All posts by Alexandra

Shield

To protect cable circuits in steel or plastic sheaths from electromagnetic influence (EMI), a tape shield is laid over the wrapping. In case aluminium or copper sheath, the sheath takes shield functions due to the less electrical resistance: the first because of the high electrical conduction, the second – relatively large thickness.

A shield can be made of soft copper or aluminium tape. It is laid on the core spirally or longwise. The last method is more processable because it isn’t connected with rotational motion. In spiral laying it is overlapping by width 10…15%. Longwise shield can be smooth or ridged. Ridged shield is more resistive to multiple curving, though it takes additional 10…15% of material as to smooth. The flute high is 0.8…1.0 mm.

Aluminium shield is wide used. Soft aluminium foil paper (0.10…0.15 mm thickness, more rarely up to 0.20 mm) is a material for it. The basic design data of aluminium foil is shown in table below. UTS is minimum 40 MPa (kgf/mm2), elongation percent is minimum 3%. The foil is made with width 10….500 mm and 5 mm intervals.

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To prevent the connection of steel and aluminium in steel cover cable, a separation sheathing (paper or plastic tapes) is laid.

Cross shield made of alumopolyethylene stripe has much more high physical and mechanical properties. It is constitute an aluminium foil, covered with polyethylene film (thickness 0.03±0.01) on one or both sides. Double-coated tape is laid on wrapping with metal-side inwards and polyethylene – outboard.

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Shield of double- or tree-layered alumopolyethylene type is used with polyethylene cladding, The shield is welded from the inside while the overlapping process of polyethylene cladding. When a shield core comes through the extruding head (200 C° in it ), polyethylene layer of alumopolyethylene tape heats and welds with a sheath, formed in the head. This way a monolithic construction of aluminium sheath, covered with metal pad from the inside, appears. This type of sheath is called aluminium polyethylene sheath.

For example, an aluminium-polyethylene double coated tape (aluminum foil thickness 0.10 and 0.15 mm) is used in Russia. Tape width is 1.1…1.2 from core perimeter.

Aluminium shield, which is one unit with the sheath, is characterized as a one with higher repeated bending resistibility, if to compare with a single shield. Also it increases sheath moisture resistance, because it works as a barrier between cable inside and vapor diffusion of wet.

Wrapping

Wrapping is laid above stranded core for form saving and fastening, and also for mechanical and thermal protection from the next processes: shielding, filler adding, cable sheath laying, cable armouring (hot bitum spraying). Because of wrapping, electric strength of insulated cores  increases relative to a metal sheath or a cable screen.

In air-space paper insulation cables it is used as wrapping: 10-100 pairs – two tapes of telephone-cable paper (thickness 0.05 mm); more than 100 pairs – cable paper (thickness 0.12 mm). In cables with polyethylene insulation as a wrapping it is used one or two plastic tapes (depends on cable’s diameter): polyvinilchloride, polyethylene and polyethylene terephthalate. Polyvinilchloride plasticised (PVC) film, polyethylene (PE) or polyethylene terephthalate (PET) film.

Fundamental properties of the films (PVC, PE, PET) are shown in the table below.

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Polyethylene film has a good insulation properties, though because of the high softening temperature (108…112 °C) it sweats while passing an extruder orifice in the process of polyethylene cable-sheath laying. The most common is polivinilchloride film. It is possible to use two types of film (for example: PVC and PET).Wrapping is laid spirally, with overlapping by width 15…50%, or lengthwise. A winding angle can change in a wide range: from 80º for the thinnest cables and to 20º for the thickest cables.

Water-blocking filler

To protect cables from moisture it is used longitudinal sealing.

Ways of sealing:

  • filling the whole space – void space – in the core with hydrophobic compound;
  • compound hydrostable corks, which periodically placed through the cable length;
  • dry powder (cellulose-based, for example), which is applied on insulated cores. The powder bloats when there is some wet inside, and it prevents the longitudinal propagation of wet;
  • appending water-absorbing elements into a cable core.

As a cable’s filler it is used composition of products of oil’s distillation – petroleum jelly with polyethylene addition. For example, there is water-blocking filler, which consists of paraffin, ceresin and mineral oil with polyethylene addition.

