[Trade Journal]
Publication: Canadian Electrical News
Toronto, Ontario, Canada
vol. 16, no. 9, p. 258-262, col. 1-2
Line Construction for Overhead Light and Power Service
BY PAUL SPENCER.
An electric light and power system can broadly be divided into three parts, the generation, the distribution, and the utilization of the current. The progress made in two of these departments, namely, the generating and the utilization of the current, has been constant and rapid. The greatest possible care and thought have been given to designing and building the power station and to its equipment with machinery that will insure uninterrupted service.
The improvement in the reliability of arc and incandescent lamps, of motors, and of all other appliances for transforming the current into useful work, has also been a matter for careful engineering study. But the central link in the chain, the distribution of the current from the power station to the consumer, has received much less attention.
Granted conditions where underground construction is economically feasible, the distribution problem stands a chance of intelligent consideration and of being satisfactorily solved along engineering lines, with proper consideration given to reliability of service, freedom from accidents, safety to the public and employees, and the future growth and development of the situation.
But the possibility of underground distribution is limited to more or less thickly settled territories. Considering electric light companies as a whole, the largest extent of the territory covered must be reached by overhead lines. And when it comes to overhead distribution, the engineer seems to have thought the matter too trifling for his efforts, and to have left the problem to solve itself or to be worked out by the rule-of-thumb methods of the line gang.
The result is shown in the generally poor construction of overhead lines throughout the country and by the general belief that overhead service is much more unreliable than underground service, and is responsible, in a great measure, for the agitation in favor of placing wires underground, even in localities where the cost of the necessary underground construction is out of all proportion to the revenue to be obtained in the territory. The writer does not believe that the objections to overhead lines can be altogether eliminated. A pole line can never be said to be, in itself, an artistic creation, a thing of beauty, or an ornament to the landscape; but with more care given to the construction, such lines can be made safe and reliable and their unsightliness reduced to a minimum, so as to be unobjectionable as compared with the benefits of the electric service, which only their use will permit.
The problem of constructing a satisfactory overhead line is not an easy one. There are many conditions that are not altogether in the control of the line super-, intendent. Suitable pole locations are frequently difficult to obtain. The highway along which the line must run is generally, to some extent at least, obstructed by trees which can only be trimmed sparingly, and may also be occupied by the lines of other companies, whose construction must be taken into consideration to avoid an unsightly and dangerous tangle of poles and wires.
The back alleys of some of the smaller cities, where alley construction is in vogue, illustrate the hopeless mess which results when two or more electric light and telephone companies build their lines without reference to the lines of the other fellow. To avoid such conditions and to construct well-built lines should be the endeavor of every electric light manager whose service, in whole or in large part, is supplied by overhead construction.
The points to be considered in line construction in the order of their importance are the following :—
First and foremost, to which all others must be entirely secondary—the safety of the public and the company's employees.
Second—the reliability of the company's service. Third—sightliness of the construction.
The question of cost has been omitted altogether, as the cost of a well-built line over that of a poorly-built line should be of no moment as compared with the much greater safety and reliability of the former construction.
These first two requirements, safety and reliability, can be considered together, the first embracing the second, for safe construction implies reliability of service. To meet these requirements we should use, first of all, structurally sound material, of ample strength to withstand, under all conditions of service, the strains to which the line may be subjected.
The poles should not have less than 7 inch tops, should be set not less than 5 feet in the ground and be held firmly, by substantial guying against side or end pulls. The cross-arms should be of sound, honest wood, and not the sap-wood variety, which are covered with so-called red paint to hide their defects, and which are made up of the leavings of the mills after the good wood has been cut into building timber and flooring. They should be firmly bolted to the pole and should be braced.
The line wires should not be less than No. 6 B. & S. gauge in size, and they should be strung with ample clearance over highways and footways, and should be inaccessible to the general public from bridges or buildings.
The guy wires should be of stranded cable and not solid wire. They should be insulated, and, so far as possible, installed so they cannot be easily reached from the ground.
