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{{Short description|Final stage of electricity delivery to individual consumers in a power grid}}
[[File:Polemount-singlephase-closeup.jpg|thumb|upright|A 50&nbsp;kVA pole-mounted distribution transformer]]

'''Electric power distribution''' is the final stage in the [[Power delivery|delivery of electricity]]. Electricity is carried from the [[Electric power transmission|transmission system]] to individual consumers. Distribution [[Electrical substation|substations]] connect to the transmission system and lower the transmission voltage to medium [[voltage]] ranging between {{val|2|ul=kV}} and {{val|33|u=kV}} with the use of [[transformer]]s.<ref name=":0"/> ''Primary'' distribution lines carry this medium voltage power to [[distribution transformer]]s located near the customer's premises. Distribution transformers again lower the voltage to the [[utilization voltage]] used by lighting, industrial equipment and household appliances. Often several customers are supplied from one transformer through ''secondary'' distribution lines. Commercial and residential customers are connected to the secondary distribution lines through [[service drop]]s. Customers demanding a much larger amount of power may be connected directly to the primary distribution level or the [[subtransmission]] level.<ref name=HSW/>

[[File:Electricity Grid Schematic English.svg|thumb|upright=1.2|General layout of [[Grid (electricity)|electricity networks]]. The voltages and loadings are typical of a European network (in Canada, for example, Extra High Voltage can mean 735kV.) ]]
The transition from transmission to distribution happens in a power [[Electrical substation|substation]], which has the following functions:<ref name=HSW>{{Cite web|url=http://science.howstuffworks.com/environmental/energy/power5.htm|title=How Power Grids Work|website=HowStuffWorks|access-date=2016-03-18|date=April 2000}}</ref>
* [[Circuit breaker]]s and switches enable the substation to be disconnected from the [[Electrical grid|transmission grid]] or for distribution lines to be disconnected.
* Transformers step down transmission voltages, {{val|35|u=kV}} or more, down to primary distribution voltages. These are medium voltage circuits, usually {{val|600|-|35000|u=V}}.<ref name=":0">{{Cite book|title=Electric Power Distribution Handbook|last1=Short|first1=T.A.|publisher=CRC Press|year=2014|isbn=978-1-4665-9865-2|location=Boca Raton, Florida, USA|pages=1–33}}</ref>
* From the transformer, power goes to the [[busbar]] that can split the distribution power off in multiple directions. The bus distributes power to distribution lines, which fan out to customers.
Urban distribution is mainly underground, sometimes in [[common utility duct]]s. Rural distribution is mostly above ground with [[utility pole]]s, and suburban distribution is a mix.<ref name=":0" />
Closer to the customer, a distribution transformer steps the primary distribution power down to a low-voltage secondary circuit, usually 120/240 V in the US for residential customers. The power comes to the customer via a [[service drop]] and an [[electricity meter]]. The final circuit in an urban system may be less than {{convert|50|ft|m|order=flip}}, but may be over {{convert|300|ft|m|order=flip}} for a rural customer.<ref name=":0" />

==History==
{{further|History of electric power transmission}}
[[File:Brush Company arc light madison square new york 1882.png|thumb|upright|The late 1870s and early 1880s saw the introduction of [[arc lamp|arc-lamp]] lighting used outdoors or in large indoor spaces, such as this [[Brush Electric Company]] system installed in 1880 in [[New York City]].]]

Electric power distribution didn't become necessary until the 1880s when electricity started being generated at [[power stations]]. Before that, electricity was usually generated where it was used. The first power-distribution systems installed in European and US cities were used to supply lighting: [[arc lamp|arc lighting]] running on very-high-voltage (around 3,000 V) [[alternating current]] (AC) or [[direct current]] (DC), and [[incandescent lamp|incandescent lighting]] running on low-voltage (100 V) direct current.<ref>Quentin R. Skrabec, The 100 Most Significant Events in American Business: An Encyclopedia, ABC-CLIO – 2012, page 86</ref> Both were supplanting [[gas lighting]] systems, with arc lighting taking over large-area and street lighting, and incandescent lighting replacing gas lights for business and residential users.

