Autility pole, commonly referred to as atransmission pole,telephone pole,telecommunication pole,power pole,hydro pole,telegraph pole, ortelegraph post, is acolumn or post used to supportoverhead power lines and various other public utilities, such aselectrical cable,fiber optic cable, and related equipment such astransformers andstreet 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, anddistribution 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.[1] Utility poles are usually made out ofwood,aluminum alloy,[2] metal, concrete, or composites likefiberglass. AStobie pole is a multi-purpose pole made of two steel joists held apart by a slab of concrete in the middle, generally found inSouth Australia.
The first poles were used in 1843 by telegraph pioneerWilliam Fothergill Cooke, who used them on a line along theGreat Western Railway. Utility poles were first used in the mid-19th century in America withtelegraph systems, starting withSamuel Morse, who attempted to bury a line betweenBaltimore andWashington, 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.[3]
Utility poles are commonly used to carry two types of electricpower lines:[4]distribution lines (or "feeders") andsub transmission lines. Distribution lines carry power from localsubstations to customers. They generally carry voltages from 4.6 to 33kilovolts (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. Aservice 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 astransmission 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.
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.[5] 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, theNational Electrical Safety Code, published by theInstitute of Electrical and Electronics Engineers (IEEE) (not to be confused with theNational Electrical Code published by theNational Fire Protection Association [NFPA]), sets the standards for construction and maintenance of utility poles and their equipment.
Most utility poles are made of wood, pressure-treated with some type ofpreservative 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, includingDouglas fir,jack pine,lodgepole pine,western red cedar, andPacific silver fir.
Traditionally, the preservative used wascreosote, but due to environmental concerns, alternatives such aspentachlorophenol, coppernaphthenate andborates 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 byANSI,ASTM, andAmerican 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.[6][7][8]Woodpecker damage to wood poles is the most significant cause of pole deterioration in some parts of the U.S.[9]
Other common utility pole materials are aluminum, steel and concrete, with composites (such asfiberglass[citation needed]) also becoming more prevalent.[10] One particularpatented utility pole variant used in Australia is theStobie pole, made up of two vertical steel posts with a slab of concrete between them.
On poles carrying both electrical and communications wiring, the electricpower 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 thesupply space.[5] The wires themselves are usually uninsulated, and supported byinsulators, commonly mounted on a horizontal beam (crossarm). Power is transmitted using thethree-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 alightning rod, providing a lowresistance path to ground thus protecting the phase conductors from lightning.
Distribution lines use two systems, either grounded-wye ("Y" onelectrical schematics) 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, theneutral, 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 aservice drop, there is a pole-mounted step-downdistribution transformer to transform the high distribution voltage to the lower secondary voltage provided to the customer. InNorth America, service drops provide 240/120 Vsplit-phase power for residential and light commercial service, using cylindrical single-phase transformers. In Europe and most other countries, 230 Vthree phase (230Y400) service drops are used. The transformer's primary is connected to the distribution line through protective devices calledfuse cutouts. 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 bylinemen using a long insulated rod called ahot stick to disconnect the transformer from the line.
The pole may begrounded with a heavy barecopper orcopper-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.[4][5] It provides similar protection in case offlashovers and lightning strikes. Asurge arrester orlightning 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.
Ifuninsulated conductors touch each other due to wind or fallen trees, the resultant sparks can startwildfires. To reduce this problem,aerial bundled conductors are being introduced.
The communications cables are attached below the electric power lines, in a vertical space along the pole designated thecommunications space.[5] The communications space is separated from the lowest electrical conductor by thecommunication worker safety zone, which provides room for workers to maneuver safely while servicing the communication cables, avoiding contact with the power lines.[5]
The most common communication cables found on utility poles are copper orfibre-optic cable (FOC) fortelephone lines andcoaxial cable forcable television (CATV). Coaxial or optical fibre cables linkingcomputer networks are also increasingly found on poles in urban areas. The cable linking thetelephone exchange to local customers is a thick cable lashed to a thin supporting cable, containing hundreds oftwisted pairsubscriber lines. Each twisted pair line provides a single telephone circuit orlocal 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.
Utility poles may also carry other equipment such asstreet lights, supports fortraffic lights andoverhead wires forelectric trolleys, andcellular network antennas. They can also carry fixtures and decorations specific for certain holidays or events specific to the city where they are located.
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.
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The primary purpose of pole attachment hardware is to secure the cable and associated aerial plant facilities to poles and to help facilitate necessary plant rearrangements. An aerial plant network requires high-quality reliable hardware to
Functional performance requirements common to pole line hardware for utility poles made of wood, steel, concrete, or Fiber-Reinforced Composite (FRC) materials are contained in Telcordia GR-3174,Generic Requirements for Hardware Attachments for Utility Poles.[11]
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In some countries, such as the United Kingdom, utility poles have sets of brackets arranged in a standard pattern up the pole to act as hand and foot holds so that maintenance and repair workers can climb the pole to work on the lines. In the United States, such steps have been determined to be a public hazard and are no longer allowed on new poles.[citation needed] Linemen may use climbing spikes called gaffs to ascend wooden poles without steps on them. In the UK, boots fitted with steel loops that go around the pole (known as "Scandinavian Climbers") are also used for climbing poles. In the US, linemen usebucket trucks for the vast majority of poles that are accessible by vehicle.
