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ANALYSIS OF DUCT PLACEMENT FACTOR
IN FIBER OPTIC PULLING

Many fiber optic outside plant installations involve long "straight" runs of cable pulled into duct. The pulling equations have shown good tension prediction ability in the installation of heavy cables in straight runs. However, these same pulling equations predict that fiber can be pulled much longer distances than today’s reality. Based on simple friction theory, it should be possible to pull fiber cable, with a weight of 100 to 200 lbs/1000 feet (150 to 300 Kg/Km), in the range of 15,000 to 30,000 feet (4.5 to 9.0 Km) with pulling forces under 600 lbs. This assumes friction coefficients under .2, which are routinely measured with our Polywater® Fiber Optic Lubricants. But experience shows that maximum pulling distances of 3000 to 6000 feet (1 to 2 Km)--at under 600 lbs (2.7 Kn)--are more realistic, even with a new, properly placed duct system. What produces this error of a factor of 5 or more?

Consensus is that a large part of the increased tension over estimates is due to "undulations" or displacement in the continuous pipe-style duct. While the run is "straight", the reel memory in the duct introduces displacements that represent many small bends. These bends add tension over long pulls, producing higher tension than expected, and shorter pulls at acceptable tensions.

This type of bending can be analyzed if we treat it as a "repeating" circular wave running down the conduit. This wave can be described by its "amplitude", or maximum displacement from a straight line, and its "period", or repeating distance from peak to peak. In field placements, these variables depend on the type, wall thickness, and size of duct, as well as the placement method. For coiled (reeled) thin wall innerduct dropped into an open trench, a wave with an amplitude as high as 12 inches (300 mm) and a period of 20 feet (3 m) seems reasonable. The same continuous duct pulled into 4" (100 mm) rigid outer duct might only have an amplitude of 0.75" (20 mm) every 20 feet (3 m). Looking at duct displacement in this fashion enables us to calculate the "effective bend" per unit length of duct. The table below presents such calculations for a few different duct wave amplitudes and periods.

As expected, we see that the higher the amplitude and the shorter the period, the higher the effective bend. The table shows effective bends that vary from 0.5 degree to as high as 15 degrees per foot of duct (2 to 50 degrees/meter). While this may not seem like much, it means hidden bend of 2,500 degrees to 75,000 degrees in a 5,000 foot (1,500 m) fiber optic pull!!

The effect of these extremes can be determined by using a modification of our Pull Planner™ Software to estimate pulling tension. The table below shows results when the continuous effective bend was inserted into the calculations. The cable weight used was 100 lbs/1,000 feet (149 Kg/Km) and the friction coefficient was 0.15.

The results of this analysis are in reasonable agreement with field observations. It is usually not possible to pull cable much more than a thousand feet into coiled innerduct laid in an open trench. And even at the lower end of "displacement," pulls in the 5,000 foot (1,500 m) range show 600 lbs (2.7 Kn) of tension.

If you are interested in learning more about this analysis, or fiber cable pulling in general, contact us. We’d be happy to discuss the above in greater detail with you.

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