Due to the high voltages used in arc lighting, a single generating station could supply a long string of lights, up to {{convert|7|mi|km|adj=off}} long.<ref>{{cite journal|journal=Journal of the Society of Telegraph Engineers|last1=Berly|first1=J.|publisher=Institution of Electrical Engineers|volume=IX|date=1880-03-24|title=Notes on the Jablochkoff System of Electric Lighting|url=https://books.google.com/books?id=lww4AAAAMAAJ&pg=PA143|issue=32|page=143|access-date=2009-01-07}}</ref> And each doubling of voltage would allow a given cable to transmit the same amount of power four times the distance than at the lower voltage (with the same power loss). By contrast, direct-current indoor incandescent lighting systems, for example [[Pearl Street Station|Edison's first power station]], installed in 1882, had difficulty supplying customers more than a mile away. This was due to the low voltage (110 V) it used throughout, from generation to end use. This low voltage translated to higher current, requiring thick copper cables for transmission. In practice, Edison's DC generating plants needed to be within about {{convert|1.5|mi|km}} of the farthest customer to avoid even thicker, more expensive conductors.

=== Introduction of the transformer ===
The problem of transmitting electricity over longer distances became a recognized engineering roadblock to electric power distribution, with many less-than-satisfactory solutions tested by lighting companies. But the mid-1880s saw a breakthrough with the development of functional transformers that allowed AC power to be "stepped up" to much a higher voltage for transmission, then dropped down to a lower voltage near the end user. Compared to direct current, AC had much cheaper transmission costs and greater [[economies of scale]] — with large AC generating plants capable of supplying whole cities and regions — which led to the use of AC spreading rapidly.

In the US the competition between direct current and alternating current took a personal turn in the late 1880s in the form of a "[[war of currents]]" when [[Thomas Edison]] started attacking [[George Westinghouse]] and his development of the first US AC transformer systems, highlighting the deaths caused by high-voltage AC systems over the years and claiming any AC system was inherently dangerous.<ref>{{cite book|first1=Webb B.|last1=Garrison|title=Behind the headlines: American history's schemes, scandals, and escapades|url=https://archive.org/details/behindheadlinesa00garr|url-access=registration|publisher=Stackpole Books|year=1983|page=[https://archive.org/details/behindheadlinesa00garr/page/107 107]|isbn=9780811708173}}</ref> Edison's propaganda campaign was short-lived, with his company switching over to AC in 1892.

AC became the dominant form of transmission of power with innovations in  Europe and the US in [[electric motor]] designs, and the development of engineered ''universal systems'' allowing the large number of legacy systems to be connected to large AC grids.<ref name="Thomas Parke Hughes 1930, pages 120-121">{{cite book |last1=Parke Hughes |first1=Thomas |title=Networks of Power: Electrification in Western Society, 1880–1930 |publisher=JHU Press |year=1993 |isbn=9780801846144 |pages=120–121}}</ref><ref name="Raghu Garud 2009, page 249">{{cite book |last1=Garud |first1=Raghu |title=Managing in the Modular Age: Architectures, Networks, and Organizations |last2=Kumaraswamy |first2=Arun |last3=Langlois |first3=Richard |publisher=John Wiley & Sons |year=2009 |isbn=9780631233169 |page=249}}</ref>

In the first half of the 20th century, in many places the [[electric power industry]] was [[vertical integration|vertically integrated]], meaning that one company did generation, transmission, distribution, metering and billing. Starting in the 1970s and 1980s, nations began the process of [[deregulation]] and [[privatization]], leading to [[electricity market]]s. The distribution system would remain regulated, but generation, retail, and sometimes transmission systems were transformed into competitive markets.

