A utility pole, commonly referred to as a transmission pole, telephone pole, telecommunication pole, power pole, hydro pole, telegraph pole, or telegraph post, is a column or post used to support overhead power lines and various other public utilities, such as electrical cable, fiber optic cable, and related equipment such as transformers and street lights while depending on its application. They are used for two different types of power lines: sub transmission lines, which carry higher voltage power between substations, and distribution lines, which distribute lower voltage power to customers. Electrical wires and cables are routed overhead on utility poles as an inexpensive way to keep them insulated from the ground and out of the way of people and vehicles. Utility poles are usually made out of wood, aluminum alloy, metal, concrete, or composites like fiberglass. A Stobie pole is a multi-purpose pole made of two steel joists held apart by a slab of concrete in the middle, generally found in South Australia. The first poles were used in 1843 by telegraph pioneer William Fothergill Cooke, who used them on a line along the Great Western Railway. Utility poles were first used in the mid-19th century in America with telegraph systems, starting with Samuel Morse, who attempted to bury a line between Baltimore and Washington, D.C., but moved it above ground when this system proved faulty. Today, underground distribution lines are increasingly used as an alternative to utility poles in residential neighborhoods, due to poles' perceived ugliness, as well as safety concerns in areas with large amounts of snow or ice build up. They have also been suggested in areas prone to hurricanes and blizzards as a way to reduce power outages.
Use
Utility poles are commonly used to carry two types of electric power lines: distribution lines (or "feeders") and sub transmission lines. Distribution lines carry power from local substations to customers. They generally carry voltages from 4.6 to 33 kilovolts (kV) for distances up to 30 mi (50 km), and include transformers to step the voltage down from the primary voltage to the lower secondary voltage used by the customer. A service drop carries this lower voltage to the customer's premises. Subtransmission lines carry higher voltage power from regional substations to local substations. They usually carry 46 kV, 69 kV, or 115 kV for distances up to 60 mi (100 km). 230 kV lines are often supported on H-shaped towers made with two or three poles. Transmission lines carrying voltages of above 230 kV are usually not supported by poles, but by metal pylons (known as transmission towers in the US). For economic or practical reasons, such as to save space in urban areas, a distribution line is often carried on the same poles as a sub transmission line but mounted under the higher voltage lines; a practice called "underbuild". Telecommunication cables are usually carried on the same poles that support power lines; poles shared in this fashion are known as joint-use poles, but may have their own dedicated poles.
Description
The standard utility pole in the United States is about 35 ft (10 m) tall and is buried about 6 ft (2 m) in the ground. In order to meet clearance regulations, poles can, however, reach heights of at least 120 feet (40 meters). They are typically spaced about 125 ft (40 m) apart in urban areas, or about 300 ft (100 m) in rural areas, but distances vary widely based on terrain. Joint-use poles are usually owned by one utility, which leases space on it for other cables. In the United States, the National Electrical Safety Code, published by the Institute of Electrical and Electronics Engineers (IEEE) (not to be confused with the National Electrical Code published by the National Fire Protection Association [NFPA]), sets the standards for construction and maintenance of utility poles and their equipment.
Pole materials Most utility poles are made of wood, pressure-treated with some type of preservative for protection against rot, fungi and insects. Southern yellow pine is the most widely used species in the United States; however, many species of long straight trees are used to make utility poles, including Douglas fir, jack pine, lodgepole pine, western red cedar, and Pacific silver fir. Traditionally, the preservative used was creosote, but due to environmental concerns, alternatives such as pentachlorophenol, copper naphthenate and borates are becoming widespread in the United States. In the United States, standards for wood preservative materials and wood preservation processes, along with test criteria, are set by ANSI, ASTM, and American Wood Protection Association (AWPA) specifications. Despite the preservatives, wood poles decay and have a life of approximately 25 to 50 years depending on climate and soil conditions, therefore requiring regular inspection and remedial preservative treatments. Woodpecker damage to wood poles is the most significant cause of pole deterioration in some parts of the U.S. Other common utility pole materials are aluminum, steel and concrete, with composites (such as fiberglass) also becoming more prevalent. One particular patented utility pole variant used in Australia is the Stobie pole, made up of two vertical steel posts with a slab of concrete between them.
Power distribution wires and equipment
On poles carrying both electrical and communications wiring, the electric power distribution lines and associated equipment are mounted at the top of the pole above the communication cables, for safety. The vertical space on the pole reserved for this equipment is called the supply space. The wires themselves are usually uninsulated, and supported by insulators, commonly mounted on a horizontal beam (crossarm). Power is transmitted using the three-phase system, with three wires, or phases, labeled "A", "B", and "C". Sub transmission lines comprise only these 3 wires, plus sometimes an overhead ground wire (OGW), also called a "static line" or a "neutral", suspended above them. The OGW acts like a lightning rod, providing a low resistance path to ground thus protecting the phase conductors from lightning.
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