Cable laying rules. Some practical tips.
Cable laying rules. Some practical tips
Cable laying rules when mechanically pulling it through the pipe requires strict adherence to the instructions. Armor made of thin steel tapes spirally wound around the cable is not designed to withstand significant tensile forces.Rules for laying cablesprescribe, with relatively small efforts, to fasten the steel cable to the end of the cable by the armor using a wire stocking or canvas belt. For large tensile forces, it is necessary to fasten the cable to the cable cores directly or using a clamp. When fastening to the cores, it is necessary to take measures to ensure that the tensile forces are transmitted to all cores evenly and that moisture does not penetrate to the end of the cable. If the cable has wire armor specially designed to withstand tensile forces, then it is pulled by the wire armor using a clamp. Similar standards apply when drawing up an electrical wiring plan for an apartment.
When pulling cables through pipes with block sewers or through other long pipes, as dictated by the rules for laying cables, only in sections up to 50 m long is it permissible to use armoured cables in a lead or aluminium sheath without an outer covering of cable yarn. In other cases, only unarmoured cables in a reinforced lead sheath should be laid. To reduce the effort when pulling cables into pipes, generous lubrication of the cable at the entrance to the channel is used. In addition, it is necessary to control the magnitude of these efforts using a dynamometer so that they do not exceed the maximum permissible for a cable of a given cross-section. General rules for laying cables contain several short points suitable for use in various cases:
— the diameter of the input cable should be approximately 2 times smaller than the internal diameter of the tube;
— the mass of the optical cable should be within 0.1-0.3 kg per linear meter, and its rigidity (flexibility) should be consistent with known brands of input cables or determined by calculation or experiment for the input range from one technical installation;
— optical cables with the maximum delivery construction length should be used. With modern methods, laying a cable with a uniform length of 4 to 6 km is becoming the norm and far from the limit;
— when designing a route, it is necessary to strive for its maximum possible straightness. This factor has a double meaning, since in addition to the general reduction in length, it allows to improve the quality of channels and the associated efficiency in the construction and operation of communication lines.
Cable laying conditions
When designing and installing, it is important to remember — cable laying conditionsmust not be violated, otherwise it will be contrary to safety standards. Cable lines are laid in earthen trenches, in underground cable structures (tunnels, channels, shafts, cable collectors, etc.), directly on building surfaces and structures or on special cable structures, not cables, on trays, in metal pipes, openly on overpasses, etc., similar to how open and outdoor electrical wiring is carried out.
Cable laying is carried out in two stages: preparation of routes for cable laying and cable laying along prepared routes. Cable laying conditionsalong prepared routes are regulated by general rules, compliance with which ensures the preservation of the level of electrical and mechanical strength of the cable that was achieved at the factory during its manufacture. It is important to take into account such a parameter as the cable bending rate. Excessive deformation creates unfavorable cable laying conditions. After all, the insulation and sheath of the cable may subsequently be damaged if the bends of the cable radius are significant. On cables with paper insulation, displacement and rupture of paper tapes, formation of wrinkles on paper tapes are possible. Plastic and rubber insulation of cables is also damaged by sharp bends. Cracks appear on the protective sheaths. Another important criterion is the permissible difference in levels. When laying paper-insulated cables on vertical and inclined sections of the route, the maximum difference in levels established by the Electrical Installation Code must be observed. When using cables with paper depleted impregnated insulation, the permissible difference in levels is 100 m, and for cables with rubber and plastic insulation.
Cable laying temperature
Cable laying temperaturemust be above zero degrees. Unwinding, carrying and laying cables with paper or plastic insulation is allowed only if the cable temperature is positive. The calculation of the fire safety risk during installation takes this indicator into account. Low cable laying temperature during bending poses a great danger to its insulation — both for paper and plastic, since at low temperatures impregnated paper and plastic become inelastic — when bending, breaks inevitably form. Normal The cable laying temperature is achieved by preheating it. The fact is that a cable that is not under load takes on the ambient temperature, and if the ambient temperature changes, the cable temperature gradually equalizes with the ambient temperature. The rate of temperature equalization depends on the presence of jute covers over the armor, the thickness of the insulation, the number of cable layers on the drum, and the presence of a sheath on the drum. Temperature equalization does not occur simultaneously on the entire cable, but first on the outer layers, and then on the inner ones. Heating the cable with a tarpaulin tent and an air blower takes 12-24 hours. The drum with the cable must be turned every 20-30 minutes. The temperature of the heated air supplied by the air blower must be continuously monitored to avoid fire. The cable is heated faster by electric current. The current source is welding transformers or machines, as well as special transformers for heating cables.