==Generation and transmission==
<imagemap>
File:Electricity grid simple- North America.svg|thumb|380px|right|Simplified diagram of AC [[electricity delivery]] from generation stations to consumers' [[service drop]].
rect 2 243 235 438   [[Power station]]
rect 276 317 412 556   [[Transformer]]
rect 412 121 781 400   [[Electric power transmission]]
rect 800 0 980 165   [[Transformer]]
desc bottom-left
</imagemap>

Electric power begins at a generating station, where the potential difference can be as high as 33,000 volts. AC is usually used. Users of large amounts of DC power such as some [[railway electrification system]]s, [[telephone exchange]]s and industrial processes such as [[aluminium]] smelting  use [[rectifier]]s to derive DC from the public AC supply, or may have their own generation systems.  [[HVDC|High-voltage DC]] can be advantageous for isolating alternating-current systems or controlling the quantity of electricity transmitted. For example, [[Hydro-Québec]] has a direct-current line which goes from the [[James Bay]] region to [[Boston]].<ref>{{Cite web|url=http://www.hydroquebec.com/learning/transport/grandes-distances.html|title=Extra-High-Voltage Transmission {{!}} 735 kV {{!}} Hydro-Québec|website=hydroquebec.com|access-date=2016-03-08}}</ref>

From the generating station it goes to the generating station's switchyard where a step-up transformer increases the voltage to a level suitable for transmission, from 44 kV to 765 kV. Once in the transmission system, electricity from each generating station is combined with electricity produced elsewhere. Electricity is consumed as soon as it is produced. It is transmitted at a very high speed, close to the [[speed of light]].

== Primary distribution ==
Primary distribution voltages range from 4&nbsp;kV to 35&nbsp;kV phase-to-phase (2.4&nbsp;kV to 20&nbsp;kV phase-to-neutral)<ref name="eep-pdvl">{{cite web|url=http://electrical-engineering-portal.com/primary-distribution-voltage-levels|title=Primary Distribution Voltage Levels|last1=Csanyi|first1=Edvard|date=10 August 2012|website=electrical-engineering-portal.com|publisher=EEP – Electrical Engineering Portal|access-date=9 March 2017}}</ref> Only large consumers are fed directly from distribution voltages; most utility customers are connected to a transformer, which reduces the distribution voltage to the low voltage "utilization voltage", "supply voltage" or "mains voltage" used by lighting and interior wiring systems.

=== Network configurations ===
[[File:NCPC Power Plant Yellowknife Northwest Territories Canada 08.jpg|thumb|Substation near [[Yellowknife]], in the Northwest Territories, Canada]]

Distribution networks are divided into two types, radial or network.<ref>{{cite book|last=Abdelhay A. Sallam and Om P. Malik|title=Electric Distribution Systems|date=May 2011|publisher=IEEE Computer Society Press|isbn=9780470276822|page=21}}</ref> A radial system is arranged like a tree where each customer has one source of supply. A network system has multiple sources of supply operating in parallel. Spot networks are used for concentrated loads. Radial systems are commonly used in rural or suburban areas.

Radial systems usually include emergency connections where the system can be reconfigured in case of problems, such as a fault or planned maintenance. This can be done by opening and closing switches to isolate a certain section from the grid.

Long feeders experience [[voltage drop]] ([[power factor]] distortion) requiring [[capacitor]]s or [[Voltage regulator|voltage regulators]] to be installed.