Ground wires should not be installed unless they can be connected to a permanent and effectual ground. A ground wire connected to a poor ground not only fails when it becomes charged to give the protection for which it is supposed to be installed, but becomes a positive source of danger to the passer-by.
For the safety of employees, pole wiring should be carried out in a systematic manner, so as to leave space on the pole for climbing and working. To protect the trimmer, series arc lamps should have absolute cut-outs.
Due consideration should be given to the wires of other companies in the territory. We must remember that the telephone and fire-alarm wires have much less mechanical strength than the line wires used by electric light companies, and that in case of sleet-storms they are the ones that are likely to break and come down, making possible contact with the electric light wires.
The writer does not believe in installing guard wires as a protection against such possible crosses with other wires. The stable and proper installation of such guard wires in a general distribution system is impracticable, and as they would generally have to be installed, they would increase rather than lessen the chances of trouble. When electric light, fire-alarm and telephone wires must be run in proximity, the best results to all concerned and to the public will be obtained by having the electric light wires on top and above all other wires, and when they must be run on the same side of a highway, joint occupancy of a single pole line is preferable to separate and conflicting pole lines.
Sightliness of construction will be best obtained by having the work done in a systematic manner and by using poles of a uniform height and size, set and maintained perpendicularly. The cross-arms should be of uniform length.
Wires should be pulled up with similar sag, and should not be left with any greater amount of sag than is necessary to relieve the strain due to contraction at low temperature. Systematic pole wiring, with taps for transformer connections and for branch circuits led across the pole horizontally and dropped perpendicularly, will do a great deal to help the look of things. Nothing is more unsightly than a pole with wires crossing it and leaving it in all directions and at all angles.
After the line is completed there remains the necessity of constant inspection and maintenance in order to keep it in good condition.
The above points have been enumerated in order to call attention to some of the most important items that enter into the problem of line construction. The subject is one of endless detail, and it may be of interest to quote at some length from the line specifications recently prepared for the electric companies of the United Gas Improvement Company and of the Public Service Corporation of New Jersey. Omitting many paragraphs which deal with minor details, the principal sections are as follows:—
POLES.
Specification.—All poles used must be purchased under, and conform to, the company's standard specification. Round chestnut poles should be used where possible, but wooden piles other than chestnut may be used in localities where it is difficult to obtain chestnut poles. If municipal regulations require a finished specification therefor, may be installed.
Chestnut poles should be of sound, live, straight chestnut, squarred at both ends, well proportioned from butt to top, peeled, and with knots trimmed close.
The poles should be of the following dimensions:—
Sawed octagonal poles should be made of long-leaf yellow pine, sound, straight grain, and free from sapwood and unsound or large knots.
The dimensions are as follows:—
The cross-section of the finished poles, at any point, is a true octagon.
Poles are shipped unpainted, but are given one coat of boiled linseed oil before shipment.
Poles are inspected at point of delivery, and all poles not in accordance with these specifications should be rejected.
Height.—Unless taller poles are required by municipal ordinance, or by exceptional conditions, the standard height in cities or thickly settled localities should be 35 feet for poles to carry either one or two cross-arms, 4o feet for poles to carry three or four cross-arms, and 45 feet for poles to carry over four cross-arms. For lines in suburban districts 3o-foot poles may be used to advantage, and their use is recommended. In general, stability of construction is sacrificed by using poles higher than necessary. The height of a pole is always considered as the total length over all.
Trimming.—Before being set, poles should be well trimmed and shaved, every effort being made to have their appearance when set as unobjectionable as possible. The top of each pole should be roofed at an angle of 45 degrees, as shown in Fig. x.
Cross-arm Gains.—Gains for the cross-arms up to the expected carrying capacity of the line should be cut in a pole before the latter is sq. Gains should be cut square with axis of pole, and with all other gains; they should be 4-1/2 inches wide to securely fit the cross-arms, and should be j2 inch deep and spaced 24 inches apart on centres, as shown in Fig. I. The gains for ten-pin cross-arms are 5 inches wide. The distance from the peak of the pole to the top of the upper gain is 9 inches.
