EuroWire March 2015

Technical article

Where: t f is time to failure (lifetime) t p is proof test time σ p is proof test stress σ a is applied stress F is failure probability N p is the proof test break rate L is the length under tension m d is the Weibull m parameter from dynamic fatigue n is the stress corrosion parameter The expression is complex, but we can make a few observations. Figure 1 shows that the greater the applied stress, the greater the failure probability. Thus, the failure probability term in the equation, F, is directly related to the applied stress term, σ a . The traditional rule of thumb that has been used to derive 20 per cent of the proof stress as a long-term maximum allowable load assumes these two variables are independent, which is not consistent with Figure 1 . Hundreds of kilometres of fibre must be tested to fully understand the relationship between the failure rate and the applied stress. Table 1 gives the results comparing three scenarios. The first is 0.69 GPa proof-tested fibre with a long-term load of 20 per cent of the proof-test load. Generating the data we used following values substituted into Equation 1 : n d =20 m d = 2.5 t p = 0.05 seconds N p = 1 break every 250km The table shows that an optical fibre meeting the conservative criteria above would exhibit reasonable mechanical performance for the 0.69 GPa at 20 per cent of the proof test level. The second case shows that the same fibre was maintained at 40 per cent of the proof test level. In this case, the 1ppm failure rate would be reached in less than a year. The third case is 1.38 GPa proof-tested fibre with a long-term load of 20 per cent of the proof test level. For this set of conditions, 1ppm failure probability is met in less than six years. Note that data in Table 1 is representative of fibre in a non-aggressive environment. Terms such as zero stress ageing, macro bends, abrasion and other factors can greatly reduce the fibre lifetime. 5 Discussion Fibre lifetime is the sum of the intrinsic and extrinsic failure probability. This paper focuses on long lengths of fibre under axial load in a regime where failure is dominated by extrinsic failures. The results shown in Table 1 highlight the error in the common requirement for optical cables,

Failure probability of 1km of optical fibre

0.069 GPa proof tested fibre 20 per cent long-term load

0.069 GPa proof tested fibre 40 per cent long-term load

1.38 GPa proof tested fibre 20 per cent long-term load

1.0ppm per km 1,600 years 1.0ppm per 100km 16 years

0.0 years 0.0 years

530 years*

5.3 years* * The failure rate varies greatly, with the change in proof-test going from0.69 GPa to 1.38 GPa ▲ ▲ Table 1 : Comparison of failure probabilities (1ppm lifetime)

the optical fibre will affect reliability and should be agreed to by the cable supplier and end user, and that more precise fibre reliability models should be considered. 7 Conclusions This paper has shown that modern cable designs are pushing the design limits for allowable long-term strain in optical cables. Under these new boundary conditions, the old rule of thumb allowing up to 20 per cent of the proof-test level as a long-term strain may no longer be appropriate. A new recommendation requiring the long-term load be limited to 0.14 GPa is proposed as an alternative criterion. This new criterion should be included in upcoming revisions of fibre cable standards. Particularly critical designs are high-strain cable types such as drop cable, and overhead cables including OPGW and ADSS cables. n 8 Acknowledgments Special thanks to Peter Hasløv (OFS), Hiroshi Nakamura (Furukawa) and Peter Pondillo (Corning) for their helpful discus- sions on fibre lifetime. [1] Steven R Schmid, et. al, ‘Development and Characterisation of a Superior Class of Micro bend Resistant Coatings for Today’s Networks’, Proceedings of the 58 th IWCS, (2009), 72-78 [2] Glaesemann, G S, and Gulati, S T, ‘Design Methodology for the mechanical reliability of optical fibre’, Optical Engineering, June 1991, Vol 30 No 6, 709-715 [3] Castilone, Glaesemann G S, and Hanson, T, NFOEC-2000, 1-9 (August 2000) [4] IEC TR 62048 Power Law Reliability Paper courtesy of the 62 nd International Wire and Cable Symposium, North Carolina, USA, 10 th to 13 th November 2013. 9 References

which holds that the long-term load on the optical fibres is simply 20 per cent of the proof-test level. If the fibre break rate was the same for the 0.69 GPa and 1.38 GPa proof tested fibre, then both fibres would have the same 1ppm life-time. We know this is not the case from the data of Figure 1 . When this knowledge is included in the analysis, the results change dramatically. Typically, long-term reliability expectation for optical cables is that the fibre failure probability should be less than 1ppm in 30 years. Using this criterion, the example given in Table 1 can be simplified as follows: • 0.69 GPa fibre at 20 per cent long-term load will provide reliable performance • 0.69 GPa fibre at 40 per cent long-term load will not provide reliable performance • 1.38 GPa fibre at 20 per cent long-term load will not provide reliable performance Though it is apparent that proof-testing at higher levels greatly improves the performance of the cables, the value commonly used in cabling standards – 20 per cent of the proof test level – can lead to false expectations about the long-term reliability of the optical cables. 6 Recommendations The information described in this document indicates that, though 20 per cent of the proof-test load for a long-term load on optical fibre may be a reasonable criterion for optical fibre proof-tested at 0.69 GPa or less, it may produce optimistic estimations for optical fibre proof-tested at higher levels. Currently, most major optical fibre standards, including those in ITU-T, IEC, and TIA, require the fibre to be proof-tested at 0.69 GPa. Cable standards in IEC, ICEA and IEEE should align with this criterion. It is thus recommended that the documents be modified to simply require maximum long-term load of 0.14 GPa (20 kpsi) on the cabled optical fibre after deployment, regardless of the proof-test level. A note could be added to the requirement stating that when optical fibre with proof-test levels higher than 0.69 GPa is deployed, higher strains on

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March 2015