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Technical article

March 2015


Long-term cable reliability

design criteria

By David Mazzarese, Mike Kinard and Kariofilis (Phil) Konstadinidis, OFS, Norcross, Georgia, USA


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


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


2 Deployment of low macro bend loss

fibres (G.657) and micro bend-resistant


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


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



. 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


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


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.