Painting.—Poles that are to he painted are given a priming coat of standard green pole paint before being taken from the yard, special attention being taken to paint thoroughly the roof, gains and parts of the pole to be set in the ground. After the pole is set, and construction line work thereon has been completed, the pole is given a second or finishing coat of standard green pole paint. Cross-arm braces, pins, switch-boxes, wooden pole steps and other pole fixtures are painted at the same time.
Pole Numbering.—In order that complete records of the locations and number of poles in use may be kept, it is necessary that every pole belonging to the company, and every pole that is the joint property of the company and of some foreign company, be numbered and the initial letters of the company marked thereon.
Spacing.—For heavy trunk lines to carry three or more cross-arms, the spans should not exceed If o feet. For main lines to carry two cross-arms, the spans should not exceed 125 feet. For branch lines that will never carry more than one cross-arm, the spans should not exceed 140 feet.
Street Rights of Way.—Pole lines on streets are preferable to those over private property. Where possible, poles should be located at the corners of intersecting streets. Lines should be laid out to follow one side of the street, so that the number of street crossings shall be a minimum. In laying out a new line, care should be taken to obtain an unobstructed right of way. Select the side of the street most free from trees and avoid erecting pole lines that will conflict with existing pole lines of other companies. Objection should always be made to the erection by other companies of pole lines paralleling and on the same side of the street as existing pole lines of this company.
Line Level.—The lengths of poles are so proportioned to the contour of the country, or to adjacent poles of exceptional height set to clear obstacles, that abrupt changes in the level of the wires will not occur.
Pole Setting.—Poles should be set in the ground to depths specified in Table I. At line terminals, corners, curves, and other points of excessive strain, poles are set in the ground an additional 6 inches. They should be set to stand perpendicularly when the line is completed. Exception can be taken to this rule in that a very slight lean against the strain can be given to poles at line terminals, corners, curves, and other points of excessive strain.
Crib-Bracing.—Poles which cannot be strongly guyed and which must be set in soft ground, may be given additional stability by crib-bracing, as shown in Fig.2. This consists in placing at the points of maximum strain two logs, about 5 feet long and not less than 8 inches in diameter. These furnish considerable extra bearing surface, tending to hold the pole in position. The top brace alone, or both braces, can be used according to the amount of additional stability required.
Poles to be stepped.—All poles carrying branch cutouts, incandescent lamps or other attachments that may require frequent attention, as also all testing poles, are stepped to facilitate climbing them. For the same reason, it will be found convenient to step poles carrying transformers. The location of steps on a pole is shown in Fig. 3. They should always be placed on a line with the street on which the pole is located.
POLE GUYING.
When to Use Guys.—Guys should be used whenever they can be located so as to counteract the strain of the wires attached to a pole, and so prevent the pole from being pulled from its proper position in a line. The following general instructions cover some of the special cases where guying may be required. On straight lines carrying more than one cross-arm; poles should be head-guyed at convenient intervals, i.e., guys should extend from the top of a pole to the butts of the adjacent poles in the line on either side. If possible, this same pole should be side-guyed, i.e., guys should extend from the top of the pole on either side at right angles to the line to guy stubs or other supports.
On street lines, side guying can be employed only in comparatively few instances. Straight line guying is for the purpose of giving additional stability to a line in case of severe storms, and is illustrated in Fig. 4. Line terminal poles are head guyed, and on heavy lines the two poles next to the terminal poles are head-guyed to assist the latter in taking the terminal strain, as in Fig. 6.
Poles at the terminals of long spans are guyed to counteract the extra strain on the pole due to the long span, as shown in Fig. 5. In turning a corner with one pole guys are preferably placed as shown in Fig. 7. In turning a corner with two poles, guys are preferably located as shown in Fig. 8.
On curved lines carrying not more than one cross-arm, the guys are located in a line with the radius of the curve on every pole with an offset of more than 10 feet. On lines carrying more than one cross-arm, a guy is located on every pole having an offset of more than 5 feet. Poles on steep hills are head-guyed.
