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2010.03 R&D Page 1/4

EXPECTED FIBRE AND CABLE LIFETIME

Optical fibre cables are designed and manufactured to ensure stable and consistent fibre

performances for a predicted operating lifetime of at least 30 years under the prevailing environmental

conditions (underground and/or aerial installation).

All the optical fibre cable elements such as loose buffer tubes, strength members, plastic sheaths and

dielectric or metallic protections are suitably designed to provide adequate means of fibre location,

identification and protection during cable manufacture, storage, installation and termination. The

structure of these elements and the materials used in their manufacture do not have any long term

detrimental effects on fibre performance during the service life of the cable; moreover the materials of

the cables are not hazardous to environment and personnel.

In order to ensure full reliability of the optical cables, the following aspects have been taken into

account and described in details below:

1 - Fibre reliability

2 - Appropriate cable design

3 – Appropriate installation and working conditions

1- Fibre reliability

Each fibre is tested after the drawing process with a strain of at least 1%, according to the “Fibre

Proof Test” in IEC 60793-1-30. During this test the entire length of the optical fibre is subjected to a

constant strain with the purpose of ensuring that the fibre does not include flaws which could produce

fractures or breaks, when an instantaneous strain is applied during the life of the fibre.

Also a Tensile Strength Test is carried out on long fibre samples, according to IEC 60793-1-31 with

the purpose of ensuring that the mechanical strength of the fibres is consistent and high. Typical test

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results, in a Weibull distribution form, are given in Figure 1 (The Weibull distribution is one of the most

widely used tool to predict lifetime reliability).

A third parameter “Stress corrosion susceptibility” is measured according to IEC 60793-1-33; the

typical value of the (dynamic) susceptibility factor "n" is about 21 ; short fibre lengths (typically 300

mm) are subjected to increasing loads, with four different strain rates, up to the break.

On a logarithmic plot (see figure 2) the breaking load versus the strain rates will result in a straight

line, whose slope is 1/(n+1).

2- Cable design

In optical fibre cable design, it is particularly important to consider the protection of fibre from external

stresses under all conditions of use: manufacture, storage, installation and operation.

In loose type construction, fibres are protected from external stresses, either axial or lateral, being

loosely contained by a structure (plastic tube) with a high degree of mechanical strength.

The cables are designed, and also the manufacturing process is controlled, in order to assure a

longitudinal fibre strain less than 0.05% (zero in practice) in the long term, and generally limited to 1/3

of the proof test (i.e. 0.33%) in the short term (e.g. during installation).

Furthermore the minimum bend radius of the optical fibre, after cabling process, is normally limited to

50 mm. Under these conditions, according to the Power Law theory as stated in IEC 62048 – TR Ed1,

the calculated lifetime is far greater than 30 years with an insignificant failure probability.

3- Installation and working conditions

Normal installation practices and equipment can be used for duct, direct buried or aerial installation,

providing that all the manufacturer’s recommendations about tensile load, crush, impact, bend,

temperature and other ambient conditions like presence of water, chemicals, animals (insects,

rodents, etc.) are fulfilled.

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2010.03 R&D Page 3/4

FIGURE 1 - WEIBULL PLOT

Nm : 1673 m = 21 Xo[Gpa] = 5,258

Note: SMr Neon G652 Fiber 20 m samples lenght

-4,66323

-3,66323

-2,66323

-1,66323

-0,66323

0,33677

-0,52288 -0,32288 -0,12288 0,07712 0,27712 0,47712 0,67712

strength (GPa)

cu

mu

lati

ve

failu

rep

rob

ab

ilit

y

0.3 0.4 0.5 4.03.53.02.52.01.50.7 1.00.6 5.0 6.0

0.90000

0.80000

0.70000

0.60000

0.50000

0.40000

0.30000

0.20000

0.10000

0.05000

0.04000

0.03000

0.02000

0.01000

0.00500

0.00400

0.00300

0.00200

0.00100

0.00080

0.00055

0.00005

0.00030

0.99999

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FIGURE 2 – STRESS CORROSION SUSCEPTIBILITY FACTOR

(DYNAMIC CONDITIONS)

G652 FOS FOS natural fibern=23±1;

0,55

0,60

0,65

0,70

0,75

0,80

0,85

-0,50 0,00 0,50 1,00 1,50 2,00 2,50 3,00

Log. Speed [mm/min]

Lo

g.L

oa

d[G

pa

]

Every Point Mean of 10 data

5.728 GPa

5.355 GPa

4.761 GPa

4.293 GPa

21/11/2007