The
mechanical design of the distribution system, and its several parts, must not
only be adequate to sustain the normal stresses and strains, but must safely
sustain them during abnormal conditions brought about by the vagaries of nature
and people. While design criteria for overhead systems are substantially
different from those for underground systems, in both instances prudent design
takes into account economic and other nontechnical considerations.
For overhead
systems, the supports for the conductors and equipment must withstand the
forces imposed on them, while the conductors themselves must be sufficiently
strong to support their own weight and the forces imposed on them.
National
Electric Safety Code (NESC)
Minimum
design criteria are suggested in the National Electric Safety Code (NESC)
issued by the Institute of Electrical and Electronics Engineers (IEEE). The
NESC has received wide acceptance by utilities and other industries in this
country and elsewhere.
In general,
the code specifies:
1.
Clearances between conductors and surrounding structures for different operating
voltages and under different local conditions
2. Strength
of materials and safety factors used in proposed structures
3. Perhaps
the most basic, the probable loading imposed on the conductors and structures
based on climatic conditions, approximately defined by geographic areas of
offsets and bends in the lines, and of the pressure of wind blowing against
them. Both vertical and horizontal loadings include the effects of ice
collecting radially about the conductors.
The vertical
force on the pole is the dead weight of the conductors with their coatings of
ice, cross arms, insulators, and associated hardware. This vertical force
exerts a compressive stress that may be considered uniformly distributed over
the cross section of the pole.
This loading,
however, is almost always overshadowed by the requirements of the horizontal
loadings, and is usually not given further attention. Even a very light pole
can safely carry the dead weight of a multicircuit, large-conductor line.
The NESC
divides the country into heavy, medium, and light load areas. The heavy loading
area comprises roughly the northeast quarter of the “lower 48” states, and
Alaska; the medium loading area comprises the northwest quarter plus a strip
across the middle of the country; the light loading area comprises California
and all the southern part of the country to a depth of some 300 to 400 mi, and
Hawaii.
The degrees of loading are indicated in Table
5-1, and the geographic areas for the continental United States in the map
shown in Figure 5-1. The values and areas of demarcation are approximate and
should be subject to other practical considerations, including probable
deviations based on actual experience, local codes and regulations, and other
environmental requirements.
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