FTTU Drop Cable Reliability and Applications Considerations

Self-supporting aerial drop cables have recently become a leading choice for fiber to the user applications. When placed in an environment where the cables will be subjected to wind and ice loads, these cables are typically designed to allow some controlled level of fiber strain as the cable elongates. In most aerial drop cable designs, the fibers and the cable are coupled together with the use of filling gel. However, the level of coupling with filling gel is often times insufficient. This paper discusses how fiber strain can lead to retraction of the fibers from splice closures into the cable, when the cable is elongated due to wind and ice loading. The fiber retraction can result in undesirable signal loss and in the worst case, fiber breaks within the closure. Included in this paper are results of tests which characterizes fiber retraction in aerial drop cables as well the development of installation techniques and methods to prevent fiber retraction.If fibers are sufficiently coupled to the cable, either by design or as a result of using installation techniques, the fibers will undergo tensile strain as the cables elongate due to wind and ice loading. This tensile strain in the fibers will inevitably lead to a predictable occurrence of fiber breakage within the cable during service. The failure rate can be expressed in terms of the number of fiber breaks per number of drops, over a declared service life of the cable. If managed properly, the failure rate can be negligible. If not managed properly, through cable design and fiber quality, the break rate may be unacceptable. Fiber reliability models have been developed to predict the fiber break rate, knowing the level of fiber strain. This paper discusses these fiber reliability models and how they take into account fiber attributes such as weak flaw distributions and fatigue behavior, as well as consideration of the specific deployment conditions, including fiber count per cable, cable span length, the number of days per year during which the cable is under ice and wind load, and the desired lifetime of the cable. Cable designs may then be adjusted to bring failure rates to acceptable levels.