EuroWire January 2019

Technical Article

Solving difficult problems while customising cables to reduce installation cost By Welch McCollough, David A Seddon and Mike Gimblet, Corning Optical Communications, Hickory, North Carolina, USA

Abstract Even though the common goal is fibre to the home, global network owners have unique problems where they cannot always use currently available products. Corning has solved a unique technical problem while customising a cable assembly that allows for reduced installation cost. One major design criterion specified by the customer was a substantial span length with a desired minimal fibre strain and cable break load, which presented a unique technical trade-off between operating under environmental loading conditions with minimal fibre strain and allowing the cable to strain sufficiently to break at a desired minimal load. This requirement protects the network from greater damage when poles shift, creating too much sag for proper clearance of moving vehicles that may snag a cable. Another significant requirement from the customer was the development of a combination coupling/ clamp solution as the specific aerial cable design requested by the customer did not have an off-the-shelf device for managing fibre coupling. Corning was tasked to develop a multi-purpose clamp that held the cable, and additionally isolated any fibre strain within the aerial span caused by installation tension and ice/wind loading from the hardened outside plant connector plugging into the network access point on the pole and subscriber end. In addition to satisfying major cable design and performance criteria, there is an ever-present desire to reduce field craftsman labour and tooling costs. The customer had time and performance

specifications around cable installation, lobe separation, and access into the twisted-pair and optical core. There was also a request to eliminate a cost-prohibitive lobe separation tool by making it possible to separate the cable by hand. the implementation of easily accessible technology through co-extrusion and specific tooling designed to extrude with a pressure set-up on the optical lobe and a tubing set-up on the twisted-pair lobe, all while managing compound flow to the centre web section for lobe separation. The resulting cable and clamp designs met the customer’s requirements and are currently being installed across the provider’s network. 1 Introduction A cable infrastructure provider wanted a specific cable design for its overhead drop cables. The design included an optical lobe containing a flame-retardant sub-unit and single-mode fibre as well as a twisted-pair lobe for optional DSL service. A standard run for this overhead drop cable would be a connection to a multi-port at the top These requirements led to

of a pole, passing through a clamp as the attachment to the pole, along the aerial span to the other end going through a clamp at the premises and running down the outside wall to an access box where the sub-unit is accessed and sent through the outer wall to the optical network terminal. This paper will address the challenges of span length versus fibre strain in a cable with a spec limiting break strength in addition to a specialised clamp design and other cable prepping techniques for installation cost and time savings. Figure 1 shows the cable design.

2 Design

requirements

2.1 Span length The design span length from the customer requirement was 55m with an exceptional span length of 68m. Stringing was done with tension priority such that the installation tension for all span lengths was maintained at 150 N. Most spans would be attached to wooden poles, a large number of which were rated with a maximum applied lateral load of 4 kN.

▼ ▼ Figure 1 : Cable design

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January 2019