EuroWire March 2015

Technical article

Long-term cable reliability design criteria By David Mazzarese, Mike Kinard and Kariofilis (Phil) Konstadinidis, OFS, Norcross, Georgia, USA

Abstract This paper investigates the current requirements for allowable axial load on optical cables. It is shown that the current criterion found in many optical cable standards – that the allowable long-term load should be less than 20 per cent of the proof test stress – may be optimistic in some cases. Instead, a new criterion – that the long-term load be standardised as 0.14 GPa (20 kpsi) – is recommended. 1 Introduction In overhead cables, there is a set of conflicting design requirements that must be optimised. One objective is to minimise the strain on the optical fibres. A second objective is to minimise the cable diameter to reduce wind and ice loading. A third is to minimise the sag in each span. Aramid yarn added to the cable minimises strain and sag, but the added material increases the diameter of the cable, which in turn increases the wind and ice loading. One key variable in the optimisation of these parameters is the allowable strain on the optical fibre. A common rule of thumb, which has been used for years, is to allow a maximum of 20 per cent of the proof test stress as a long-term strain on the optical fibres in the cable. This criterion appears in many of the current standards documents and has proven to be reasonable for the current generation of cables manufactured with 0.69 GPa (100 kpsi) proof-tested fibre. The criterion, which was developed to provide 30-year mechanical reliability and is based on the excellent overall reliability performance of deployed overhead cables, appears sound. With cables being developed closer to their design limits, it is worth exploring these limits and the rules of thumb that are used in cable design to ensure that, in the future, deployed optical cables will provide similar or better reliability performance than their predecessors.

2 Impact of modified cable designs on reliability 2.1 General observations The traditional design boundaries for the manufacture of optical cables have changed in the past ten years. Some of these changes include: 1 Deployment of higher fibre count cables 2 Deployment of low macro bend loss fibres (G.657) and micro bend-resistant coatings 3 Cutting costs by minimising material in the cable and reducing design margins 4 Higher proof-tested fibres (1.38 GPa [200 kpsi]) These changes in cable design trends can impact the overall reliability of optical cables. Each will be discussed separately to show that, when combined, they could have a dramatic impact on long-term reliability if not managed correctly. 2.2 Deployment of higher fibre counts Many overhead cables fall into the drop cable category. These small cables tie the access network to an individual dwelling. These are typically low fibre-count cables. Excluding these drop cables, however, there is a general trend to deployment of higher fibre-count cables. This is driven by the high cost of rights of way and installation. In many higher fibre-count cables, half the weight of optical cables comes from the optical fibres. The higher weight requires higher tension on the cable to minimise cable sag. Aramid yarns and fibreglass composites (FRP) are used to carry the bulk of this load, with the residual load being taken up by the optical fibres. Further, the more fibres in an optical cable, the larger its diameter becomes. Larger diameter cables have greater wind- and ice-loading, making the situation more difficult. As a result, higher fibre-count cables have the potential for more strain on the optical fibres.

2.3 Deployment of G.657 fibres and micro bend-resistant coatings It is no surprise that we are seeing greater deployment of G.657 fibres in the optical network. Recent data from CRU has shown that more than six per cent of optical fibre being deployed today falls into this category. [Private communications Patrick Faye of CRU.] The G.657 fibres are being deployed because of their superior macro bend performance. One further benefit of G.657 fibres is the improved micro bend performance, making them less sensitive to cabling conditions. Another key development in optical fibres is the deployment of micro bend-resistant coatings [1] . This new generation of optical coatings show two to four times improvement in loss due to micro bending, as compared to those deployed five to ten years ago. Together, these two improvements to the optical fibre have a huge impact in observed cable attenuation, even under aggressive conditions. The superior fibre and coating properties can ‘mask’ the impact of a poor cable design or installation. When optical cables using traditional G.652 fibres are deployed with high residual strain on the fibre, higher attenuation is often observed. By default, the cable manufacturer is required to control the strain on the fibre to ensure the cable can meet the qualification requirements. When G.657 fibres with micro bend- resistant coatings are used for the same cable design, then the measured attenuation will improve and the same cable design may pass this optical requirement. The net result of using G.657 fibres is that the cable will pass this qualification test. However, after deployment, higher fibre strain could pose a long-term reliability risk. In short, if the cable is designed properly, G.657 fibres and micro bend coatings are a huge benefit to the optical performance of the deployed cable. But if the cable is designed poorly, the improved optical fibres can mask the strain issue from the end user, which could pose a long-term mechanical reliability risk.

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March 2015