CHAPTER TX 



TKEE-LINE CONSTEUCTION 

 Section 58 General Principles 



With a telephone line passing through a heavy stand of timber, the possibility of 

 trees falling across the line makes it essential that the line wire should be able to give 

 way without breaking. The rigidity so necessary for standard pole-line construction 

 is therefore undesirable and even inimical to proper tree-line construction. 



The. tree-line method is generally used when poles are scarce or inaccessible, when 

 ground conditions are unsuitable to the setting and maintenance of poles, or where 

 there is not enough money available for the construction and maintenance of a 

 standard pole-line. 



The essential features of the tree-line method are the use of the split insulator 

 and a suspending wire support. The former allows the line wire to draw through it 

 when there is a pull from one side; the latter permits the wire to be attached 'to the 

 tree itself. 



Metallic-circuit tree lines are seldom used. Short circuits, resulting from broken 

 wire ties and from trees falling across the lines, make this method of construction im- 

 practicable under ordinary conditions. Where the danger from high-voltage trans- 

 mission lines makes necessary the use of a metallic circuit, but other conditions make 

 the use of a tree line desirable, the wires should be strung on separate trees and the 

 standard methods of transposition followed. Under no circumstances may a metallic 

 tree line be undertaken without authorization by the inspector. 



Section 59 Selecting Trees and Route 



Judgment and care should be used in selecting the trees to support the line, and 

 also in determining the tying place on each tree and the method of tying. Only sound 

 trees should be selected, of sufficient diameter to minimize the swaying, but large and 

 smooth tret^i that are difficult to climb should be avoided. The course of the line 

 should be varied to take advantage of trees that will lessen the cost of construction; 

 but if suitable trees are not available, poles should be used. 



In tree lines the spans should not exceed 175 feet. If possible the average span 

 should approximate 100 feet, and may be shorter if necessary. The span should be 

 equalized, that is adjacent spans should be as nearly of equal length as possible and 

 no abrupt changes in length of spans should be made The crooks and turns of the 

 trail should not be followed unless to do so would mean more economical and better 

 construction. The line wire should never touch the trunk of a tree, and care should 

 be taken when attaching the insulators to see that the pull of the wire is away from 

 the tree and not against it. On side-hill slopes the line will be strung, if practicable, 

 below the trail, so that in case the wire comes down it will not fall on the trail. 

 Wherever possible avoid crossing the trail. 



In selecting trees for ties two systems of alignment are followed both of which 

 give good results. In the first system (Fig. 30) the effort is made to select the trees so 

 that they follow a regular zigzag course, each tree being on the opposite side of the 

 right of way from the two adjacent ones. The amount of pull between adjacent trees 

 on the same side of the right of way should be 6 to 8 feet. The wire then forms a 

 zigzag over the cleared right of way and pulls away evenly from each tie tree. The 

 insulator, of course, is attached on the inner, or concave, side of each crook in the line. 



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