The level of filler and polyethylene insulation compatibility depends on swell value. The higher insulation swelling, the less polyethylene’s density. Level of insulation swelling ΔH is defined by weighing samples before and after ageing in filler.

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Graded-density polyethylene’s swelling at 70C° temperature in petroleum jelly

 

Service capacity rises from C’ to C” when the core free space filling is 100% (solid insulation δ‘, thickness d). Insulation’s thickness can be increased to save service capacity on C’ level (δ”>δ‘), but an outer cable’s diameter will be also increase at least by 15%. The mass of solid filled cables is bigger than the mass of unfilled cables in average 35%.

It is possible to leave the insulation thickness unchanged under C’=const condition: just replace solid insulation with foam or solid-foamed insulation. But the cables’ mass will increase by 5…10%. (table below)

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Silicon rubber-based compound is used to create water-blocking corks while periodic fractionary core filling. Compound is injected in dose into the stranded core by port injection. The length of cork is about 0.2 m, and the interval is near 4 m. Because of the correlation, the level of free core space filling is 5%, which practically has no effect to form factors and electrical parameters of cables. This method is usable for solid, foamed and solid-foamed insulation.

Over the last years, other compositions are used as water-blocking filler.

Polyethylene Insulation

Polyethylene insulation is lapped on the core by the extrusion method. There are tree types of it: solid, pored and solid-pored. The stuff for it includes polyethylene, polyethylene-based and thermostabilizing additives, etc.

Polyethylene is a solid high-molecular material of alkene of ethylene polymerization C4H4. Depending on the production process there are high-pressure polyethylene (HPP) and low-pressure polyethylene (LPP). To get HPP the pressure have to be 140…250 MPa and the temperature 70…100ºC. Density of HPP is 0.918…0.930 g/sm3 and it is low-density polyethylene. Density of LPP is 0.949…0.967 g/sm3 and it is the high-density polyethylene. The way of polyethylene getting stipulates not only the density, but also the molecular structure – i.e. general properties.

Solid insulation consists of LPP based composition. It is also allowed to use HPP based compositions.

General properties of polyethylene compositions are shown in the table:03

Foamed polyethylene insulation is a result of extruding polyethylene and some foaming concentrate on a core. Foaming concentrate is a high-pressure (low-density) polyethylene with evenly spread gas developing agents, gas developing agents decompounding activators and stabilizators in it.

PE foam’s density depends on the degree of porosity ƒ, which is equal to the ratio of air bubbles volume and solid dielectric in the foam material.

Dependency of relative of expanded polyethylene inductive capacitivity εr from the degree of porosity f (and density) is shown in the pic.1:
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Advantages of foamed polyethylene insulation over solid insulation consist in lower specific inductive capacity and smaller cable diameter and mass as a result. The disadvantages: major moisture capacity, minor mechanical and electric strength.

Solid-foamed polyethylene insulation consists of two layers. Inner layer (80% of total thickness) provides equivalent specific inductive capacity insulation lowering. Outer layer is continuous and its function is to prevent moisture ingress for saving foam insulation’s characteristics in case the wet get into the cable or filler.

 

Air-Space Paper Insulation

Strip paper insulation appear when the power core is loosely wraped with telephone-cable or cable insulating paper. Every next turn is overlaid the previous one (20-25% from tape width). The width and the pitch of taping are designed in a such way that there are an air slot between the paper tube and the power core (pic.1, a).

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Pic. 1

Strip paper insulation is made by multiway vertical machines with central paper-lapping head.

The pulp insulation is formed on the core with cellulose. It looks like a cylindrical layer of paper mass with inner pores, which filled with air (pic.1, b). Wood pine unbleached sulphate cellulose is a raw material for the paper mass, which is also used for manufacturing various cable papers and electrical insulating boards.

Cable or phone papers wastes can be added to the cellulose. Basic specifications of insulating materials are shown in the table:

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The manufacturing method of insulating consists of next operations:

  • beating (shortening and splitting) of cellulose fibres in water;
  • mass mixing and cleaning;
  • watering up to 0.1…0.2% concentration;
  • dosed supply of water paper pulp suspension into the insulated part of paper pulp aggregate – a buth, where the cage cylinder is rolling.

