ELECTRIC DISTRIBUTION AND WIRING. 2/9 



Stresses in the wire. In stringing a wire on poles two things 

 in particular should be provided for, namely, (a) an approximate 

 equality of wire tension on the two sides of each insulator, and 

 (b) a certain maximum tension in the wire when it is shortened 

 by the coldest winter weather. 



The first condition is desirable not only because it relieves the 

 pins, cross-arms, and poles from unnecessary stress, but also 

 because it is difficult to tie a line wire to an insulator so that it 

 cannot slip lengthwise through the tie, unless the line wire is bent 

 which it should not be if it can be avoided. The horizontal com- 

 ponents of the wire tension can always be made equal on the 

 two sides of an insulator ; but in the case of a pole line on a 

 grade the vertical component of the wire tension will be somewhat 

 greater on the down-hill side of an insulator when the horizontal 

 components are equal. 



The second condition is explained in the following discussion. 



Pole line on a level. The calculation of the tension in a span of wire in terms 

 of length of span, vertical sag at the center of the span, and weight of the wire, or 

 the calculation of the sag corresponding to a prescribed tension, is based upon the 

 equation to the curve formed by the wire. When the sag is a small fraction of the 

 length of the span, say one twentieth or less, the curve formed by the wire is sensibly 

 a parabola and the working formulae are : 



*= <> 



and 



'-/+? (40 



in which T is the tension of the wire in pounds, / is the length of the span in feet, h 

 is the sag at the center of the span as shown in Fig. 161, s is the length in feet of the 

 wire in a span, and w is the weight of the wire in pounds per foot. Equation (39) 

 gives the tension of the wire at the center of the span. The tension at the ends of 

 the span is wh pounds greater than at the center ; but this difference amounts to only 

 2 per cent when the sag is one twentieth of the length of the span, and it is always 

 negligible. The important use of equation (40) is in making allowance for the 

 effects of changes of temperature. 



Equation (39) when solved for / gives : 



where T f represents the maximum safe tension of the wire in pounds, which is equal 

 to the breaking tension 7& in pounds divided by the factor of safety. See following 



