OVERHEAD LINEWORK The advantages of overhead linework as compared with under ground Iinework are that it is much less expensive; it is more readily and more quickly installed; and it can be more readily inspected and repaired.
Its principal disadvantages are that it is not so permanent as underground linework; it is more easily deranged; and it is more unsightly.
For large cities, and in congested districts, overhead linework should not be used. However, the question of first cost, the question of permanence, and the municipal regulations, are usually the factors which determine whether overhead or underground linework shall be used.
The principal factors to be considered in overhead linework will be briefly outlined.
The size and number of conduct ors, and the potential of the line work, determine to a great extent the distance between the poles; the smaller the size, the less the num ber of conductors; and the lower the potential, the greater the distance between the poles may be made. Of course, the exact location of the poles is subject to variation because of trees, buildings, or other obstructions. The usual method employed in locating poles, is first to make a map on a fairly large scale, showing the course of the line work, and then to locate the poles on the ground according tc the actual conditions.
The proper height of pole to be used depends upon conditions. In country and suburban districts, a pole of 25 to 30 feet is usually of sufficient height, unless there are more than two or three cross-arms required. In more densely populated districts and in cities where a
great number of cross-arms are required, the poles may have to be 40 to 60 feet, or even longer. Of course, the longer the pole, the greater the possibility of its breaking or bending; and as the length increases, the diameter of the butt end of pole should also increase. Table XI gives the average diameters required for various heights of poles, and the depth the poles should be placed in the ground. These data have been compiled from a number of standard specifications.
The minimum diameters of the pole at the top, which should be allowed, will depend largely on the size of the conductors used, and on the potential carried by the circuits; the larger the conductors and the higher the potentials, the greater should be the diameter at the top of the pole.
Poles should be shaved, housed, and gained, also cleaned and ready for painting, before erection.
Poles should usually be painted, not only for the sake of appear ance, but also in order to preserve them from the weather. It is par ticularly important that they should be protected at their butt end, not only where they are surrounded by the ground, but for a foot or two above the ground, as it is at this point that poles usually deteriorate most rapidly. Painting is not so satisfactory at this point as the use of tar, pitch, or creosote. The life of the pole can be increased con siderably by treating it with one or another of these preservatives.
Before any poles are erected, they should be closely inspected for flaws and for crookedness or too great departure from a straight line. Where appearance is of considerable importance, octagonal poles may be used, although these cost considerably more than round poles. Gains or notches for the cross-arms should be cut in the poles before they are erected, and should be cut square with the axis of the pole, and so that the cross-arms will fit snugly and tightly within the space thus provided. These gains should be not less than 41 inches wide, nor less than # inch deep. Gains should not be placed closer than 24 inches between centers, and the top gains should be at least 9 inches from the apex of the pole.
Pole Guying. Where poles are subject to peculiar strains due to unusual stress of the wires, such as at corners, etc., guys should be employed to counteract the strain and to prevent the pole from being bent and finally broken, or from being pulled from its proper position.