WCA MAY 2015

Wire & Cable ASIA – May/June 2015

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The intrinsic strength and n values are typically specified

by end users to ensure long-term reliability of the cable.

Unfortunately, the extrinsic portion, shown as region II,

plays an important role in characterising the long-term

reliability of an optical cable. This region contains flaws

closer to the proof-test level that are spaced at a frequency

which may be several kilometres apart.

Over time, these can become fibre breaks if the cable is left

in tension. Understanding this region requires information

that can only be gathered by measuring many kilometres

of fibre. Higher proof test levels will eliminate some of the

larger flaws in the fibre.

However, the exact impact to optical fibre reliability in

a deployed cable is hard to determine without more

information on the overall flaw distribution in the fibre.

One way to illustrate this would be to proof-test an optical

cable at a level just shy of the intrinsic strength of the

fibre, or about 3.8 GPa (550 kpsi). If a 1,000m fibre sample

generated from that experiment were left at a constant

stress of 110 kpsi, the fibre would likely break in less than

a day, or well in advance of the 40-year expected life time.

This example is an extreme case, but highlights the

importance of understanding the complex equations that

govern reliability.

4 Guidance from IEC technical report

on reliability

One of the currently accepted reliability models has been

published by the IEC

[4]

. One of the equations found in

that report is used to predict fibre lifetime – the lifetime

equation for optical fibre after proof testing. This can be

shown as the following expression:

Where:

is time to failure (lifetime)

is proof test time

is proof test stress

is applied stress

F is failure probability

is the proof test break rate

L is the length under tension

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

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

=20

= 2.5

= 0.05 seconds

= 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

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.

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 from 0.69 GPa to 1.38 GPa

❍

Table 1

: Comparison of failure probabilities (1ppm lifetime)