TeleTópicos Volume XII - A Technical Bulletin that Covers Matters of Interest in Telecommunications Construction.

TeleTópicos Volume XII - A Technical Bulletin that Covers Matters of Interest in Telecommunications Construction.

TeleTópicos Volume XII - A Technical Bulletin that Covers Matters of Interest in Telecommunications Construction.

Volume XII

As communication and data networks are installed in demanding industrial environments, more and more lightweight cables (fiber optic, shielded copper, coaxial, or hybrid) are placed in ducts for their protection. When these cables are pulled into the duct, there are a number of significant differences in relation to the installation of its counterpart in an external plant. These TeleTópicos will review these differences. Although the data presented corresponds to the case of fiber optic cables, the principles apply for most light cables.

Indoor cables are thinner and lighter in weight than outdoor cables. This lower weight makes them much easier to install and manipulate. However, the tensile strength and the maximum pulling tension for indoor cables are much lower than for external plant cables.

The maximum tension of a cable is normally expressed in units of pound force (lbf) or Newtons (N). The Newton is a metric measure of force equal to 100,000 dynas, or 4.45 lbf. To convert a force in Newtons to a force in pound force, divide by 4.45. For example, a maximum draw voltage of 100 Newtons equals 22.5 lbf.

Six hundred lbf (2,700 N) is a typical maximum voltage for external plant fiber optic cables. The stresses for internal plant cables depend on the construction and design of the cable itself. Voltages for indoor plant fiber optic cables can be as low as 25 lbf (110 N), and are often lower than 100 lbf (440 N). An installer can easily exert a force of 25 lbs (110 N) with only one of his arms. Installers should take special care and care not to exceed the specified voltage limits for internal premises wiring during assembly.

Pulling Multiple Cables Simultaneously

There are some precedents regarding the maximum voltage of groups of cables in the installation of groups of electrical control cables, where the total voltages for all cables is reduced by 20 or 50% depending on the specific case. Check with your cable manufacturer for recommendations on maximum pulling tension for single cables or groups of cables.

Cable Jacket

Polyethylene with carbon black (HDPE, MDPE ) is the predominant jacket in external plant cables. The inner wires, however, usually come in multiple colors (orange, yellow or white). Jackets from the latter tend to be made of fire-resistant materials such as PVC, PTFE and PVDF. Due to these jackets differences, the pulling properties and friction characteristics of indoor cables differ greatly from those of external plant.

Duct and Lubricant Viscosity

When the outer plant cables are pulled out, the pipeline systems may be underground. The pipeline in most cases is continuous HDPE (for the inner duct), PVC, or the multicellular type.

Pouring lubricants, such as Polywater F® or Polywater SL® with Silicone are very used. It is very simple and convenient to pour the pulling lubricant into the cable feeder tube or into the pipeline itself before starting the pulling and as needed during the pull.

Tension Reduction

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Keeping everything else the same (cable weight, pipe installation, etc.), the best way to reduce pulling tension is to reduce the friction of the cable jacket against the duct wall. Friction is measured by a non-dimensional constant called Friction Coefficient (FDC). Studies have shown that the CDF varies with the type of cable jacket, type of duct, temperature, and type of pulling lubricant.

The data given below are CDF for inner and in ducts. The test was performed with multiple bends or curvatures. This test has control over the return voltage in the cable, and measures the pulling tension through a series of ducts curvatures. From these measurements, a CDF can be determined. See the Teletopes Volume X for a more detailed theoretical discussion of this method.

The trials were conducted with a large number of distribution cables and pulling lubricants. There is lots of information to present in this limited space. The theoretical CDF calculated against the return voltage of the cable has been plotted on the graph.

The graph shows that the cable has a CDF when it has no lubricants in the pipeline, of approximately 0.33 to 0.36. American Polywater Premise Loop ™ (a gel) lubricant reduces CDF more than the lubricant most often used for outer plant (liquid-build) cables. The Premise Loop ™ lubricant was designed to work with the jackets and ducts commonly used in interior wiring. Both lubricants significantly reduce the CDF compared to the unlubricated case and therefore reduce the pulling forces required to pull the cable. But by how much?

Relating Friction to Stress

The cable pull equations relate the CDF to the pulling tension of the cable. A simplified form of the equations clarifies the following:

Note the significance of the CDF (μ) in determining the voltage. In curvatures, the CDF is an exponent that multiplies the input voltage. In pulls with several curvatures, the reduction of the CDF by 50 to 65% (as can be seen in the graph), can reduce pulling tension in multiples of 5 to 10 !!

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