Зображення сторінки
PDF
ePub

a half turns, with a pair of clamps similar to those employed in twisting the Mclntire connector. The joint produced is practically equal to the original wire in both tensile strength and electrical conductivity.

Tests made at the Columbia University showed the fusing points of pure aluminum wires suspended horizontally in the open air to be 180 amperes for No. 8, 135 amperes for No. 10, and 60 amperes for No. 14 A. VV. G. For aluminum alloyed with 1 per cent copper, the fusing-points were 163 amperes for No. 8, and 54 amperes for No. 14 wire.

The use of aluminum as an electrical conductor may be summed up as follows: It is especially advantageous for bare overhead lines, because it weighs only one-half as much as copper for the same resistance and length, thus reducing to one-half the weight to be carried by insulators, cross-arms, and poles. Its tensile strength is about one-half as great as that of copper; but its specific gravity is less than one-third (.3) as much, so that it has an advantage in this respect also. On the other hand, its diameter is 1.3 times that of an equivalent copper wire, so that it exposes correspondingly greater surface to wind surface and to the accumulation of ice. The electrostatic capacity of an aluminum line is higher than for copper of the same resistance and length on account of its greater diameter, as is evident from the formulae on pages 137 and 138. But the capacity being a logarithmic function of the diameter would not be much augmented by increasing the latter by 30 per cent. For example, the diameter of No. 1 wire is 42 per cent greater than that of No. 4 wire ; but the capacity of a circuit composed of two of the former, placed 18 inches apart, is only 7£ per cent greater than if the latter were used. For overhead lines the electrostatic capacity of an aluminum conductor would not be more than about 5 per cent higher than that of an equivalent copper wire. Moreover, capacity does not play an important part except in very long transmission lines.

Aluminum is also a very suitable material for 'bus bars or other conductors that do not require to be covered with insulation; or in other words, bare conductors that are carried upon insulating supports, which applies to overhead lines as well. In such cases the fact that aluminum would have about 30 per cent more surface is an advantage in dissipating heat.

On the other hand, if aluminum conductors are to be covered with insulating material, as in the case of ordinary wiring in buildings, or especially with underground and submarine cables, then the fact that 30 per cent greater diameter and circumference are required is a disadvantage, since it increases the cost of insulation in about the same proportion. The lead covering or iron armor of cables would also be correspondingly augmented in weight and cost, and the space occupied would be greater to the extent of about 67 per cent in cross-section.

Sag and Stress in Overhead Conductors. — A wire suspended freely between two supports hangs in a curve called a catenary. The exact determination of the sag and other facts is somewhat difficult; but for electrical lines in which the sag is usually small compared with the span, very closely approximate results may be obtained by assuming the

curve to be a parabola. A A C

wire stretched between the ^
points A B may be repre- .E

v 3 r Fig. 182. Sag of Overhead Wires.

sented by the parabolic curve

A E B. The horizontal distance A C B is called the span H in feet, the vertical distance D is the deflection or sag of the lowest point in feet, L is the actual length of the wire measured along the curve; T is the tension in pounds in the wire at its lowest point, and W if the weight of the wire in pounds per foot. We have the following approximate relations : —

D = ^> (81)

[graphic]
[ocr errors]

z = /f + §§\ (8a)

With a given span //the tension ^is a minimum when the sag D is one-third of H. In practice, the sag is made much less than this, being usually one to two per cent of the span, in order to avoid the strains and chances of making contact with other wires due to excessive swinging.

Expansion and contraction by changes of temperature produce considerable effect upon the sag and tension of overhead wires. For this reason a greater sag should be allowed for wires laid in warm weather, in order to allow for the contraction in winter. The actual length L, of a copper wire at a given temperature / in centigrade degrees compared with its length at 20° C. is given by the following expression : —

L, = L„ [1 + .000017 (/ - 20)] (84)

The sag with the increased or decreased length may be found by solving (83) for D, which gives:

D = sjlE^L^H) (85)

The following table may also be used for determining the variations in sag, due to temperature changes. The sag in inches is given for every 10° between 30° and 100° F., being the limits between which lines are likely to be laid.

