WCA March 2019

2.4 Alternative strength member material Since it is not feasible to use non-metallic strength member materials, the use of metal-based materials is the next choice to consider. High-tensile steel wire can be heat treated to control the yield point and to a certain extent the breaking strain of the wire. Small steel wires plated with brass have been used in metallic drop cables for many years, and these wires have a relatively low breaking strain compared to the non-metallic alternatives. By careful choice of wire count and gauge, it is possible to design a cable with just the right level of tensile stiffness whilst also meeting the break load requirement. This is the material that was chosen to provide strength in this new cable design. 2.5 Performance modelling A model was developed to attempt to predict the cable performance under the required span and loading conditions. A summary of this modelling is shown in Figure 2 . The normalised tensile stiffness is plotted against span length and in this performance space limits for cable break load, and relative expected fibre break rate are shown for both 100 and 200 kpsi proof-tested fibre. Using tensile testing to verify the tensile stiffness of the cable and its breaking strength, the maximum fibre strain was calculated along a range of span lengths and shown in Figure 2 . Using a maximum short-term fibre strain of 0.67 per cent for a 100 kpsi fibre, the maximum span length was calculated to be less than the customer request. This led to making sure the customer understood the risks, and ultimately reducing the span length specification. It also led the team to add another safety factor of using 200 kpsi fibre. The model was tested under simulated loading conditions at an outdoor test facility where the cable was aerially

2.2 Environmental loading conditions The environmental loading conditions provided by the end user under which the cable was required to operate were wind speeds up to almost 100kph. Although some ice loading is possible, the induced load for the ice load condition was less than the wind induced load; for the purposes of this paper, only wind load will be considered. 2.3 Maximum break load requirement Typical outside plant drop cables are often based on the use of non-metallic strength members. These strength members are manufactured from materials such as glass-reinforced plastic composite materials or aramid fibres in various forms. One characteristic these materials have is a relatively high breaking strain typically in the 2-4% range. This characteristic presents an interesting dilemma for the cable designer: how to achieve the required break load from the intrinsic failure of the strength member whilst also controlling the level of cable, and in particular fibre strain, when the cable is subjected to environmental loading conditions such as that from lateral wind pressure and/or ice accretion. One option was to employ a device at the pole that would break once the maximum cable load was reached, but this option was disallowed by the end user and a cable solution had to be found. It is clear that the use of non-metallic strength member materials is not possible due to the implied low level of tensile stiffness dictated by the break load requirement. This level of tensile stiffness would result in a cable design with very high elongation under environmental loading conditions. It was not feasible to accommodate this level of cable strain from building strain margin into the cable, so a different material for the strength member was sought.

installed between two poles. Attenuation, fibre strain, sag, and cable movement within the clamps were monitored throughout. The cable was loaded and unloaded at specific cycles requested by the customer. The cable not only performed as expected and met the customer requirements for final sag and strain, but the testing demonstrated that the model matched the real world very well. Actual measurements were within three per cent of the modelled system illustrated in Figure 2 . 3 Installation time savings A combination of cable design aspects listed and developing a close coordination with the customer led to a 50 per cent installation time savings over the previous deployment methods and products.

❍ ❍ Figure 2 : Span length and fibre strain modelling

Design A – 097kph wind + 0.0mm ice – 150N installation tension

Cable breaking load exceeded in this region

Cable designed to operate in this region

Absolute maximum fibre strain exceeded in this region

Normalised tensile stiffness

Target maximum fibre strain exceeded in this region

Span (m)

60

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Wire & Cable ASIA – March/April 2019