The efficient location of structures on the profile is an important component of line design. Structures of appropriate height and strength must be located to provide adequate conductor ground clearance and minimum cost. In the past, most tower spotting has been done manually, using templates, but several computer programs have been available for a number of years for the same purpose.
Manual Tower Spotting
A celluloid template, shaped to the form of the suspended conductor, is used to scale the distance from the conductor to the ground and to adjust structure locations and heights to (1) provide proper clearance to the ground; (2) equalize spans; and (3) grade the line.
The template is cut as a parabola on the maximum sag (usually at 49#C) of the ruling span and should be extended by computing the sag as proportional to the square of the span for spans both shorter and longer than the ruling span. By extending the template to a span of several thousand feet, clearances may be scaled on steep hillsides.
The form of the template is based on the fact that, at the time when the conductor is erected, the horizontal tensions must be equal in all spans of every length, both level and inclined, if the insulators hang plumb. This is still very nearly true at the maximum temperature.
The template, therefore, must be cut to a catenary or, approximately, a parabola. The parabola is accurate to within about one-half of 1% for sags up to 5% of the span, which is well within the necessary refinement.
Since vertical ground clearances are being established, the 49#C no-wind curve is used in the template. Special conditions may call for clearance checks. For example, if it is known that a line will have high temperature rise because of load current, conductor clearance should be checked for the estimated maximum conductor temperature.
One crossing over a navigable stream was designed for 88#C at high water. Ice and wet snow many times cause weights several times that of the 1/2-in radial ice loading, and conductors have been known to sag to within reach of the ground.
Such occurrences are not normally considered in line design, and when they occur, the line is taken out of service until the ice or snow drops. Checks made afterward have nearly always shown no permanent deformation.