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ELECTRIC POWER DISTRIBUTION EQUIMPENT AND SYSTEMS
2
Overhead L
ines
Along streets, alleys, through woods, and in backyards, many of the dis-
tribution lines that feed customers are overhead structures. Because over-
head lines are exposed to trees and animals, to wind and lightning, and to
cars and kites, they are a critical component in the reliability of distribution
circuits. This chapter discusses many of the key electrical considerations
of overhead lines: conductor characteristics, impedances, ampacity, and
other issues.
2.1 Typical Constructions
Overhead constructions come in a variety of configurations (see
Figure 2.1
)
.
Normally one primary circuit is used per pole, but utilities sometimes run
more than one circuit per structure. For a three-phase circuit, the most com-
mon structure is a horizontal layout with an 8- or 10-ft wood crossarm on a
pole (see
Figure 2.2
)
. Armless constructions are also widely found where
fiberglass insulator standoffs or post insulators are used in a tighter config-
uration. Utilities normally use 30- to 45-ft poles, set 6 to 8 ft deep. Vertical
construction is also occasionally used. Span lengths vary from 100 to 150 ft
in suburban areas to as much as 300 or 400 ft in rural areas.
Distribution circuits normally have an underbuilt neutral — the neutral
acts as a safety ground for equipment and provides a return path for
unbalanced loads and for line-to-ground faults. The neutral is 3 to 5 ft below
the phase conductors. Utilities in very high lightning areas may run the
neutral wire above the phase conductors to act as a shield wire. Some
utilities also run the neutral on the crossarm. Secondary circuits are often
run under the primary. The primary and the secondary may share the
neutral, or they may each have their own neutral. Many electric utilities
share their space with other utilities; telephone or cable television cables
may run under the electric secondary.
33
Copyright © 2006 Taylor & Francis Group, LLC
34
Electric Power Distribution Equipment and Systems
(a)
FIGURE 2.1
Example overhead distribution structures. (a) Three-phase 34.5-kV armless construction with
covered wire.
Copyright © 2006 Taylor & Francis Group, LLC
Overhead Lines
35
(b)
FIGURE 2.1
Continued. (b) Single-phase circuit, 7.2 kV line-to-ground.
Copyright © 2006 Taylor & Francis Group, LLC
36
Electric Power Distribution Equipment and Systems
(c)
FIGURE 2.1
Continued. (c) Single-phase, 4.8-kV circuit.
(d)
FIGURE 2.1
Continued. (d) 13.2-kV spacer cable.
Copyright © 2006 Taylor & Francis Group, LLC
Overhead Lines
37
4
3
−
8
3
−
8
4
1
−
6
4
8
2
−
0
9
Position of guy
Neutral
FIGURE 2.2
Example crossarm construction. (From [RUS 1728F-803, 1998].)
Wood is the main pole material, although steel, concrete, and fiberglass
are also used. Treated wood lasts a long time, is easy to climb and attach
equipment to, and also augments the insulation between the energized con-
ductors and ground. Conductors are primarily aluminum. Insulators are pin
type, post type, or suspension, either porcelain or polymer.
The National Electrical Safety Code (IEEE C2-2000) governs many of the
safety issues that play important roles in overhead design issues. Poles must
have space for crews to climb them and work safely in the air. All equipment
must have sufficient strength to stand up to “normal” operations. Conduc-
tors must carry their weight, the weight of any accumulated ice, plus with-
stand the wind pressure exerted on the wire. We are not going to discuss
mechanical and structural issues in this book. For more information, see the
Lineman’s and Cableman’s Handbook
(Kurtz et al., 1997), the
Mechanical Design
Manual for Overhead Distribution Lines
(RUS 160-2, 1982), the
(IEEE C2-
(Clapp, 1997).
Overhead construction can cost $10,000/mi to $250,000/mi, depending on
the circumstances. Some of the major variables are labor costs, how devel-
oped the land is, natural objects (including rocks in the ground and trees in
the way), whether the circuit is single or three phase, and how big the
conductors are. Suburban three-phase mains are typically about $60,000 to
$150,000/mi; single-phase laterals are often in the $40,000 to $75,000/mi
range. Construction is normally less expensive in rural areas; in urban areas,
crews must deal with traffic and set poles in concrete. As Willis (1997) notes,
upgrading a circuit normally costs more than building a new line. Typically
this work is done live: the old conductor has to be moved to standoff brackets
while the new conductor is strung, and the poles may have to be reinforced
to handle heavier conductors.
NESC Handbook
Copyright © 2006 Taylor & Francis Group, LLC
NESC
2000), and the
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