Transcript
COPPER CABLES Construction
Introduction Copper conductor Tele-communications cables transmit voice, signals or data. The application re-quirements determine the cable type. The cable laying conditions determine the type of sheathing, protective coating/covering or the armouring. Conductor insulation Insulation materials can be paper or plastic. Paper is wrapped onto the wire with an air space. Plastic insulations are extruded onto the conductors. Stranding elements Single conductors are stranded for cables which have to transmit d.c. signals and low frequency a.c. signals. Two insulated conductors twisted together form a pair. Generally, the core of the cables consists of stranded twisted pairs. In a star quad four insulated conductors are twisted together in a way placing them symmetrically in the corners of a square. Two diagonally opposed conductors form a sidecircuit. Sub-units generally consist of five star quads or 10 pairs. Stranding methods Two stranding methods are currently practiced : unit stranding and stranding in concentric layers.
Cable fillings The core of cables with polyethylene insulated conductors can be filled with a compound ensuring longitudinal water-tightness. The compound is pressed into the interstices between the stranding elements, thus preventing longitudinal penetration of water or moisture into the cable. Cable sheaths To protect the cable core from environmental damage, the core is covered with a sheath. The outer sheath can be preceded by a protective layer of lead, aluminium or corrugated steel tape. The outer sheath of thermoplastic material is extruded onto the cable core, the armouring or bonded to a copolymer coated aluminium tape. Protective coverings Protective coverings or coatings prevent damage of metal sheaths from mechanical or chemical envi-ronmental impact. They consist of low viscosity compounds, pre-soaked fibre-fabric tape and/or jute, plastic and/or rubber tapes, PE or PVC. A thermoplastic coating is, in any case, an excellent corrosion protection for the metal sheath or armouring. Armourings Cables exposed to exceptional strain in the ground will be equipped with an armouring. Depending on the type of stress and the cable diameter, the armouring will consist of steel wire or steel tape applied in
concentric layer(s) onto the cable which has to be protected. The rodent protection is also a steel tape armouring. Reduction factor Short-circuit currents in high voltage power lines can jeopardize telecommunications cables installed closely. A protective effect is obtained by means of a conductive metallic jacket applied around the core. The protection level is de-termined by the reduction factor, which measures the ratio between the voltage induced in the conductors and in the metallic jacket :
rk = Ea
Em
A reduced d.c. resistance in the cable sheath (metallic jacket and steel tape armouring) improves the reduction factor. Coaxial cables For Radiofrequency applications (up to several GHz), coaxial pairs are used. One insulated conductor is surrounded by a continuous tubular metallic sheath, which forms the second conductor of the pair. These coaxial pairs are normally well insulated from external electromagnetic fields, but can be made to radiate by punching holes in the external metallic conductor. In that case, the radiating coaxial cable acts as a kind of special antenna.
