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Example overhead distribution structures
FIGURE 2.1 - Example overhead distribution structures. (a) Three-phase 34.5-kV armless construction with covered wire.

Along streets, alleys, through woods, and in backyards, many of the distribution lines that feed customers are overhead structures.

Because overhead 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. 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 common 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 configuration.

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.

(b) Single-phase circuit, 7.2 kV line-to-ground
FIGURE 2.1 Continued. (b) Single-phase circuit, 7.2 kV line-to-ground
FIGURE 2.1 Continued. (c) Single-phase, 4.8-kV circuit
FIGURE 2.1 Continued. (c) Single-phase, 4.8-kV circuit


FIGURE 2.1 Continued. (d) 13.2-kV spacer cable
FIGURE 2.1 Continued. (d) 13.2-kV spacer cable

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 conductors and ground.

Conductors are primarily aluminum. Insulators are pin type, post type, or suspension, either porcelain or polymer.

FIGURE 2.2 - Example crossarm construction.
FIGURE 2.2 - Example crossarm construction.

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. Conductors must carry their weight, the weight of any accumulated ice, plus withstand the wind pressure exerted on the wire. We are not going to discuss mechanical and structural issues in this book.

Overhead construction can cost $10,000/mi to $250,000/mi, depending on the circumstances. Some of the major variables are labor costs, how developed 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.

SOURCE: El. Power Distribution Equipment and Systems – A. Short

About Author //

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Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears.Professional in AutoCAD programming and web-design.Present on

8 Comments

  1. […] the widespread manufacturing capacity and cost effectiveness.For all but local distribution, copper-based overhead lines are more costly because of the copper conductor material costs. Copper (BS 7884 applies) has a very […]

  2. […] ConstructionWhere general appearance, economics, congestion, or maintenance conditions make overhead construction inadvisable, underground construction is specified.While overhead lines have been ordinarily […]

  3. […] shown in table below are chosen to match the requirements of the high-voltage transmission grid of overhead lines.Table – Technical Data for 420kV and 550kV GIL Transmission NetworksTypeValueNominal voltage […]

  4. […] Meter40 Meter132KV40 Meter50 Meter230KV50 Meter60 Meter400KV60 Meter60 Meter Distance between overhead lines to gas pipelines at intersectionsVoltage KVMin. Distance20KV8 Meter63KV9 Meter132KV10 Meter230KV11 […]

  5. […] conductor between support points under all wind, ice, and temperature conditions must be known.Bare overhead transmission or distribution conductors are typically quite flexible and uniform in weight along their length. […]

  6. […] platforms in alleyways, or alongside buildings, or on ground-level pads protected by fencing.Overhead construction is still the most economical choice in rural areas, but it has the disadvantage of susceptibility […]

  7. […] produces radio noise and in lesser extent television ( TV ) disturbances around high-voltage transmission lines. This can be easily observed by all of us when we drive under a high-voltage line. The radio […]

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