Route Surveys for Overhead Electrical Distribution
and Transmission Lines
The reconnaissance survey for electrical power lines employs many of the same principles and practices that you studied for highway work; however, the design considerations are different. For a power line, the design engineer considers principles that you studied in chapter 2 of this TRAMAN to select one or more tentative routes over which the line will pass. For convenience, those principles are listed as follows:
1. Select the shortest possible route.
2. Follow the highways and roads as much as possible.
3. Follow the farmer’s property or section lines.
4. Route in the direction of possible future loads.
5. Avoid going over hills, ridges, swamps, and bottom lands.
6. Avoid disrupting the environment.
During the reconnaissance phase, you should first study all available maps of the area to gain a general understanding of the landscape. If a portion of the line is off the military installation, determine the owner-ship of the lands through which the line will pass. That is necessary to obtain permission to run the line. Look for any existing utilities that may already exist in the area. If there are existing utilities, then look for existing utilities maps. Visit the area to examine the terrain and look for any natural or man-made features that may hinder or help the construction. In short, gather all information that the engineer will need to select one or more general routes for the power line.
With the tentative route or routes selected, you are ready to conduct a preliminary survey from which a map is prepared showing the country over which the line will pass. Since the final location is not known, a wide strip of land needs to be mapped. When running the preliminary survey, incorporate all pertinent topographic information into the field notes. Note particularly any existing overhead or underground lines and indicate whether they are power or communications lines. Locate such features as hills, ridges, marshes, streams, forests, roads, railways, power plants, buildings, and adjacent military camps or bases.
When the preliminary mapping is completed, the engineer selects the final route. Again, the engineer considers the principles listed above to select the route.
POLE LINE SURVEYS.—When the route has been selected, a plan and profile are plotted. The plan shows the route the line will follow and the significant topography adjacent to the route. The profile shows the ground elevation along the line and the top eleva-tions of the poles. These elevations are set in accord-ance with minimum allowable clearances specified in the National Electrical Safety Code (NESC), ANSI C2, and the most recent edition of the National Electrical Code ® (NEC ®).
For distribution lines, poles should be placed on the side of the street that is most free of other lines and trees. Try to keep off the main streets. As much as possible, you should use the same side of the road throughout the length of the line. For straight portions of lines, the usual spacing between poles is about 125 feet (100 feet minimum and 150 feet maximum); however, to make the poles come in line with property lines or fences, the span length may need to be adjusted. The engineer will determine the spans. Along roads, poles should be placed 2 feet from the inside edge of the curb or 2 feet from the edge of the road surface where curbs do not exist. On openroadways or highways, poles should be set 18 inches from the outside of fences.
For transmission lines, poles should be located in high places so that shorter poles can be used and still maintain the proper ground clearance at the middle of the span. Avoid locating poles along the edge of embankments or streams where washouts can be expected. In rolling country, the grading of the line should be considered when determining pole locations. A well-graded line does not have any abrupt changes up or down the line and will appear nearly horizontal regardless of small changes in ground level. Sometimes, by shifting a pole location a few feet, a standard length pole can be used where otherwise an odd-sized pole would be needed. In addition, transmission line poles should be located at least 2 feet from curbs, 3 feet from fire hydrants, 12 feet from the nearest track of a railroad track, and 7 feet from railway sidings.
When you are staking pole locations, the center ofeach pole is marked with a hub on the line; the hub may be offset. On the guard stake, you put the pole number, the line elevation, and the distance from the top of the hub to the top of the pole obtained from the profile.
TOWER LINE SURVEYS.—High-voltage lines are often supported by broad-based steel towers. For a tower line, construction economy requires that changes in direction be kept at a minimum. That is because a tower located where a line changes direction must withstand a higher stress than one located in a straight direction part of the line. In general, tower construction is cheaper in level country than in broken country; however, the line may be run over broken country to minimize changes in direction, to make the distance shorter, or to follow a line where the cost of obtaining right-of-way is inexpensive. Lines should be located adjacent to existing roads, whenever practical, to provide easier access for construction and future maintenance. When a change in direction in a tower line is unavoidable, it should be made gradually in as small-angular increments as possible. Suppose, for example, a change in direction of 90° is required. Instead of an abrupt change in direction of 90°, towers should be set so as to cause the line to follow a gradual curve in a succession of chords around an arc of 90°.