Publication: The Journal of Electricity
San Francisco, CA, United States
THE CONSTRUCTION OF TRANSMISSION LINES.
BY JOHN S. EASTWOOD.
(A paper read before the Santa Cruz Convention of the Pacific Coast Electric Transmission Association, August 17, 1897.)
In presenting the following elementary notes, I beg the indulgence of the members of this association for, while there may be nothing new in this paper to those who have built transmission lines and whose experiences may exceed that of the writer in all directions, if in any way I shall have added to the practical literature on the subject and shall have given information to those contemplating new work, I shall feel amply repaid.
When the plans for a transmission plant are under consideration, it may be taken as a rule that ordinarily the plans and specifications of the pole line will receive less consideration than those for any other part of the plant. This is due largely to the fact that the hydraulic and electrical equipments are accompanied by the plans and specifications of the engineers of the manufacturing companies, the apparatus being guaranteed as to workmanship and efficiency, while the pole line material, being usually furnished by a number of different parties, is frequently thrown together haphazard under the direction of a lineman and a carpenter. This laxness is largely due to the fact that literature bearing on line construction practices are extremely meager.
The modern electric transmission line is an evolution of the telegraph line and the later telephone and electric lighting distribution lines, and the tendency has been to follow the loose methods and bad workmanship of the earlier constructions. Like any other part of an electrical transmission plant, the pole line should be designed to fit local conditions, but there remains general paints that should be carried out in all lines. In the earlier discussions of the problems of power transmission the great question of doubt was in the maintenance of uninterrupted service on the line. The general consensus of opinion was that the dangers a interruption would be in direct proportion to the length of the line, but in practice this feature of transmission work will depend, to a very large degree, oil the character of the line constructions. The specifications of an electrical transmission line should be based on the same sound engineering principles as those of any other permanent work, and the line should be so designed as to cost the least for maintenance, coupled with economic cost of construction. Good engineering consists of good judgment, mingled with technical knowledge, and good engineering work consists of construction wherein all the factors bearing on the case are given their proper value in the resulting equation and the results are embodied in a work costing the least money for construction consistent with the most economical maintenance.
The general features entering into the problem of line engineering are line location, line materials and line construction. In weighing these factors, they should be considered in their relation to the prime idea of electrical transmission, namely, permanent continuity of service.
Consider first the factor of line location, one of the most important features of which is that of the alignment. A general rule that is wise to follow when at all possible, is one similar to that laid down by the Czar, who, when asked to locate the route for a rail- way from St. Petersburg to Moscow, did so by drawing on the map with a rule a straight line between the two cities.
The factors that enter into the problem of location to prevent straight alignment are found in such impassable barriers as very wide streams, lakes, inaccessible canons and mountains, and, in many cases, the difficulty and expense of obtaining rights of way. In computing the relative value of these factors, they must be weighed by a consideration of the additional cost of line, the additional cost of maintaining, the additional drop in the voltage of the line by reason of length added, the additional cost-of angles and of the added line, and the cost of the right-of-way. Another feature of importance brought out by the experience with the San Joaquin Electric Company's lines is that the danger of malicious interruption is very largely decreased by having lines remote from public highways. Our line has its general direction of south 30 degrees west, and we found that we could much better pay $250 per mile for right-of- way than to take a free right-of-way on the section line around the lands in question, even when counting in only the additional cost of the line and leaving out the features of added drop, angles and maintenance.
In the location of the line over rolling and hilly ground there is very little to be gained by one location over another, and as a rule it is better to keep as straight as possible, regardless of the character of the ground. On very rough ground there is a slightly increased cost to be added for the delivery of line material in difficult places, but this cost will be more than offset by the shortening of the line. Measurements for line work should be taken- on the slope of the ground as that represents the true distance. In running over undulating ground, it will be found that the upward pull will occur on very few of the poles as the hill slopes are almost all vertical curves, giving a downward pull; the upward pull occurring on one or two poles at the ravine crossings. All sharp, horizontal angles should be avoided in locating transmission lines. If necessary to turn a corner at right angles. the angles should be broken in two or three parts and the poles set close together to take up the additional strain.
As to the next question, the selection of line material is one of vital importance in line construction, :Is upon it. more than any other feat ore depends the life of the line and, consequently, the cost of maintenance. A wise rule to follow in this regard is that the best material obtainable is none too good. The vital element in the life of the line is in the selection of the poles. We of the Pacific Coast are exceedingly fortunate in having an abundant variety of excellent materials for poles from which to choose. In selecting our poles the central idea was that of making the most permanent line and after many years of close observation the conclusion has been reached that the sap-wood of any variety of timber is absolutely worthless as to its lasting qualities in the ground, and that its use is a menace to any transmission line in that it is liable to be counted on for strength; which disappears in a few years, and besides, the decaying portion leaves the pole loose in the ground, subjecting it to greater strains and leading to the decay of the more resistant heart-wood. Having seen round cedar telegraph poles along the line of the Southern Pacific railway that have rotted entirely off after having been in the ground for twelve years and having seen sawed redwood poles that were still sound after standing in the ground for forty years, it did not take long to reach the decision to use sawed redwood poles, especially as the price was nearly alike on equal sizes. As the poles for most lines are ordered but a short time before being put to use and are delivered direct from the woods, the use of tar on the butts is of very doubtful value, as applying it at that time encloses all the sap contained in the green wood and moreover the tar will not penetrate the wood to preserve it. A much better procedure to set the pole in the line and after a couple of years of seasoning to dig away two feet or so of earth, then clean the pole and apply the tar hot, during the dry season. This method will afford ample protection, as the decay is greatest at or near the surface and for about two feet downward.
