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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, optical fiber, and related equipment such as transformers and 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, Aluminium 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.
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 . 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.
Traditionally, the preservative used was creosote, but due to environmental concerns, alternatives such as pentachlorophenol, copper naphthenic acid and 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 utility pole variant used in Australia is the Stobie pole, made up of two vertical steel posts with a slab of concrete between them.
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.
Distribution lines use two systems, either grounded-wye ("Y" on Circuit diagram) or delta (Greek letter "Δ" on electrical schematics). A delta system requires only a conductor for each of the three phases. A grounded-wye system requires a fourth conductor, the neutral, whose source is the center of the "Y" and is grounded. However, "spur lines" branching off the main line to provide power to side streets often carry only one or two phase wires, plus the neutral. A wide range of standard distribution voltages are used, from 2,400 V to 34,500 V. On poles near a service drop, there is a pole-mounted step-down distribution transformer to transform the high distribution voltage to the lower secondary voltage provided to the customer. In North America, service drops provide 240/120 V split-phase power for residential and light commercial service, using cylindrical single-phase transformers. In Europe and most other countries, 230 V three phase (230Y400) service drops are used. The transformer's primary is connected to the distribution line through protective devices called . In the event of an overload, the fuse melts and the device pivots open to provide a visual indication of the problem. They can also be opened manually, usually by linemen using a long insulated rod called a hot stick to disconnect the transformer from the line.
The pole may be grounded with a heavy bare copper or copper-clad steel wire running down the pole, attached to the metal pin supporting each insulator, and at the bottom connected to a metal rod driven into the ground. Some countries ground every pole while others only ground every fifth pole and any pole with a transformer on it. This provides a path for leakage currents across the surface of the insulators to get to ground, preventing the current from flowing through the wooden pole which could cause a fire or shock hazard. It provides similar protection in case of and lightning strikes. A surge arrester or lightning arrester may also be installed between the line (ahead of the cutout) and the ground wire for lightning protection. The purpose of the device is to conduct extremely high voltages present on the line directly to ground.
If Electrical fault due to wind or fallen trees, the resultant sparks can start . To reduce this problem, aerial bundled conductors are being introduced.
The most common communication cables found on utility poles are copper or fibre-optic cable (FOC) for telephone lines and coaxial cable for cable television (CATV). Coaxial or optical fibre cables linking computer networks are also increasingly found on poles in urban areas. The cable linking the telephone exchange to local customers is a thick cable lashed to a thin supporting cable, containing hundreds of twisted pair . Each twisted pair line provides a single telephone circuit or local loop to a customer. There may also be FOCs interconnecting telephone exchanges. Like electrical distribution lines, communication cables connect to service drops when used to provide local service to customers.
Solar panels mounted on utility poles may power auxiliary equipment where the expense of a power line connection is unwanted.
Streetlights and holiday fixtures are powered directly from secondary distribution.
Dead-end and other poles that support lateral loads have guy-wires to support them. The guys always have inserted in their length to prevent any high voltages caused by electrical faults from reaching the lower portion of the cable that is accessible by the public. In populated areas, guy wires are often encased in a yellow plastic or wood tube with reflectors attached to their lower end, so that they can be seen more easily, reducing the chance of people and animals walking into them or vehicles crashing into them.
Another means of providing support for lateral loads is a push brace pole, a second shorter pole that is attached to the side of the first and runs at an angle to the ground. If there is no space for a lateral support, a stronger pole, e.g. a construction of concrete or iron, is used.
Utility poles were first used in the mid-19th century in United States with telegraph systems. In 1844, the United States Congress granted Samuel Morse $30,000 () to build a 40-mile telegraph line between Baltimore, Maryland and Washington, D.C. Morse began by having a lead-sheathed cable made. After laying underground, he tested it. He found so many faults with this system that he dug up his cable, stripped off its sheath, bought poles and strung his wires overhead. On February 7, 1844, Morse inserted the following advertisement in the Washington newspaper: "Sealed proposals will be received by the undersigned for furnishing 700 straight and sound chestnut posts with the bark on and of the following dimensions to wit: 'Each post must not be less than eight inches in diameter at the butt and tapering to five or six inches at the top. Six hundred and eighty of said posts to be 24 feet in length, and 20 of them 30 feet in length.'"
In some parts of Australia, wooden poles are rapidly destroyed by , so metal poles must be used instead and in much of the interior wooden poles are vulnerable to fire. The Oppenheimer pole is a collapsible wrought iron pole in three sections. It is named after Oppenheimer and Company in Germany, but they were mostly manufactured in England under license. Nomination for Engineering Heritage Recognition: The Overland Telegraph Line 'Joining Point', Frews Pond, Northern Territory , Engineers Australia, June 2012. They were used on the Australian Overland Telegraph Line built in 1872 which connected the continent north to south directly through the centre and linked to the rest of the world through a submarine cable at Darwin.McMullen, Ron, "The Overland Telegraph", The Australian Telegraph Office (CD ROM). The Stobie pole was invented in 1924 by James Cyril Stobie of the Adelaide Electric Supply Company and first used in South Terrace, Adelaide.Rob Linn, ETSA – The Story of Electricity in South Australia, pp. 38–39, 1996.
