Quantcast Figure 2. National Electrical Safety Code district map for ice loads

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Figure 2. National Electrical Safety Code district map for ice loads: Light 0
in., Medium 0.25 in., Heavy 0.5 in. (NESC 1990).
ances by towers, poles, crossarms, and guys. In power lines, the wires and conductors are isolat-
ed electrically from the support structures by single post-type insulators in the low-voltage distri-
bution lines that typically run along streets, or insulator strings in high-voltage transmission
lines. The NESC provides design ice- and wind-load maps; however, design-load requirements
vary depending on the type of service and the grade of construction. The country is divided into
three districts for ice loads: Heavy, Medium, and Light (Fig. 2). In the Heavy loading district,
encompassing the region in the United States that was hit by the January ice storm, the uniform
ice thickness is 0.5 in. In the Medium and Light districts the uniform ice thicknesses are 0.25 in.
and 0 in., respectively. The wind pressures for these districts are 4, 4, and 9 lb/ft2, respectively. A
factor is applied to the basic ice loads that is as high as 1.5 for major transmission lines. Many
transmission line designers and utilities recognize that the ice-load district map in NESC shows
loads that occur more or less frequently, rather than extreme loads. This is in contrast to the
wind-speed map in NESC that shows fastest-mile wind speeds for a 50-year return period, taken
from the 1988 revision of Minimum Design Loads for Buildings and Other Structures (ASCE 7-
88). Thus, on the coast of Maine, for example, utilities are required to design transmission
lines to withstand hurricane winds with a 2% probability of exceedance in any year (equivalent
to a 50-year return period), but they are not required to design to get the same reliability in their
overhead lines when they are subjected to ice loads that occur as often. Because of this discrep-
ancy, some utilities have developed their own more stringent design requirements for ice loads.
The ice-load map that was developed for Guidelines for Electrical Transmission Line Struc-
tural Loading (ASCE 74) attempted to provide transmission line designers with 50-year return-
period loads. These loads, which are also specified in terms of a uniform ice thickness, range
from 0 in the extreme southern United States to 1.6 or 2.2 in. in the region hit by the January ice
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