EuroWire March 2015

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Technical article
March 2015 88
Where:
tf is time to failure (lifetime)
tp is proof test time
σp is proof test stress
σa is applied stress
F is failure probability
Np is the proof test break rate
L is the length under tension
md 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, σa. 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:
nd =20
md = 2.5
tp = 0.05 seconds
Np = 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,
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
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.
9 References
[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 58th 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 62nd International Wire
and Cable Symposium, North Carolina, USA,
10th to 13th November 2013.
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 0.0 years 530 years*
1.0ppm per 100km 16 years 0.0 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)
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