In this post I will discuss about one of the important parts of electrical power transmission and distribution and it is about the “Overhead Line”. I already postdated couple of article about electrical transmission and distribution. If you want you can have a look on it. Now come to the point. At first what is the “Overhead Power Line?”
Generally an overhead electrical power line is a structure and it use for electric power transmission and distribution system and it transmit electrical energy or power for large distances from the generation center or plant.
Overhead Electric Line Construction With Details:
Along streets, alleys, through the woods and in backyards you may find many of the distribution lines that feed customers and your home by overhead structures. Generally electric overhead lines use to stay exposed to wind and lightning, to trees and animals and to cars and kites.
For this reason it is one of the critical components in the reliability of distribution circuits system.
Normally electric overhead constructions come in a variety of configurations. Generally one primary circuit is used per pole but utilities sometimes run more than one circuit per structure. For a typical three phase circuit the most common structure is the horizontal layout with an 8 or 10 ft wooden crossarm on an electric pole.
But armless constructions can also found where fiberglass insulator standoffs or the post insulators are used in a tighter configuration. Utilities generally use 30 ft to 45 ft poles and set 6 ft to 8 ft deep. Vertical construction is also occasionally used in some area for special reason.
Span lengths can be varying from 100 ft to 150 ft in suburban areas and 300 ft or 400 ft in rural areas. Distribution circuits generally have an under built neutral system and the neutral use to acts as a safety ground for the equipment and provides a return path for any unbalanced loads and due to line to ground faults.
The neutral is use to stay 3 ft to 5 ft below from the phase conductors. But utilities in very high lightning areas the neutral wire can stay above the phase conductors to act as a shield wire to protect for lightning. Some utilities system also runs the neutral on the crossarm.
Secondary circuits use to run under the primary circuit. But the primary and the secondary circuit system may share the neutral or they can have individual own neutral system.
Sometime many electric utilities share their space with other utilities like telephone or cable television cables, internet cable may run under the electric secondary system.
Generally wood is the main pole material although steel, concrete and fiberglass are also used for making electric pole. But treated wood lasts a long time and it’s easy to climb and attach equipment with it and also augments the insulation between the energized conductors and ground.
Conductors are primarily aluminum metal. But insulators can be 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. Electric poles must have space for crews to climb them and work safely in the air.
All equipment must have to stay sufficient strength to stand up to normal operations. Conductors must carry their own weight and the weight of any accumulated ice plus withstand the wind pressure exerted on the wire.
Overhead line construction can cost $10,000/mi to $250,000/mi, depending on the circumstances like area, load type, distance and others. But one of the major variables is labor costs and how developed the land and natural objects including rocks in the ground and trees in the way.
Then the circuit system is single or three phase and how long the conductors are. Suburban three phase mains are typically cost about $60,000 to $150,000/mi or may be more and for single phase laterals are often in the $40,000 to $75,000/mi range (Approximate).
But line construction is usually less expensive in rural areas but in urban areas crews must deal with traffic and set poles in concrete. Typically this work is done live and the old conductor has to be moved to standoff brackets while the new conductor is strung but the poles may have to be reinforced to handle heavier conductors for future life.