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Circuits for Electric Transmission-Circuits for Electric Distribution
Relative Importance of Insulation-Aërial Lines—Insulators-Attachment of Conductor to Insulator-Joints—Couplings—Material for Aërial Lines-Estimate for Aërial Line-Protection from Lightning-Underground Lines—Edison Mains—The Three-Wire System-Various Systems of Underground Conduits—Lead-Covered Cables.
THE question whether the line should be carried overhead or be placed underground, depends on a number of local circumstances, but as a rule it will be more economical and sufficiently safe to use aërial conductors for the transmission proper of energy, whereas for its distribution underground cables are preferable, and in some cases indispensable. The time is fast approaching, and in America may be said to have already arrived, when no further addition to the vast network of overhead telegraph and telephone wires in our towns will be permitted, and it is quite certain that no exception in favour of wires containing, so to speak, a large store of potential energy, will be made. Electric Light and Power Companies have realized this state of affairs from the beginning, and where they have come forward with definite proposals for a general supply, they have always arranged for their distributing plant to be placed underground. The case is different when electric transmission over a long distance, and possibly across country, is involved. Here the danger from breakage of an overhead wire can be almost entirely avoided by placing the supports at frequent intervals—a precaution not always possible in towns where the width of streets and places often necessitates an ex
cessively long span from one support to the other—and if a wire should break, the chances of anybody being hurt are infinitely smaller than in the crowded streets of a town. We have already seen that power can only be economically transmitted over a long distance by the employment of a high electro-motive force, and hence the proper insulation of the line becomes a matter of the utmost importance. If, in a town district supplied with current at, say, 100 volts, a small leak of a few amperes should take place—and Mr. Edison's experience in the New York Central Station seems to show that such leaks do occasionally occur--the loss of energy, as compared to the thousands of amperes sent out from the station, is very trifling, but if an equal leak should be developed in a circuit of two or three thousand volts, it might very easily absorb all the current which the generating dynamo can pour into the cables, and no energy at all could be obtained from the motor. In an overhead line faults of insulation are not so easily developed, and if they occur, are more easily discovered and repaired than in any of the underground systems, which as yet have hardly had any prolonged trial to show their practical value ; and for this reason it will be safe to assume that aërial conductors will be almost generally used for the transmission of electric energy at high potential over long distances, and that underground conductors will generally be used for the distribution of electric energy at low potentials.
Aërial Lines. The conductor is generally a naked wire or cable of copper, iron, phosphor bronze, or silicon bronze, but slightly insulated conductors are sometimes used. The
insulation gives some protection against short circuits, which might otherwise be caused by other wires, branches of trees, or other bodies falling across the leads, and it has also the advantage of increasing the cooling surface
of the conductor, thus reducing the temperature. At first sight it might seem surprising that a wire coated with insulating material, which is necessarily also a bad conductor of heat, should become less heated than a naked wire. But such is the fact ascertained by experiments, and explained on the ground that quiescent air is the very worst possible conductor of heat, whereas the material of the insulation, although relatively to metal a bad conductor, is a good one relatively to air. If the wire be exposed to wind, then air, although a bad conductor, carries heat off very fast, because each molecule of air as it becomes heated by contact with the wire is carried away and replaced by a new and cool molecule,
and in this case the insulated wire has no advantage over the naked wire.
The conductor is supported on porcelain insulators in the manner of telegraph lines, but to obtain a high degree of insulation they should be of the double-bell type, as shown in Fig. 68. The material should, when fractured, show a uniformly fine and dense grain free from pores and holes ; it must be perfectly white, and contain no cracks or other flaws. The glaze must be perfectly white, and cover the whole external and internal surface with exception of the bottom rim of the outer bell. The thread must be even and well defined, without having broken parts. The stem, Fig. 69, is cylindrical and roughened up. It is taped with yarn, served with linseed oil, and then screwed into the thread. To test the insulator electrically it is placed upside down, the inner space is filled with acidulated water, and it is then
immersed to near the rim in a bath of acidulated water. If the insulation is perfect, it must be impossible to pass a current from the liquid on the inside through the insulator to the liquid on the outside.
The wire may be attached to the insulator either on the groove at the top or at the side, the latter if there
should be a bend in the line occasioning a considerable lateral strain. The method of attachment in both cases will be seen from Figs. 70 and 71, where the views a, b, c, d and a, b, c represent respectively the different stages of
Since wire and cables can only be obtained and carried