The fiber-optical cable consists of fibres on which the signal extends, and elements of the cable design, fibres providing protection against external influences. We shall consider elements of a fibre-optical cable design within the limits of given article.

Depending on conditions of operation various demands are made to a design of a cable. The cable which is used outside of premises, first of all should have protection against atmospheric influences, such as a sunlight, a moisture, differences of temperatures. To a cable which is intended for a lining in cable wells, protection against rodents is necessary. If the cable is suspended between support of electric mains, its mechanical durability is important.

At a choice of a cable the basic attention is usually given two aspects. The first is a fire safety which necessity arises if the cable is laid inside of premises. The second aspect is an integrity and safety of fibers at storage, installation and operation of a fiber-optical cable. On each of these stages the cable is exposed to mechanical, atmospheric and other influences which can be dangerous to a fibre. Here speech does not go about physical destruction of an optical fibre.

More often for manufacturing an external environment of fiber-optical cables polythene is used. It possesses excellent physical parameters (greater durability, good wear resistance, non-reaction to ultra-violet radiation and chemical influences). Also good dielectric properties are inherent in polythene. Polythene has quite good resistibility to penetration of a moisture, low and to heats. Also polythene possesses ability to not change the physical properties under influence of an environment temperature drops.

Environments of a fiber cable differ on density of used polythene. The strongest material is High Density Polyethylene - HDPE. It is used at manufacturing cables for external application which are directly dug in in a ground or are stretched on sewer pipes. Cost of such material is high enough, therefore it is more often applied in cables with a lot of optical fibres.

Smaller cost has Medium Density Polyethylene - MDPE. The majority of cables which at rather low price differ good mechanical characteristics is made Of it. MDPE provides a sufficient level of optical fibres protection.

The least strong is polythene of low density - LDPE (Low Density Polyethylene). It is often used in a design of cables together with polythene of high density. For example, a fiber-optical cable with the metal reservation. Its external environment can be made of MDPE-polythene, and an additional internal environment which is under the steel reservation, - from polythene LDPE.

To give the raised durability to a fiber-optical cable it is possible in the event that as the top layer polyamide PA is used. It is very suitable material for manufacture of a cable of industrial application. Polyamide has excellent resistibility to chemical compounds, such as the concentrated alkalis, various oils, some solutions of organic and mineral acids. Thickness of a polyamide film does not exceed 0,5 mm. The environment has orange or black color more often.

Polyamide is subject to penetration of a moisture and influence of a ultraviolet. Therefore, polyamide is applied as an additional element of a design together with polythene. Such cable use for a lining in cable water drains or plastic pipelines. To tighten a polyamide cable simply, as factor of friction between a smooth environment from polyamide and the plastic pipeline rather low. As polyamide possesses excellent abrasive stability.

The special attention should be given fiber-optical cables which environments meet the requirements of fire safety. A basis for manufacturing corresponding environments is polythene, and necessary properties are reached by addition of special chemical additives. In the description of a fiber-optical cable abbreviation LSZH more often testifies to presence of such properties (Low Smoke Zero Halogen).

There are many standards describing necessary properties of cable environments according to the international or national requirements of fire safety technics. It is necessary to tell about them more in detail.

Condition, when the external environment of a cable does not support burning (Non propagation of flame) means, that in case of vertically located cable ignition from below its environment will not promote distribution of a flame upwards. However conformity to the given norm at all does not guarantee, that at an arrangement of a cable, for example, in tunnels or pipelines fire will not extend in a horizontal direction.

For such resistibility fire is answered with the characteristic "Non propagation of fire". It guarantees, that it does not become the reason of ignition or distribution of fire.

Absence of allocation of halogens and other toxic connections at burning provides necessary conditions for evacuation of the personnel and elimination of ignition. These are the same important properties of a fiber-optical cable, as well as ability to not allocate a smoke under influence of fire.

Presence at a fiber-optical cable of a nonflammable environment which does not allocate some halogens, noticeably increases its cost. But at making a cable inside of premises, on industrial targets, in tunnels of underground the international and national norms of fire safety needed to application of a cable of such type.

When speak, that the cable has protection against rodents presence of the metal reservation more often is meant. It speaks that such reservation is most effective at low cost, therefore it has found the widest application. It is conditionally possible to allocate some types of the metal reservation:

• The steel laminated tape which is bent lengthways that its lateral edges found one on another. A certain similarity of a tube which for increase in admissible squeezing effort also goffer turns out;
• The wire reservation from the steel zinced wire, which basic purpose - maintenance of the big admissible effort to break, and also protection against rodents. This cable is intended for digging in in a ground.

For our breadthes application of a cable and in the steel goffer-reservation which is much cheaper is admissible. The reservation in the form of diagonal the twisted steel wire, but cables with such reservation concerns to the equipment of special purpose, for example, a cable which is laid under water.

Special clamp of a cable power elements tension and its fastening in assembly muft from the company R&M Sometimes it is necessary to use a cable which completely does not have metal components. How in this case to provide reliable protection against rodents? There are fiber-optical cables with so-called "the nonmetallic reservation". It means or an external layer from especially strong dielectric material, for example, polyamide thickness half-millimeter, or a thin layer of polypropylene.

Good protection against rodents can provide and a dense layer of fiber glass strings which carry out also a role of power elements. However at a bend of a cable the tube with optical fibres can leave a layer of the longitudinal-tense fiber glass strings that places under a threat protection of a fibre. It can be avoided, if fiber glass strings are twirled in a spiral or even are bound together - thus the degree of protection raises.

All these ways of protection against rodents are based on those features of a design of a fiber-optical cable which help to strengthen its mechanical durability. There is also other way which is based on addition of special chemical components in an external environment.

For increase in an admissible stretching of a fiber-optical cable into its design necessarily enter power elements. Sizes of an admissible stretching in 1000-2000N can be reached by means of use fiber glass strings. As a rule, this parameter happens quite enough for cables of a general purpose. Strings can form a dense layer, and can and interwine. It is considered, that glass strings provide greater admissible effort to break. However fiber glass strings also protect from rodents and are a barrier to distribution of burning.

Sometimes in parallel with glass strings apply one central or pair lateral cores. Additional power elements can be dielectric or metal. The design with the central power element is characteristic for a cable with greater number of fibres which are placed by groups around of a power element.

The high admissible effort to break in special types of cables in which this size should matter tens KN, is reached by means of steel rods. In such cables optical fibres settle down not in thermo plastic, and in steel gel the filled tubes is more often.

There is one more function which can carry out fiber glass strings in cables. It is so-called "effect of memory". The basic component of an external environment of a fiber-optical cable is polythene which is delivered on manufacture in granules. After thermal processing the homogeneous weight from which the environment of a cable is extended in the form of a tube turns out.

Eventually occurs lenth-down of polythene. It is compressed, aspiring to accept the initial form caused by the molecular structure. As a result there is a longitudinal displacement of an external environment of a cable rather freely placed smooth pipes in which fibres are located. Thus the external environment is compressed, releasing on the ends of a cable sites thermo plastic pipes which can damage optical fibres inside of finishing muft or boxes. Such lenth-down it is possible to prevent, if braided under an external environment of a cable glass strings to fix on special fastening.