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PRIMARY COMPONENTS OF AN OVERHEAD TRANSMISSION LINE

The primary components of an overhead transmission line are:
  1. Conductors
  2. Ground or shield wires
  3. Insulators
  4. Support structures
  5. Land or right-of-way (R-O-W)
1) CONDUCTORS: are the wires through which the electricity passes. Transmission wires are usually of the aluminum conductor steel reinforced (ACSR) type, made of stranded aluminum woven around a core of stranded steel that provides structural strength. When there are two or more of these wires per phase, they are called bundled conductors.

2) GROUND OR SHIELD WIRES: are wires strung from the top of one transmission tower to the next, over the transmission line. Their function is to shield the transmission line from lightning strikes.



3) INSULATORS: are made of materials which do not permit the flow of electricity. They are used to attach the energized conductors to the supporting structures, which are grounded. The higher the voltage at which the line operates, the longer the insulator strings. In recent years, polymer insulators have become popular in place of the older, porcelain variety. They have the advantage of not shattering if struck by a projectile.

4) SUPPORT STRUCTURE: The most common form of support structure for transmission lines is a steel lattice tower, although wood H frames (so named because of their shape) and steel poles are also used. In recent years, as concern about the visual impact of these structures has increased, tubular steel towers also have come into use. The primary purpose of the support structure is to maintain the electricity-carrying conductors at a safe distance from the ground and from each other. Higher voltage transmission lines require greater distances between phases and from the conductors to ground than lower voltage lines and, therefore, they require bigger towers. The clearance from ground of the transmission line is usually determined at the midpoint between two successive towers, at the low point of the centenary formed by the line.

5) LAND OR RIGHT-OF-WAY: The land that the tower line transverses is called the right-of way (R-O-W). To maintain adequate clearances, as the transmission voltage increases, R-O-W widths also increase. In areas where it is difficult to obtain R-O-Ws, utilities design their towers to carry multiple circuits. In many areas of the country, it is not uncommon to see a structure supporting two transmission lines and one or more sub transmission or distribution lines. There are different philosophies on the selection of R-O-Ws.

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