Reconfiguration, by exchanging the functional links between the elements of the system, represents one of the most important measures which can improve the operational performance of a distribution system. The problem of optimization through the reconfiguration of a power distribution system, in terms of its definition, is a historical single objective problem with constraints. Since 1975, when Merlin and Back<ref>Merlin, A.; Back, H. Search for a Minimal-Loss Operating Spanning Tree Configuration in an Urban Power Distribution System. In Proceedings of the 1975 Fifth Power Systems Computer Conference (PSCC), Cambridge, UK, 1–5 September 1975; pp. 1–18.</ref>  introduced the idea of distribution system reconfiguration for active power loss reduction, until nowadays, a lot of researchers have proposed diverse methods and algorithms to solve the reconfiguration problem as a single objective problem. Some authors have proposed Pareto optimality based approaches (including active power losses and reliability indices as objectives). For this purpose, different artificial intelligence based methods have been used: microgenetic,<ref>{{Cite journal |last1=Mendoza |first1=J.E. |last2=López |first2=E.A. |last3=López |first3=M.E. |last4=Coello Coello |first4=C.A. |date=2009-09-01 |title=Microgenetic multiobjective reconfiguration algorithm considering power losses and reliability indices for medium voltage distribution network |url=https://digital-library.theiet.org/content/journals/10.1049/iet-gtd.2009.0009 |journal=IET Generation, Transmission & Distribution |language=en |volume=3 |issue=9 |pages=825–840 |doi=10.1049/iet-gtd.2009.0009 |issn=1751-8687}}</ref> branch exchange,<ref>{{Cite journal |last1=Bernardon |first1=Daniel Pinheiro |last2=Garcia |first2=Vinicius Jacques |last3=Ferreira |first3=Adriana Scheffer Quintela |last4=Canha |first4=Luciane Neves |date=2010-03-01 |title=Multicriteria Distribution Network Reconfiguration Considering Subtransmission Analysis |url=https://ieeexplore.ieee.org/document/5422823 |journal=IEEE Transactions on Power Delivery |volume=25 |issue=4 |pages=2684–2691 |doi=10.1109/TPWRD.2010.2041013 |s2cid=36322668 |issn=0885-8977}}</ref> particle swarm optimization<ref>{{Cite journal |last1=Amanulla |first1=B. |last2=Chakrabarti |first2=Saikat |last3=Singh |first3=S. N. |date=2012-01-24 |title=Reconfiguration of Power Distribution Systems Considering Reliability and Power Loss |url=https://ieeexplore.ieee.org/document/6138890 |journal=IEEE Transactions on Power Delivery |volume=27 |issue=2 |pages=918–926 |doi=10.1109/TPWRD.2011.2179950 |s2cid=21613514 |issn=0885-8977}}</ref>  and non-dominated sorting [[genetic algorithm]].<ref>{{cite journal|doi=10.3390/en6031439|doi-access=free|title=Pareto Optimal Reconfiguration of Power Distribution Systems Using a Genetic Algorithm Based on NSGA-II|year=2013|last1=Tomoiagă|first1=Bogdan|last2=Chindriş|first2=Mircea|last3=Sumper|first3=Andreas|last4=Sudria-Andreu|first4=Antoni|last5=Villafafila-Robles|first5=Roberto|journal=Energies|volume=6|issue=3|pages=1439–1455|hdl=2117/18257|hdl-access=free}}</ref>

=== Rural services ===
[[File:Red-tailed hawk on power pole-2033.jpg|thumb|High voltage power pole in rural [[Butte County, California]]]]
[[Rural electrification]] systems tend to use higher distribution voltages because of the longer distances covered by distribution lines (see [[Rural Electrification Administration]]). 7.2, 12.47, 25, and 34.5&nbsp;kV distribution is common in the United States; 11&nbsp;kV and 33&nbsp;kV are common in the UK, Australia and New Zealand; 11&nbsp;kV and 22&nbsp;kV are common in South Africa; 10, 20 and 35&nbsp;kV are common in China.<ref name="eolss">{{cite book|last1=Chan|first1=F|title=Electrical Engineering|chapter-url=http://www.eolss.net/sample-chapters/c05/e6-39a-06-01.pdf|access-date=12 March 2016|chapter=Electric Power Distribution Systems}}</ref> Other voltages are occasionally used.

Rural services normally try to minimize the number of poles and wires. It uses higher voltages (than urban distribution), which in turn permits use of galvanized steel wire. The strong steel wire allows for less expensive wide pole spacing. In rural areas a pole-mount transformer may serve only one customer. In [[New Zealand]], [[Australia]], [[Saskatchewan|Saskatchewan, Canada]], and [[South Africa]], [[Single-wire earth return]] systems (SWER) are used to electrify remote rural areas.