The fibres come down on 60 wires, which bend the cage cylinder, and form an insulating layer on each wire, the water filtered out with the cylinder warp. After that, the warps get through a former – fast rolling spiral planer, where the insulation is taken cylindrical shape. The process is finished by high-temperature drying in an electric furnace, thereby the wet is boiled off, forming pores in the mass.

Electric Conductors and Core Insulation

Electric Conductors

To manufacture the electrical conductors it is used soft copper wire with next diameters: 0.32; 0.4; 0.5; 0.64; 0.7; 0.9; 1.2 mm. Copper wire is produced by continuous drawing of stock material – copper wire rod (7.2 – 8.0 mm in diameter), which is made by method of continuous casting and rolling; it is light and need no etching operations. The basic specifications of conducting wire:

  • density γ = 8.9 t/m3;
  • UTS (ultimate tensile strength) σt MPa (kgf/mm2) – 196…274.5 (20…28);
  • elongation l minimum % – 20…25;
  • electrical resistivity ρ with temperature 20ºC – maximum 17.24 μΩ∙km;
  • conductivity σ with temperature 20ºC – minimum 58.9 MOhm/m;
  • TKC (temperature coefficient of resistance) αR ºC-t x 10-3 – 3.93.

Core insulation

As a core insulation for local network it is used spiral strip paper, solid polyethylene, pulp, foam-polyethylene insulation, and cord-styroflex insulation is used for trunk cables. The picture below shows structures of insulated cords:

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Types of core insulations: a) paper-strip; b) solid polyethylene; c) foam-polyethylene; d) solid-foam polyethylene

 

There will be more detailed overvie of insulations’ types in next articles.

 

Constructional parts of electric communication cables Prt.2

Twisted in groups, insulated cores are systematized by groups in certain way and united in one common cable core.

There are homogeneous cores (with similar structure of elementary groups – quads, twains) and heterogeneous cores (with dissimilar twisting and elementary groups diameter).

According to the core formation character there are lay twisting and bunchy twisting. In the lay twisting elementary groups are placed around central group as concentric layers. Related lays are twisted in opposing sides for the purpose of cross-coupling decline and more mechanical strength for the cable core. Bunchy twisting means that groups were firstly united in bunches before they are twisted together.

For the stability of electrical characteristic and damp proofing of cable core, it is filled with waterbloking mass. The defence from the environment is realized by hermetic cover. Depending on the material there are metal (lead, aluminium, corrugated steel), plastic (polyethylene, polyvinyl chloride) and reinforced plastic.

Outer covering is placed over the cable covering. It protects the cable from mechanical damage. Because of mechanical effects on the cable during it’s laying and running there are two armor: a pair of steel stripe, a lay of round steel wire.

To protect cables from gnawing animals, they are covered with thin-walled one-layer strap (0.1 mm) in spirally way.

A special cable construction for stringing on airlines cable supports is foreseen. It includes in-built steel rope.

In follow articles will be shown basic specifications of cables elements, which are taken from a hand book with title “Urban Telephone Cables” (A.S. Brisker, A.D. Rug, D.L. Sharle, 1984). The reason of such an old source is that a lot of different cable constructions are used in telephone network, but the basic specifications of elements were saved.

Constructional parts of electric communication cables Prt.1

Cable structurally consists of a core and a cable protection. The core represented by insulated conductors, twisted in determinate order, which create electric circuits, and proofing – the watertight sheath (metal, plastic, metal-plastic) and outer coverings (jute, armor, flexible tubing).

Generally, the cables power cores are made of copper an, as a rule, used soft copper with resistivity ρ=0.01754 μΩm and temperature resistance coefficients αR = 0.004/deg.

Copper cores with the diameter 0.9 and 1.2 mm usually used for high-frequency cables. Stranded conductors, which consists of twisted wires with different cross section, are used for underwater and radio-frequency cables.

