Fibre Optic Cabling

fibre Optic cables used in outdoor locations can be subjected to some fairly harsh conditions. It is essential that any cables used give enough protection to the fibres for the environment where they are to be used.

Some of the harmful influences that fibres need protection from are listed below; along with information on why protection is required and what protection can be give.

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Moisture

Water ingress into a fibre cable can cause the fibre coating to degrade, causing the fibres to weaken and break should the cable be moved. Should water in the cables freeze then pressure will build on the fibres causing them to stress and possibly even break. Protection can be given by aluminium tape barrier, gel filled cable, water swellable tape or waterproof sheath materials.

Temperature variation

Cables heating up and cooling down may expand and contract which could put stresses on the fibres.Protection can be given by loose tube cables and non metalic cables.

Strain

External cables can experience a lot of strain; especially during installtion. If the fibres are not isolated from the strains placed onto the main cable they would break. Protection can be given by fibreglass strength members, high tensile strength members, loose tube cables, kevlar strength members.

Abrasion

Cables can suffer from abrasion for a number of reasons, perhaps rubbing against sharp or rough edges during installation, or being placed next to another object that vibrates or moves. Protection can be given by a high density polythene sheath.

Crushing

Buried cables can suffer from crushing forces that can bend, stress and break the fibres. Protection can be given by armoured cables, elastometric sheath and coatings or loose tube cables.

Ultra-violet light

Many plastic materials will break down when subjected to ultra-violet light from the sun over a period of time. Protection by using carbon black filled outer sheath or avoiding cables with a PVC sheath.

Chemicals

In certain environments where exposure to chemicals may be likely some though will need to be given to protecting the fibres from any harmful chemicals. Protection from acid can be achieved by using lead sheathing, where hydro carbons are in use nylon sheathing materials can offer sone protection.

Rodents

Some rodents just seem to like chewing though cables. Protection can be given by using steel tape, very high density plastics or glass yarns around the cable core.

Cables used indoors tend not to suffer the same kinds of stress and strains as outdoor cables so do not tend to need to be so robust. However, there are still some important considerations when using cables indoors.

Low smoke; Zero halogen

Choices of cable are often restricted due to the need to comply with fire and/or safety regulations. The idea is to use a cable such that if it does catch fire the effects of toxic or corrosive gases are eliminated. The use of LSZH (Low Smoke Zero Halogen) cable sheath materials must be used.

Flame Retardant

Although LSZH is a very important part of your cabling choice, this alone is not enough from a safety point of view, it is essential to use cables that will not help or allow the spread of fire through a building. Use cables made with flame retardent sheath materials.

Riser Installations

Indoor cables are often run vertically through risers in a building; if your not careful with loose tube cables you may find the fibres have dissappeared down the tubes! Using tight jacket cables with all the fibres tightly bundled together will prevent fibres dropping, or if you do use loose tube cabling, put loops in the cable every 10m or so where the cable runs vertical.

Now we know some of the considerations we have to make when choosing the correct cable for our application, what types of cable do we have to choose from?

Primary Coated fibre

This fibre has no other protection other than the primary coating applied during manufacture. Very light and compact, probably the cheapest fibre to buy, often used for test leads. However, care must be taken as there is very little protection of any kind!

Ribbon fibre

Primary coated fibres, perhaps 4 or more, are layered side by side between layers of tape. The ribbon is then incorporated into other cable constructions. Ribbon fibre is very compact and with the right tools all the fibres in the ribbon can be spliced in one go. However, specialist tools and equipment may be required for terminating and splicing. Used in high fibre counts for telecoms companies, or can come pre fitted with connectors for use in data communication applications.

Secondary Coated fibre

A secondary protective coating, perhaps nylon or PVC, is applied over the primary coating with a diameter of around 900 microns. Used for pig tails or test leads secondary coated fibre is light weight, compact, reasonably cheap and can withstand more handling than prinmary coated fibres. However, these fibres can be prone to kinking and have little tensile strength.

Single Ruggedised

A secondary coated fibre is surrounded by a layer of kevlar before being sheathed, perhaps with a PVC outer. This cable is idealfor fitting connectors too and where the cable will be handled frequently - patch leads fo example. This is because it allows good attachment for connectors, is very rugged and flexible. The main problems with these cables is that they can be expensive due to the kevlar, and usually come with a limited number of fibres - 1 or 2 perhaps.

Distribution Cable

Secondary coated fibres are placed around a central strength member, a layer of kevlar is placed around the fibres before an outer sheath is applied. This type of cable is frequently used for datacomms applications and many other indoor applications. The cable is compact and, provided a termination box or patch panel is provided, can be directly terminated. Available with flame retardnet and LSZH sheaths this cable is ideal for use indoors and especially for running up risers. Care needs to be taken when using cable ties to hold cable in position - too tight and the fibres can be damaged.

Breakout Cable

Individual rugerdised fibres are stranded together, bound with tape and over sheathed with either PVC or other similar materials. Each rigerdised fibre can be fitted directly with a conn3ector without the need for termination boxes and plugged directly into equipment. This cable can however be quite bulky.

Single Loose Tube

In this cable construction the sheath forms a loose tube containing primary coated fibres. Relatively cheap construction and very compact, but with suseptible to crushing and low tensile strength.

Composite Loose Tube

A single, gel filled, loose tube contains primary coated fibres. Some strengthening of the tube is applied before an outer sheath is applied. Good for building to building links this cable benefits from light weight, relatively inexpensive and compact size.

Stranded Loose Tube

Numerous tubes, each containing primary coated fibres, are placed around a central strength member. They are then bound with tape before an external sheath is applied ontop of an aluminium tape moisture barrier. Used for outdoor runs through cable ducts, usually waterproofed, strong enough to allow long cable pulls and can have very high fibre counts.

Armoured cable

Cables for use externally can often be supplied with armour for additional protection. The armouring is usually wire braid. Offersexcellent protection, but heavy, expensive and can be difficult to source.
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