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Ensuring Fibre Optic Cable Longevity as well as Reliability & Speed

Is Product Longevity as Important as Reliable Day 1 Connectivity?

The fibre optic cable industry has historically been conservative and cautious, seeking to prove to customers that its products won’t fail during at least 20 years of service, even when subjected to harsh environments and treatment. But what if customers prioritise other issues over text-book longevity? For many parts of the World and in some applications it is more important to achieve reliable day 1 connectivity and to provide speedy service than to insist cable performance is unchanged after several decades.

ITU-T initiative L.dsa

Recently ITU-T produced a draft Recommendation for ‘Optical fibre cables for direct surface application (DSA)’. Unfortunately, the title is somewhat misleading in that surface mounted cables have existed for decades (routed around buildings, on gantries and on roofs). What it is really aiming at is cables that can be deployed straight onto the ground surface, albeit with occasional lifting up to avoid obstructions.

Let’s look at the essential features of such a product, some optional ones and some products that might achieve this.

Firstly, since the product is to be de-reeled onto the surface it requires a degree of tensile reinforcement; it may be pulled off the reel, frequently by hand and so is ‘uncontrolled’. During service the cable may be unsupported and either a central strength member or twin members in the cable walls will be effective. Depending on the area for deployment it may be wise to use non-metallic reinforcement to minimise lightning strikes.

Since the cable is not – at least initially – protected by soil the cable should be able to withstand (i) limited UV exposure (not mentioned in L.dsa) (ii) the local temperature range but as a minimum -10 to +60C would be sensible and (iii) anticipated mechanical effects.

Desirable Features

So having a cable that can be deployed and won’t disintegrate under the local weather conditions is a good start but insufficient. One of the most significant variables is the crush loading the product is required to tolerate and the best means to achieve it. Since the cable is exposed, as a minimum it should tolerate being stepped upon by a person which translates to a load of 500N over 100mm (IEC 60794-1-21 E3A). Objects are likely to be dropped on the cable so an impact resistance of at least 1J (IEC 60794-1-21 E4) is advisable. Higher ratings for both will be needed if vehicles cross the cable and/or the cable is sited in an industrial environment.

Mechanical reinforcement is most obviously provided by the use of steel wire or steel tape armour; indeed, this is the method addressed in L.dsa. Undoubtedly this can give a well-protected cable but it has 3 significant drawbacks (i) The cable weight is increased (ii) The cable is no longer all-dielectric and may be vulnerable to lightening strikes and (iii) depending on the exact nature of the steel armour an impact of crush load can generate permanent deformation and the possibility of fibre damage.  As we will see below, alternative approaches may be more desirable.

There is one other contentious issue that needs to be considered, the topic of water penetration into surface cables. The ONLY way to create a completely dry cable is to provide a continuous, longitudinal, metallic barrier along the length of the cable. However, as we’ve seen metal containing surface cables have their disadvantages. As an alternative, filling compounds can be used to retard water transmission along the cable. However, they too have major issues, they increase cable weight and cost and are messy to deal with at terminations, but worst of all the do very little to prevent water migration through what can be a thin cable sheath. So it is often a better compromise to use a thicker sheath and omit the filling material as long as the cable is ‘stopped’ at its termination points.

One of the issues ITU-T faces in producing such a recommendation is the conflicting requirements it poses. The customer wants a rapid-deployable (maybe) temporary fibre link but there is the possibility that such a cable may need to perform above and beyond existing cables. For instance, what may be primarily a cable laid on the ground may end up being used to cross ravines unsupported, be laid into water courses (either accidently or intentionally) so we are likely see a whole gradation of performance amongst these products

An interesting approach

One way to overcome many of these issues is to uses a reinforced sheath with loose filled fibre units inside.

Product examples include:

QWKConnect for single fibre count

RTRYVA for high fibre count

Cable in Duct for high fibre count

The approach used here is to provide temperature resistance via embedded FRP strength members which resist the normal shrinkage and contraction of simple polymer sheaths. At the same time a relatively thick walled sheath confers excellent water resistance and considerable resistance to crush and impact incidents. The product is reasonably lightweight so can in principle be strung aerially over limited span lengths. In such situations the supports used to carry the cable are as important as the cable itself since added ice and wind load on the cable can affect the supports and clamps.

This concept also allows the fibre component to be pre-connectorised with standard LC or SC connectors although in principle other types could be fitted. Pre-connectorised cables inevitably provide excess cable and where a reinforced cable is used it can inconvenient and unsightly to store the excess in the originating or terminating location. With this loose cable-in-tube approach, the excess lengths can be reduced to the 1.1mm core fibre unit by removing the tube and storing this very small sub unit in a tight coil the size of which will also depend on the fibre type used.

L.dsa does not currently address pre-connectorised variants. Historically both IEC as well as ITU-T has tended to separate connector and fibre/cable topics. This separation needs to be reconciled for products such as this and customers can help suppliers by highlighting gaps such as this and other ones. For hand-deployed rapid cables it will also be very useful to hear customer experience of the packaging supplied for these cables and whether it suits their way of installing!

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