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.
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.