Guy Wire.—The material used for guying should be standard cable composed of galvanized iron or steel wire. The standard guy cable consists of seven strands of No. 12 B.W.G. galvanized iron wire. A smaller cable may be used for guying cross-arms and light poles, but no cable of less diameter than one-quarter inch should be used, nor should solid iron wire be used for guying poles or cross-arms. In connection with the stranded cable, galvanized iron guy clamps and thimbles should be used. Wrapped joints should not be made in guy wire when clamps can be employed.
Guy Attachment.—All guy wires are preferably attached to poles, guy stubs, trees or other ungrounded supports, and when so attached should not reach within 8 feet of the ground. Unless such attachment be absolutely unavoidable, guy wires should not be attached to rocks, stone foundations, iron structures or other grounded supports, and such attachments to structures are made only with the consent of the owner, and in such a manner that there is no danger of any damage to, or interference with, the free use of the structure.
On poles carrying extra-high-potential wires, guys should not be attached to the cross-arms carrying these wires, nor to the pole at or above these cross-arms. When two or more guy wires run to a pole, guy stub or other support in close proximity to each other, the attachment of one guy should never overlap that of another, but be entirely independent. In new construction work, and in rebuilding old lines, guy wires are placed and pulled to the required tension before the lines are strung.
Stub-Guying.—When a line cannot be properly guyed by means of other poles in the vicinity, guy stubs are set, as shown in Fig. 2. Guy stubs should be of sound chestnut, at least 8 inches in diameter, and of sufficient length to raise the attached guys to the proper height from the ground or from obstacles as herein specified. They are set in the ground to a depth of at least 6 feet, leaning away from the pole to be guyed, and set in the ground according to specifications applying to poles. Special stability of guy stub setting may be obtained by the use of crib-bracing, as indicated in Fig. 2, or by concrete setting.
Anchor Guys.— An anchor guy may be employed to guy poles, but must not be installed when it might interfere with surface traffic. It is constructed as shown in Fig. to. A 34-inch iron eye-holt about 7 feet long is attached at the middle of, and at right angles to, a wooden anchor consisting of a cross-log of sound chestnut, not less than 8 inches in diameter, and about 5 feet long. This anchor is set in the ground so that the eye of the guy rod stands about 1 foot above the ground, the guy rod being in line with the wire attached to it. The guy rod is attached to the anchor by means of a washer and nut.
Tree-Guying.--When neither poles nor guy stubs can be obtained to which to fasten wires, conveniently located trees may sometimes be used. Guy wires should not be attached to trees without permission of the owner or other proper authorities. Tree guys are preferably attached to tree trunks. When this is impossible, attachment may be made to a live sound limb, close to the tree trunk, provided the limb is not less than 8 inches in diameter.
Tree trunks and limbs should always be protected from injury by the use of tree-blocks between the tree and the wire attached to it. Tree-blocks should be of chestnut, and should be placed around a tree trunk or limb sufficiently close together to prevent the wire from touching it. To avoid injury to the tree, guy wires should not be wrapped continuously around it, but should simply pass around the tree, supported on blocks, as shown in Fig. 9.
Clearance.—Guys should be attached to poles so as to interfere as little as possible with workmen climbing or working thereon. Every guy which passes either over or under any electric wires other than those attached to the guyed pole should be so placed and maintained as to provide a clearance of not less than 24 inches between the guy and such electric wires, under all conditions of temperature and sag. As changes in temperature will affect the sag of the wires more than that of the guy, the latter being under strain, allowance must be made for this at the time the guy is installed.
Guy Insulation.—All guy wires attached to poles carrying electric light or power wires should be insulated by the insertion of at least one strain insulator. In the case of head guys and side guys, the insulator is boated at the upper end of the guy, and at least six feet from the pole, measured horizontally. Where any portion of the guy passes under electric light or power wires, other than those attached to the guyed pole, a second strain insulator is used, placed 6 feet from the lower end of the guy.
Guy wires which are attached to a non-insulating support, such as a rock, iron pole, bridge, or any other structure, and anchor guys, should have inserted in them two strain insulators, one being so placed as to protect a man working on the pole from coming in contact with that portion of the guy beyond the insulator, and the other placed so as to be out of reach from the ground or structure.
