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Jonathan Rogers
Jonathan Rogers

Aberdare Cables Facts And Figures Pdf Free

14 diameter and single core cables with a space of > 0,5 x overall diameter between themselves and the vertical wall or surface supporting them as per IEC 287 If they are installed in direct contact with the wall then the current rating given should be reduced by 5% as a rough guide line provided there is a space of 150 mm or six times the overall diameter of the cable whichever is the greater between adjacent cables or cable groups in the case of single core cables. If the installation fails to comply with this requirement then the derating factors in the relevant sections should be applied. Where the ambient temperature along a route varies, the highest value should be taken to select the cable size Cables Installed in Ducts The air within a pipe or duct will increase the thermal resistance of the heat dissipation path. Consequently the current rating or a cable run in a duct (pipe) is lower than that for an equivalent cable in the ground or in free air. The ratings given can be applied to cables laid in concrete, asbestos, pitch fibre, PVC, earthenware or cast iron pipes which are the more common materials encountered. It should be noted that single core cables forming a part of an a.c. system should not be individually installed in cast iron pipes due to the heavy losses incurred by eddy current induction. Generally the size of the duct (pipe) chosen should depend upon the ease of pulling in, or out, the cable. It should be borne in mind that a larger cable may be required in the future to cater for increased load growth. Common pipe sizes used in South Africa are 100 mm and 150 mm internal diameters. When groups of cables are run in pipes along the same route, they should be derated according to the factors given in the relevant tables Composite Cable Routes It frequently happens that a cable run is made partly in air, partly direct in the ground and partly in ducts. The latter conditions lead to the lowest rating and it is here that attention must be focused. Very little heat travels longitudinally along the cable, the main dissipation being vertically through the duct wall and surrounding ground. Any rating where the route is part ground, part duct must therefore be treated with care. Where the length of ducting does not exceed 5 metres per 100 m of route length, the cable rating may be assumed to be that for direct burial in the ground Intermittent Operation Certain types of loads have an intermittent characteristic where the load is switched on and off before the cable has time to cool completely. Depending upon the load cycle it may be possible to select a smaller cable for intermittent operation than would be the case if the load were continuously applied. When a current in excess of the normal rated current is applied, the heating of the cable will be a correspondingly quicker operation than the cooling. 10

aberdare cables facts and figures pdf free


27 Figure 4 (a) Cable Roller (b) Corner Roller (c) Cable Stocking (d) Pulling Eye Ensure that: (vi) Cable covers are available at convenient points. (vii) Any objects that may fall into the trench and damage the cable during the pull and prior to backfilling have been removed. (viii) If the ambient temperature is below 10 C or has been so for the past 24 hours, the cable on the drum will have to be covered with a tarpaulin and heated with suitable lamps or heaters for at least 24 hours under close supervision. Ensure that sufficient ventilation exists, and pay the cable off the drum slowly and carefully. The drum should be lagged with only a few of the bottom lags removed during the heating process. (ix) Place the drum at a convenient point prior to the pull on strong jacks and on sound footing (as mentioned earlier) with the arrow on the drum flanges POINTING IN THE OPPOSITE DIRECTION to the rotation when the cable is being pulled. (x) Before pulling, cut the inner end of the cable free. (xi) Remove the drum battens carefully and from the bottom. (xii) Inspect the cable ends for any sign of leakage (especially Paper insulated cables). If a leak is suspected, it can be proved by heating the cap until just too hot to touch and insulating oil will exude out, the cap should then be removed and the extent of the damage assessed, by means of the dielectric test (See Section 4.2). Cables with extruded dielectrics should be sealed and free from moisture. (xiii) The cable must be payed off from the top of the drum but take care not to bend it too sharply. 23

31 4.0 Paper InsulatedAnd Lead-covered 6.35/11 kv Cables The following chapter generally covers 6.35/11 kv PILC cables. For higher voltages or single core applications please consult our application engineers for specialised technical or installation information. The cables described in this section are manufactured according to SANS 97. (For greater details see brochure covering this product). 4.1 Notes on impregnating compound Present day paper insulated cable are mass impregnated with non-draining compound (MIND). This Poly-Iso-Butylene compound remains in a solid state at normal operating temperatures and melts at approximately 100 C. Compound migration, as was experienced in earlier rosin-oil impregnated cables installed vertically or on inclines, has thus been eliminated by the use of this non-draining compound. 4.2 Moisture in Paper cables If cable is damaged and the lead sheath or end cap is punctured, moisture almost invariably penetrates into the insulation and, if not detected immediately and removed, may cause trouble at a later date. In every such case, therefore, a moisture test should be carried out and the cable cut back until all traces of dampness are removed. The following simple, but reliable test is recommended: MoistureTest Heat about 1 litre of oil compound (or melted paraffin wax)in a saucepan to a temperature of 150 C (check by thermometer). Remove individual paper tapes from the cable under test and immerse them in the hot compound. If any moisture is present, it will boil out of the paper and form bubbles or froth, which will rise to the surface of the liquid. If no moisture is present, the hot compound will be undisturbed. When carrying out the above test do not handle the portion of the paper tapes to be immersed in the compound, as moisture from the hands may give rise to false conclusions. As moisture is most likely to travel along the cable under the lead sheath or along the conductors, the papers next to the sheath and conductors are those most likely to contain moisture. To minimise the penetration of moisture into the cable from the atmosphere or other sources, the cores should be moisture-blocked at each end, by sweating them solid or using solid centre ferrules. Table 4.1 : Current Rating Parameters Maximum sustained conductor temperature Ground Temperature Ambient air temperature (free air-shaded) Ground Thermal Resistivity Depth of laying to top of cable or duct 70 C 25 C 30 C 1,2 K.m/W 800 mm 27


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