Copper cores with diameter 0.32; 0.4; 0.5; 0.64 or 0.7 mm are used for town cables.

In the coaxial cables outer conductors presented by circular copper tubes with longitudinal seam, corrugated or braided tubes, and also aluminum tubes.

For communication-cables insulation, beside paper, it is used polymeric plastic – polystyrol for trunk cables, polyethylene – for areal and local cables.

The process of cables insulation engineering is aimed to minimize the amount of solid-state dielectric, which provides stability of insulation and stiffness of the cable structure, and to maximize the amount of area for it is the best dielectric. This kind of insulation structure is accepted in trunk cables systems.

Types of solid or composite insulation that are used:

  • Sleeve – paper or plastic web beaded as tube;
  • Insulating cord – consists of insulating cord spirally overlaid on conductor and thin tape above the insulating cord;
  • Solid – made by continuous layer of plastic;
  • Bubble-formed – by spongy layer of polyethylene;
  • Foam-skin – spongy polyethylene with thin continuous polyethylene layer;
  • Balloon – thin-walled plastic tube, inside which the conductor is situated. The tube is clamped with hot tool in periodic points or spirally. After solidification, cord is latched up in the centre of insulation;
  • Beaded – made in disk-form from solid-state dielectric, set on the conductor in certain intervals;

Next insulation types are used widely nowadays:

  • For symmetrical HF cables – сord-polystyrene insulation, solid sponge-polyethylene;
  • For UTN (urban telephone network) and RTN (rural telephone network) — solid polyethylene, spongelike paper;
  • For coaxial cables — beaded, balloon and sponge-polythylene;
  • For station cables — solid polyvinyl chloride.

The most usable ways of insulated conductors group, twisting in symmetrical cables:

  • Pair twisting – two insulated conductors are twisted in certain spacing (100…300 mm).
  • Star-type – four insulated cores, that placed at the corners of square, are twisted in 150…300 mm spacing.
  • Double pair twisting – two previously twisted pairs are twisted together in a quad with 150…300 mm spacing.
  • SZ twisting – provides transposition of cores with certain spacing and intermediate parallel section.

Classification of communication cables

Electric communication cable is a cable which contains one or more isolated electric circuit in the sheath. Depending on laying conditions and usage environment it can have a  special protective covering above.

Electric cables are classified by the areas of application, transmission frequency spectrum, laying and usage conditions.

According to the areas of use cables are divided into trunk cables, areal or intraregional cables, local cables (town and country), office cables, structure (intrafacility) cable systems.

Depending on transmission frequency spectrum cables are classified as low-frequency (up to 10 KHz) and high-frequency (more than 10 KHz).

Because of laying conditions and usage areas, there exist buried cables (placed in ductbank), cables for stringing on airline holdups, underwater cables, station cables, cables for in-house phone networks and structure systems.

Сircuits of electric cables can be symmetrical or coaxial – it depends on construction and relative positions of electrical conductors.

Symmetry circuit (symmetrical pair) consists of two insulated conductors with similar structural and electric properties.

Coaxial circuit (pair) has an inner conductor which concentrically placed in outer conductor that has a shape of cored cylinder. Inner conductor insulated from outer with different layers (washers, balloon, insulating cord, continuous layer).

For easy of classification and using electric communication cables assigned with alphanumeric name – cable mark, which allows to recognize the structure and appropriation. Different countries have different marks, though each holds the same principle: every letter of cable mark means a structural member and they ordered in the “from in to out” way.

For example, in Russia the very first group of letters shows the field of use: trunk cable, areal cable, intra-areal cable, low-frequency phone cable.

The second place filled with letter that means low-frequency station cable type: inside or distribution.

As a rule, cables have pair twisting threads. But it doesn’t mentioned in the marking.

Next one shows the type of insulation: cord-polystyrene, polyethylene. If there are no letter, it means paper insulation.

Another letter refers to the sheath material. Absence means lead covering.

The very last gives an information about the cable protection.

Group of figures means cable capacitance and its diameter of threads.

Here were given the main well-established marks that widely used in Russian cable industry, though nowadays new marks that means manufacturing enterprise and other distinctive features are being added.