[merged small][table][merged small]

At —10° F. the sag is reduced, by the contraction, to the very small values shown in the table; and the tension in the wire is raised to 30,000 lbs. per square inch, which is rather too near the breaking stress, assumed to be 60,000 lbs. per square inch. Hence it appears that the sag of about 1.7 per cent at 70° F., upon which the table is based, gives excessive tension if an overhead line, even of hard-drawn copper, is exposed to temperatures of —10° F. or less.

The stretch which occurs in wires considerably modifies the results obtained by calculations, using the ordinary formulae. This is particularly true of soft-drawn copper and aluminum, which show some permanent elongation with any considerable tension applied to them, and do not seem to have a definite elastic limit, like steel.

Poles. — In most cases wooden poles are employed to support overhead electrical conductors. But in some countries, notably in India, iron poles are used almost exclusively for telegraph and other electrical lines, because wood is rapidly destroyed by white ants. This is tme of most other tropical regions. The form of iron pole generally adopted is hollow and tapering, being similar in its general size and proportions to the natural wooden pole, but somewhat smaller in diameter compared with length. It consists of sheet iron riveted together, and may be made in convenient lengths, the ends of which are fitted into each other. These set into a cast-iron base or sole plate, which is buried in the ground. In order to protect the iron, it should be galvanized inside and out, and should also be treated with some resinous material inside and outside, as far as it is buried in the earth. The insulators are carried on iron brackets, which are bolted to the pole, making a very strong and neat construction. In this country iron poles are made of sections of wrought iron pipe, with the joints either "swaged" or rusted. Sometimes for use as anchor poles, iron lattice construction is used.

Iron bases or sockets are often employed with wooden poles, enabling the latter to be made smaller in diameter and straighter. This also overcomes the objection to iron poles, due to the fact that they offer a ground connection to the wires or to the workmen, which in the case of the latter is very dangerous with high voltages.

Wooden Poles.— Chestnut is a very good material for this purpose, especially sawed or hewn for smaller poles. For large poles, pine is suitable on account of size and straightness ; but pine, particularly southern pine and spruce, are not as durable as chestnut or cedar. The latter has long life, but is rather too crooked and knotty for first-class work, where appearance is important. In California sawed redwood is recommended.

Preservation of Timber. — Wooden poles for electrical lines or other exposed timber is liable to be destroyed more or less rapidly by decay, or by the ravages of various small forms of animal life. The chief cause of decay is the fermentation of the sap. When located continually under water, wood is hardly affected by decay, but may be attacked by the teredo navalis, or other animal enemies. But when alternately dried and wet, or when buried in the earth, it is especially liable to decay. To prevent it various things have been tried.

1. Kyanizing consists in soaking in a solution of about three per cent corrosive sublimate (Hg Cl3).

2. Burnettizing consists in impregnating timber with a 1 to 3 per cent solution of zinc chloride (Zn Cl2), formerly by soaking, but now by forcing solution into the pores under pressure. Oak absorbs about 10 and pine about 20 per cent of its volume.

The trouble with the above processes is the dissolving out of the antiseptic salt, and various means have been devised to prevent it, such as the Thilmany process, in which zinc or copper sulphate solution was first forced into the pores and then barium chloride solution to form insoluble barium sulphate (Zn S04 + Ba Cl3 = Zn Cla + Ba S04). The Wellhouse process employed glue and tannin, and the Hagen process used gypsum to retain the salt in the wood.

3. Creosoting consists in placing the timber separated by laths on cars which are run into a large cylinder closed by heavy iron doors. Live steam at 225° to 250° F. is turned on until the timber is heated through, and the albumen of the sap coagulated. A vacuum is then formed to extract the sap, and finally the cylinder is pumped full of dead oil of coal-tar, a measured quantity being introduced under a pressure of about 100 lbs. per square inch. The amount of oil is generally from 10 to 20 lbs. per cubic foot of timber, the oil weighing 8.8 lbs. per gallon. Besides possessing antiseptic qualities, the oil is insoluble in water, and is not washed out or displaced by it. The oil usually only penetrates a little below the surface, hence this skin should not be removed by subsequent work upon the timber.

Creosoted telegraph poles in England showed no sign of decay

« НазадПродовжити »