WCA MAY 2015

58 / 68

56 Wire & Cable ASIA – May/June 2015
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.