One of the early Bell System lines was the Washington DC–Norfolk line which was, for the most part, square-sawn tapered poles of yellow pine probably treated to refusal with creosote. "Treated to refusal" means that the manufacturer forces preservatives into the wood, until it refuses to accept more, but performance is not guaranteed. Some of these were still in service after 80 years. The building of pole lines was resisted in some urban areas in the late 19th century, and political pressure for undergrounding remains powerful in many countries.
In Eastern Europe, Russia, and third-world countries, many utility poles still carry bare communication wires mounted on insulators not only along railway lines, but also along roads and sometimes even in urban areas. Errant traffic being uncommon on railways, their poles are usually less tall. In the United States electricity is predominately carried on unshielded aluminium conductors wound around a solid steel core and affixed to rated insulators made from glass, ceramic, or poly. Telephone, CATV, and FOCs are generally attached directly to the pole without insulators.
In the United Kingdom, much of the rural electricity distribution system is carried on wooden poles. These normally carry electricity at 11 or 33 kV (three phases) from 132 kV substations supplied from pylons to distribution substations or pole-mounted transformers. Wooden poles have been used for 132 kV for a number of years from the early 1980s one is called the trident they are usually used on short sections, though the line from Melbourne, Cambs to near Buntingford, Herts is quite long. The conductors on these are bare metal connected to the posts by insulators. Wood poles can also be used for low voltage distribution to customers.
Today, utility poles may hold much more than the uninsulated copper wire that they originally supported. Thicker cables holding many twisted pair, coaxial cable, or even fibre-optic, may be carried. Simple analogue or other outside plant equipment have long been mounted against poles, and often new digital equipment for multiplexing/demultiplexing or digital repeaters may now be seen. In many places, as seen in the illustration, providers of electricity, television, telephone, street light, traffic signal and other services share poles, either in joint ownership or by renting space to each other. In the United States, ANSI standard 05.1.2008 Standard specifications for wood poles US Department of Agriculture, Forest Products Laboratory governs wood pole sizes and strength loading. Utilities that fall under the Rural Electrification Act must also follow the guidelines set forth in RUS Bulletin 1724E-150 (from the US Department of Agriculture) for pole strength and loading.
Steel utility poles are becoming more prevalent in the United States thanks to improvements in engineering and corrosion prevention coupled with lowered production costs. However, premature failure due to corrosion is a concern when compared to wood. The National Association of Corrosion Engineers or NACE is developing inspection, maintenance, and prevention procedures similar to those used on wood utility poles to identify and prevent decay.
The date on the pole is applied by the manufacturer and refers to the date the pole was "preserved" (treated to withstand the elements).
In the United States, utility poles are marked with information concerning the manufacturer, pole height, ANSI strength class, wood species, original preservative, and year manufactured (vintage) in accordance with ANSI standard O5.1.2008. This is called branding, as it is usually burned into the surface; the resulting mark is sometimes called the "birth mark". Although the position of the brand is determined by ANSI specification, it is essentially just below "eye level" after installation. A rule of thumb for understanding a pole's brand is the manufacturer's name or logo at the top with a two-digit date beneath (sometimes preceded by a month).
Below the date is a two-character wood species abbreviation and one- to three-character preservative. Some wood species may be marked "SP" for southern pine, "WC" for western cedar, or "DF" for Douglas fir. Common preservative abbreviations are "C" for creosote, "P" for pentachlorophenol, and "SK" for chromated copper arsenate (originally referred to salts type K). The next line of the brand is usually the pole's ANSI class, used to determine maximum load; this number ranges from 10 to H6 with a smaller number meaning higher strength. The pole's height (from butt to top) in 5-foot increments is usually to the right of the class separated by a hyphen, although it is not uncommon for older brands to have the height on a separate line. The pole brand is sometimes an aluminum tag nailed in place.
Before the practice of branding, many utilities would set a 2- to 4-digit Datenail into the pole upon installation. The use of date nails went out of favor during World War II due to war shortages but is still used by a few utilities. These nails are considered valuable to collectors, with older dates being more valuable, and unique markings such as the utilities' name also increasing the value. However, regardless of the value to collectors, all attachments on a utility pole are the property of the utility company, and unauthorized removal is a misdemeanor or felony. (California state law cited as example)
However, not all power lines follow the road. In the British region of East Anglia, EDF Energy Networks often add the Ordnance Survey Grid Reference coordinates of the pole or substation to the name sign.
In some areas, utility pole name plates may provide valuable coordinate information: a poor man's GPS. A Taiwan Power Company example; 電力座標 A British Columbia, Canada example;
At electrical operated railways, pole routes were usually not built as too much jamming from the overhead wire would occur. To accomplish this, cables were separated using spars with insulators spaced along them; in general four insulators were used per spar. Only one such pole route still exists on the UK rail network, in the highlands of Scotland. There was also a long section in place between Wymondham, Norfolk and Brandon in Suffolk, United Kingdom; however, this was de-wired and removed during March 2009.
Some chemicals used to preserve wood poles including creosote and pentachlorophenol are toxic and have been found in the environment.
The considerable improvement in weathering resistance offered by creosote infusion has long-term drawbacks. In recent years, concerns have been raised about the toxicity of creosote-treated wood waste, such as utility poles. Specifically, their biodegradation can release phenolic compounds in soil, which are considered toxic. Research continues to explore methods to render this waste safe for disposal.
Historically, pole-mounted transformers were filled with a polychlorinated biphenyl (PCB) liquid. PCBs persist in the environment and have adverse effects on animals.
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