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WEEK 13: DISTRIBUTION: EQUIPMENT Distribution: Equipment: Pole Lines Sections: Structures | Pole Lines | Conductors | Transformers | Voltage Control | Protection Line Conductors and Open-Wire Lines Conductor Factor Copper and aluminum are the metals used as conductors in distribution systems. Proportions are fixed by the combined effect of conductivity, weight, strength and cost.Conductor Materials Aluminum has the advantage of about 70% less weight for a given size, but its conductivity is only about 61% that of annealed copper. For distribution, it is commonly rated as equivalent to copper conductor two AWG sizes smaller, which has almost identical resistance. To obtain high tensile strength, aluminum is stranded around a steel core, or combine d steel with copper, known as Copperweld, or aluminum, known as Alumoweld, wires. Crossarms made of Southern pine and Douglas fir wood are used because of their characteristics: thin, straight gain, high tensile strength, and durability. Crossarms usually have a cross section3½ in wide by 4½ in high for the average distribution line. Minimum spacing of pins is 12 inches, and up to 16 inches. minimum spacing of pole pins is 30 inches to provide climbing space. Crossarms are used for supporting transformers and other equipment. Double crossarms are installed on poles at corners, at terminals, and at other points where unusual loads are to be supported. Vertical racks are installed on poles to support secondary and multiple street-lighting wires. Sag is the dip along the span of two adjacent poles due to the weight of the conductors, the length of the span and the wind and ice loadings. If the tension is known, sag of any span is found by the Rankine formula: d = S²w / 8t The sag at the center of a horizontal span, d, is in feet; S = span length, feet; w = resultant load, including weight of wire, lb/ft of conductor; and t is the tension in pounds. If span length is doubles, tension must be quadrupled in order to keep the sag the same. If tension is the same on several spans of different length, sag is different in each span. The tension should be sufficient to prevent too much sag in the spans and yet not so great to stress the wire unduly. Maximum tension in a span is limited by the strength of the wire and supports. Also, the tension at 60°F, without external load, should not exceed 35% of the conductor ultimate strength under its initial unloaded condition. Changes in sag due to expansion and contraction of conductors under varying temperature conditions are important in stringing the conductors. Lines erected in cooler months are likely to be slack during the summer unless allowances are made. Ampere loadings often require that the temperature rise of the conductor due to resistance losses must also be taken into account. Sag for Typical Distribution Conductor (Heavy Loading District - 60°F)
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