built into these systems. As technology advanced and more and more comfortable and speedier rail cars became available they replaced the earlier outdated versions. Yet the infrastructure built then continues to this day and into the future.

The fiber optic telecom network builder today need not despair. He has sufficient flexibility at hand to build a future-proof fiber optic network. He merely has to be cognizant of all the opportunities available to him and he need make no compromises on quality, durability and flexibility. There are ducts customized to meet his every need. But like the city planner of yesteryear he needs to be a visionary. Buying of the duct is investing in infrastructure. The cable that goes into the duct is like the rail cars in the Underground or Metro. As fiber optic technology advances the cable like the rail cars can be replaced. However, the duct should retain its integrity. Further, the duct should be such that it eases the installation of the cable, and when obsolete, its removal, and subsequent replacement.

Choices for Long-haul
The telecom buyer has a choice of two ducts for the long haul: both long life as well as long distance. Ducts conforming to American Standards for Testing Materials (ASTM) and those that pass the Environmental Stress Crack Resistance (ESCR) test for 1,000 hours can be considered good. This test establishes a duct life of 50 years.

New Ducts are there

Vested interests with the obsolete technology continue to promote the use of 200-meter ducts with pre-inserted ropes

Pulling of cables by ropes damages ducts

Plus, cable lengths are limited to 2 km

Permanently solid lubricated duct with jetting technology makes 4-km cables the  norm

Such ducts are ideal for cable installation through jetting technology. The permanently solid lubricated silicore lining minimizes friction between the outer wall of the cable and the inner wall of the duct. The cable is blown and flows easily through the length of the duct. The cable is not stressed as it is when installed through pulling. The minimal friction facilitates blowing of upto 2 km cable at a time. At the time of replacement the existing cable can be deblown just as easily. The other advantage of stress-free installation by blowing is that it enables the use of unarmored cable. Installation of cable by pulling on the other hand results in stress that may necessitate the use of armored cable. Unarmored cable is lower cost than armored cable.

Other ducts for the long-haul are available, which are designed for enhanced blowing performance. A network builder looking for a rapid roll-out of the network could be well-advised to go for this duct. The duct has a spirally ribbed inside construction reducing the area of contact between the cable and the duct and thus the friction. Additionally, the swirl of air through the spiral ribs provides a stronger cushion to the cable. The result is a speedier installation of cable over the same length and the possibility of blowing longer than 2 km cable in a single shot. Use of this duct will enable the use of 6-km cable drums in place of the 4-km cable drums currently in vogue. This offers the additional advantage of reducing the number of splicing points, improving the quality of the signal, and reducing overall cost. These are recurring advantages as and when the cable is replaced over the long life of the duct. Deblowing of the cable to be removed is as speedy and easy as the blowing.

Yet another type of duct is designed for easy traceability of the duct underground and also for checking the depth of the installed duct. This is the usual smooth walled duct with a copper wire co-extruded along the length of the duct to facilitate identification and location of the duct. This duct finds application within the city as well as on inter-city routes. In urban areas its use helps in pinpointing the exact duct location so time is not lost in digging up sidewalks and built-up areas on a hit-and-trial basis. The right-of-way is congested with various lines and underground installations and this duct prevents misplaced diggings and resultant penalty. It facilitates quicker maintenance and upgradation of the network.

Use of such ducts on long-haul routes along highways is strongly recommended again for ease of identification and quick location of the ducts, as drawings cannot always be relied upon since over a period of time highway expansion and route changes take place.

Ducts for Cities
The city environment poses a challenge. Digging in the city tends to be expensive. Permissions for digging are difficult to obtain. During the monsoon months many municipalities refuse to give digging permission because of the water logging problem from the rains. The network builder has to keep the network running at all times. In the case of a fault he needs immediate access to rectify the problem and have the system up again. In major metros, there are areas where due to underground congestion further underground installations are simply banned. What does the network builder do in such a situation? Fortunately there is a solution. Look at what the Delhi Metro builders are doing. Go above ground. The solution lies in the use of ducts for aerial networks. These ducts are clamped to utility poles 150 to 200 meters apart. They have in-built rigidity which prevents any catenary formation between poles. The marginally higher initial investment is easily justified by ease of maintenance of the network in the difficult and congested city environment.


It is not enough to know what duct to buy. The more important question is where to buy, or who to buy from. Buying ducts is not as simple as buying pencils. Unfortunately, some buying organizations have difficulty in making the distinction. These are the people who make the decision on the basis of the bid price alone. The buying of industrial products is a more sophisticated process and the bid price alone should never clinch the decision in favor of any particular supplier. 
A buyer of ducts should consider these aspects when selecting a supplier:

The reputation of the supplier

The manufacturing capability of the supplier

The quality control in place in the supplier organization

Ducts should be the reason for the supplier’s existence (it should be his core business, not an adjunct business)

Technology and technical competence of supplier

The supplier should offer the complete range of products

The training capabilities of the supplier

The supplier’s ability to offer the total solution including testing of the laid duct for its suitability for cable blowing, duct rectification, and finally cable installation through blowing

All the ducts we talked about so far are great for long-distance networks or networks circling major cities. What about local access networks? Let us go back to the analogy with the Underground or Metro systems. The builders of those systems built interconnecting links at specific stations from one line to another. Similarly, for local access networks, the current practice has been to provide for cable loops at various potentially high-traffic points around the city. This practice leads to a bigger investment for some future demand. Additionally, the cable has to be spliced to meet that demand. Fortunately, there is a better way to meet the needs of the local area network. The answer lies in micro ducts. These are very small ducts that, like cable, can be blown into the main duct. A micro cable consisting of a few fibers can be blown into the micro duct as and when there is a demand at a local area network. There is no longer a need for splicing from the cable loop to provide connection in the local area. No splicing means no interruption of service. No unnecessary cable loops means no more dead or dormant investment in cable and loop pits. Investment in micro cable can be deferred until such time that the demand is there and further network is built with latest available fiber technology.

