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 μΩ∙m 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.