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The poles at the end of a straight section of utility line where the line ends or angles off in another direction are calleddead-end poles in the United States. Elsewhere they may be referred to as anchor or termination poles. These must carry the lateraltension of the long straight sections of wire. They are usually made with heavier construction. The power lines are attached to the pole by horizontal strain insulators, either placed on crossarms (which are either doubled, tripled, or replaced with a steel crossarm, to provide more resistance to the tension forces) or attached directly to the pole itself.
Dead-end and other poles that support lateral loads haveguy-wires to support them. The guys always havestrain insulators 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.
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The system of suspending telegraph wires from poles with ceramic insulators was invented and patented by British telegraph pioneerWilliam Fothergill Cooke. Cooke was the driving force in establishing theelectrical telegraph on a commercial basis. WithCharles Wheatstone he invented theCooke and Wheatstone telegraph and founded the world's first telegraph company, theElectric Telegraph Company. Telegraph poles were first used on theGreat Western Railway in 1843 when the Cooke and Wheatstone telegraph line was extended toSlough. The line had previously used buried cables but that system had proved troublesome with failing insulation.[13]: 32 In Britain, the trees used for telegraph poles were either nativelarch orpine from Sweden and Norway. Poles in early installations were treated with tar, but these were found to last only around seven years. Later poles were treated instead withcreosote orcopper sulphate for the preservative.[13]: 80
Utility poles were first used in the mid-19th century inAmerica with telegraph systems. In 1844, theUnited States Congress grantedSamuel Morse $30,000 (equivalent to $1,012,400 in 2024) to build a 40-miletelegraph line betweenBaltimore,Maryland andWashington, D.C. Morse began by having a lead-sheathed cable made. After laying seven miles (11 km) 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 bytermites, so metal poles must be used instead and in much of the interior wooden poles are vulnerable to fire. TheOppenheimer pole is a collapsiblewrought iron pole in three sections. It is named after Oppenheimer and Company in Germany, but they were mostly manufactured in England under license.[14] They were used on theAustralian 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 asubmarine cable atDarwin.[15] The Stobie pole was invented in 1924 by James Cyril Stobie of theAdelaide Electric Supply Company and first used inSouth Terrace, Adelaide.[16]
One of the earlyBell System lines was the Washington DC–Norfolk line which was, for the most part, square-sawn tapered poles ofyellow pine probably treated to refusal withcreosote. "Treated to refusal" means that the manufacturer forces preservatives into the wood, until it refuses to accept more, but performance is not guaranteed.[17]Some of these were still in service after 80 years.[18] The building of pole lines was resisted in some urban areas in the late 19th century,[citation needed] and political pressure forundergrounding remains powerful in many countries.
InEastern 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 unshieldedaluminum 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 frompylons 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 forlow voltage distribution to customers.
Today, utility poles may hold much more than the uninsulated copper wire that they originally supported. Thicker cables holding manytwisted pair,coaxial cable, or evenfibre-optic, may be carried. Simple analoguerepeaters or otheroutside plant equipment have long been mounted against poles, and often new digital equipment formultiplexing/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[19] governs wood pole sizes and strength loading. Utilities that fall under theRural Electrification Act must also follow the guidelines set forth in RUS Bulletin 1724E-150[20] (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.[21]The National Association of Corrosion EngineersArchived 2010-06-19 at theWayback Machine or NACE is developing inspection, maintenance, and prevention procedures similar to those used on wood utility poles to identify and prevent decay.
British Telecom posts are usually marked with the following information:[citation needed]
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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[24] (vintage) in accordance with ANSI standard O5.1.2008.[25] 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" forcreosote, "P" forpentachlorophenol, and "SK" forchromated 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-digitdate nail 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.[26](California state law cited as example)
A practice in some areas is to place poles on coordinates upon a grid. The pole at right is aDelmarva Power pole located in a rural area of the state ofMaryland in the United States. The lower two tags are the "X" and "Y" coordinates along said grid. Just as in acoordinate plane used in geometry, X increases as one travels east and Y increases as one travels north. The upper two tags are specific to the sub transmission section of the pole; the first refers to the route number, the second to the specific pole along the route.
However, not all power lines follow the road. In the British region ofEast Anglia,EDF Energy Networks often add theOrdnance 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'sGPS.[27][28][29]
Apole route (orpole line in the US) is a telephone link or electrical power line between two or more locations by way of multiple uninsulated wires suspended between wooden utility poles. This method of link is common especially in rural areas where burying the cables would be expensive. Another situation in which pole routes were extensively used were on the railways to linksignal boxes. Traditionally, prior to around 1965, pole routes were built with open wires along non-electrical operated railways; this necessitated insulation when the wire passed over the pole, thus preventing the signal from becoming attenuated.
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 betweenWymondham,Norfolk andBrandon inSuffolk, United Kingdom; however, this was de-wired and removed during March 2009.
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Utility poles are used by birds for nesting and to rest on.[30][31] Utility poles and related structures are regarded by some to be a form ofvisual pollution[citation needed]. Many lines areplaced underground for this reason, in places of high population density or scenic beauty that justify the expense. Architects design some pylons to be pretty, thus avoiding visual pollution[citation needed].
Some chemicals used to preserve wood poles includingcreosote andpentachlorophenol are toxic and have been found in the environment.[32]
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.[33]
Historically, pole-mounted transformers were filled with apolychlorinated biphenyl (PCB) liquid. PCBs persist in the environment and have adverse effects on animals.
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