The ribbed in/out ducts are designed for use as sub-ducts in existing main duct underground systems. These ducts are the answer to fast and efficient duct and cable placement in existing systems. The outer rib on the sub-duct reduces the surface contact area and therefore the friction between the sub-duct and the internal surface of the main duct through which it is pulled during installation. The interlocking outer ribs of sub-ducts as they are pulled through the main duct reduce spiraling during installation ensuring a straight installation. The combined beneficial effect is the need for a reduced pulling force to pull the sub-ducts through the main duct. A straight sub-duct results in stress free cable installation through blowing ensuring better network quality. The inner rib reduces the area of contact and resulting friction between cable and duct further easing the cable installation.

Special-purpose Ducts
Special fire retardant ducts are appropriate for use in those applications where fire hazard and human safety are concerns. These ducts have found use in tunnels and in regeneration stations in the laying of networks. Other applications are in hospitals, hotels, mines, movie halls, high-rise buildings, and mission-critical, strategic importance buildings. Use of these ducts is recommended wherever there is a fire hazard.

The double-wall corrugated duct is another special duct. It is suitable for road and rail crossings. It is the ideal solution for replacement of galvanized iron (GI) pipes, PVC ducts, and reinforced concrete construction (RCC) pipes in network build-ups.

Finally, we come to the large diameter ducts of up to 110 mm in diameter. These ducts areappropriate for installation of copper cable for the last kilometer telecom application.

Buying Considerations
The track record of the potential supplier is important. How long has he been in the business, how many kilometers duct has he supplied, how many kilometers cable has he blown? There are four or five known instances where the duct supplier for whatever reason has failed to complete the blowing of the cable in the duct supplied by him. In such situations the network builder has had to prevail on some other reputable supplier to come with his blowing resources to complete the job.

The network builder in buying ducts should look for a supplier who has a long-term commitment to the market. Tomorrow or whenever in the future he needs support he should be able to call on the supplier. The person who has made a low bid to gain the business will have to cut corners to make a profit on the deal. The sample submitted for gaining approval as a qualified supplier may be quite different from the quality of the supply finally delivered to the field locations, as has been established by the results of tests carried out on field samples. The thickness of the duct may not be uniform throughout. Sometimes claims are made about ducts being homogeneously solid lubricated, but the high cost of such a duct belie the claim as no more than mere words. These problems can be averted through specification of third party inspection of field deliveries in purchase contracts with outright rejection of the entire quantity supplied against deficiencies.

Is he a supplier of duct alone? Or is duct selling an incremental business for him. The core business may be something else. Tomorrow he may give up on the incremental business and revert back to the core business alone. On the other hand, the supplier who has invested in manufacture of ducts alone will concentrate on making innovations in his field and in addressing his customers problems and needs through long-term relationships with them.

Look out for these Features

Independent tests conducted on field samples drawn from factory-approved supplies to some network builders found most samples failing on one or more of these tests on account of use of recycled substandard polymer:

Ovality (leads to leakage at couplers)

Elongation (less elasticity; more rigid)

Oxidation Induction Time (OIT) Test (duct life-related; indicates inadequate use of expensive antioxidant)

Impact Test (for brittleness)

Undersized Thickness (thickness of duct less than specified)

High Filler Content (use of filler instead of HDPE polymer)

Internal Coefficient of Friction (higher than specified: 0.08 instead of 0.06; increased friction makes cable blowing more difficult)

Thickness of Lubricant Layer (7 percent or 8 percent against specified 10 percent)

Vested interests with the obsolete technology continue to promote the use of 200-meter ducts with pre-inserted ropes. The problem with pulling of cables by rope is that the pulled cable and the pulling rope both damage the duct. In addition the pulling action stresses the cable with potential for damage of fibers. The short duct lengths result in the use of many more couplers with added possibility of coupler leakage. Pulling by rope requires the use of external liquid lubricant. This gradually solidifies and at the time of system upgradation cable removal and replacement becomes a major problem. The use of 1-km ducts with rope pulling aggravates the damage to both ducts and cables. With this obsolete technology cable lengths are limited to 2 km. Use of permanently solid lubricated duct with jetting technology has made the 4-km cable drum the norm in India. In the West 9-km cable drums are already in common use.

Can the supplier offer training to the duct buyer and his people on the proper techniques for installation of the duct, the testing of the duct integrity in holding pressure to establish readiness for cable blowing, and the blowing of the cable through jetting technology? This establishes the extent to which the supplier is committed to helping the buyer. The requirements of the purchase contract may be fulfilled by delivery of the duct, but the committed supplier will be concerned to ensure that the buyer receives full value for his money through a long-life easily installable quality network.

The buyer is not interested in merely buying duct. He is looking for the total solution. He wants a speedy roll-out of his network so that it is up and running. His core business is running the network with maximum up-time. Whoever can help him achieve this by providing quality ducts, installing and testing the ducts, and blowing cable through them is the ideal supplier. The buyer would be happiest placing the order on the party providing the total solution.

The key to buying duct therefore lies in selecting the appropriate duct for your application and more importantly in selecting the duct supplier. Select the best supplier who can meet all your needs, address all your requirements, and work with him. Of course, once you have selected the supplier, negotiate the best possible package with him.

Ashok Kapur
director,  Duraline–Plumettaz Academy

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