It is well to bear in mind that when a pole becomes rotten there are no means available of ascertaining its exact condition and it will surely give way at some time of unusual strain when least expected. Were it possible to determine the condition of a decaying pole it might be replaced by a new one without interruption of service, but the great difficulty is that it is impossible to locate the weakness until it has manifested itself in a breakdown. In the matter of the size and length of poles that should be used, the ultimate weight of the line, the length of spans, and the wind strains will determine the size to be required for safety, while the length is a matter that is determined principally by franchise requirements. As to the height of wires, the high line has the advantage of reducing earth induction and of avoiding malicious disturbance, but, on the other hand, it possesses the disadvantage of increased leverage on the butts of the poles and of increased cost.
The cross arms should be made of the toughest timber available, to avoid splitting and not warp. They should be painted with lead and oil paint to prevent weathering and cracking, which is a weak point cause of the holes they contain and of their being subject to constantly changing stains. The cross arms of the San Joaquin Electric Company are made of Oregon pine, 4x5 inches, dressed and rounded on the upper side with holes for six pins and for the pole and brace bolts. Bore only such holes for pins as are needed, as each hole is a source of weakness. One great defect in putting on cross arias is that usually the bolt washers are too small and crush into the wood, weakening the arm. The strains on cross arms are mainly due to the tortional strain on the pin, which generally exceeds the weight to be borne, hence cross arms should be nearly square ill section. Braces of 1x1-3/4 inch iron, bent to give a square seat on the cross arm and poles, bolted with a 3/8 inch bolt through the cross aria and lag bolted to the pole, have proven very effective.
The question of pins and insulators is the most intricate problem of line construction. Pins are a mechanical device which can be made to meet the requirement of furnishing a stable support for the insulator. They should have a stein that will not pull out of the cross arm, a base or shoulder to resist-lateral strain and a thread to securely hold the insulator. These conditions are best met by a composite pin consisting of a bolt passed through a wooden-threaded pin with a broad base, or, which is better, a lead thread on a bolt supported on a metal base and tightened by a nut and broad washer under the cross arm. One of the great objections to wooden pins is that in order to give them strength, a very large hole must be bored through the cross arm, which weakens it, and that during the dry season the pins shrink and loosen while during the wet season they swell and crack the cross arms, hence the great advantage of a bolt lies in the fact that it possesses neither of these defects and always remains in place. Pins should never be made so that the threaded portion touches the insulator, as it is liable to expand and split it.
The insulator is the most important feature of line construction and is now receiving universal attention. The experience of the few years of high-tension transmission have taught that it is only the mixture of insulators and water that has given special trouble. Almost any type of insulator will make a fair weather insulator, but fogs and storms and the wettest kind of weather must be prepared for, and manifestly the type of insulator that will absolutely insure a dry gap between the lines and the ground under all possible climatic conditions, is the one being sought for. Great stress has been laid on points in various insulators concerning the amount of insulated surface the current would have to travel over to reach the pins, but the vital point of imperviousness to water has been, to a great extent, overlooked. In California we have all the conditions of weather that exist anywhere and in some localities its climatic conditions are worse, so far as long duration is concerned, than in the western climates, and especially is this so in the occurrence of daily and long continued sea and land fogs. The portion of our lines on the plains were submerged in fog for fifteen consecutive days and nights last winter, ending with a diverging rain. These fogs drifted slowly along the ground and condensed on all intervening objects, so that everything that would absorb moisture, became thoroughly water-logged on the side facing the breeze, though they sometimes remained comparatively dry on the leeward side.
Viewing the problem in this practical way, leads to the point of determining the question of insulator design from a mechanical standpoint. The insulator must be impervious to water, as it must act as a water shied or roof to keep the body of the insulator dry. It must, therefore, have a dry gap between the roof and the pin and it must be of sufficient mechanical strength to resist the strains to which the line subjects it.
Other details of construction require careful consideration. The right-of-way should be cleared of all brush and grass at least ten feet on each side of the line and all trees of whatever kind that might reach the line in falling should be cut down. The right-of-way should be at least twenty feet in width so as to give ample room for construction, maintenance, and the protection of the line from fires. The distance between poles must be determined from the topography of the country, from the size of the poles and the weight to be carried on them as well as from other strains. The excavation of pale holes forms an important element in the cost of line construction and 11w method to be pursued depends upon the character of the ground. A large proportion of the country crossed by the lines of the San Joaquin Electric Company is of hard or soft granite, rock, or hard pan overlaid by a shallow bed of earth and the use of giant powder proved to be the cheapest means of loosening both rock and earth. Rock was loosened by two shots of giant powder while in earth surface, the top was removed by ordinary means and the bottom loosened by a single shot, consisting of a stick or a portion of a stick of giant powder put into the bottom of a hole drilled by means of a churn drill operated by two men.
It is difficult to get insulators erected properly on wooden pins, as the pins must be nailed in during dry weather and insulators with a single groove for the line wire will often lack a quarter of a turn when fully screwed in, of being in alignment, in which case the lineman will turn the insulator back until the groove is in line, making the insulator loose on the pin. The San Joaquin Electric Company has had as much line trouble from this one cause as from all others combined, and while it does not often short circuit the line, it necessitates the shutting down of one line to reset the insulator. Obviously, if bolted pins are used the insulator can be screwed down tight and the nut of the pin tightened when the insulator is in alignment.