Three phase service provides power for large agricultural facilities, petroleum pumping facilities, water plants, or other customers that have large loads (three-phase equipment).
In North America, overhead distribution systems may be three phase, four wire, with a neutral conductor. Rural distribution system may have long runs of one phase conductor and a neutral.<ref>Donald G. Fink, H. Wayne Beatty (ed), '' Standard Handbook for Electrical Engineers, Eleventh Edition'', McGraw Hill, 1978, {{ISBN|0-07-020974-X}}, page 18-17</ref>
In other countries or in extreme rural areas the neutral wire is connected to the ground to use that as a return (single-wire earth return).

== Secondary distribution ==
[[File:World Map of Mains Voltages and Frequencies, Detailed.svg|thumb|upright=3|World map of mains voltage and frequencies]]
{{main|Low-voltage network}}
{{See also|Mains electricity}}
Electricity is delivered at a frequency of either 50 or 60&nbsp;Hz, depending on the region. It is delivered to domestic customers as [[single-phase electric power]]. In some countries as in Europe a [[Three-phase electric power|three phase]] supply may be made available for larger properties. Seen with an [[oscilloscope]], the domestic power supply in North America would look like a [[sine wave]], oscillating between −170 volts and 170 volts, giving an effective voltage of 120 volts RMS.<ref>{{Cite web|url=http://science.howstuffworks.com/environmental/energy/power2.htm|title=How Power Grids Work|website=HowStuffWorks|access-date=2016-03-18|date=April 2000}}</ref> [[Three-phase electric power]] is more efficient in terms of power delivered per cable used, and is more suited to running large electric motors. Some large European appliances may be powered by three-phase power, such as electric stoves and clothes dryers.

A [[Ground (electricity)|ground]] connection is normally provided for the customer's system as well as for the equipment owned by the utility. The purpose of connecting the customer's system to ground is to limit the voltage that may develop if high voltage conductors fall down onto lower-voltage conductors which are usually mounted lower to the ground, or if a failure occurs within a distribution transformer. [[Earthing system]]s can be TT, TN-S, TN-C-S or TN-C.

=== Regional variations ===

====220–240 volt systems====
Most of the world uses 50&nbsp;Hz 220 or 230 V single phase, or  400&nbsp;V three-phase for residential and light industrial services. In this system, the primary distribution network supplies a few substations per area, and the 230&nbsp;V / 400&nbsp;V  power from each substation is directly distributed to end users over a region of normally less than 1&nbsp;km radius. Three live (hot) wires and the neutral are connected to the building for a three phase service. Single-phase distribution, with one live wire and the neutral is used domestically where total loads are light. In Europe, electricity is normally distributed for industry and domestic use by the three-phase, four wire system. This gives a phase-to-phase voltage of {{nowrap|400 volts}} [[Three-phase electric power#Three-wire and four-wire circuits|wye]] service and a single-phase voltage of {{nowrap|230 volts}} between any one phase and neutral.  In the UK a typical urban or suburban low-voltage substation would normally be rated between 150&nbsp;kVA and 1&nbsp;MVA and supply a whole neighbourhood of a few hundred houses. Transformers are typically sized on an average load of 1 to 2&nbsp;kW per household, and the service fuses and cable is sized to allow any one property to draw a peak load of perhaps ten times this.
For industrial customers, 3-phase {{nowrap|690 / 400 volt}} is also available, or may be generated locally.<ref>{{cite web  | url = https://www.en-powered.com/blog/the-bumpy-road-to-energy-deregulation  | title = The Bumpy Road to Energy Deregulation  | publisher = EnPowered  | date = 2016-03-28  | access-date = 2017-04-07  | archive-date = 2017-04-07  | archive-url = https://web.archive.org/web/20170407145323/https://www.en-powered.com/blog/the-bumpy-road-to-energy-deregulation  | url-status = dead  }}</ref> 
Large industrial customers have their own transformer(s) with an input from 11 kV to 220 kV.

==== 100–120 volt systems ====
Most of the Americas use 60&nbsp;Hz AC, the 120/240 volt [[Split-phase electric power|split-phase]] system domestically and three phase for larger installations. North American transformers usually power homes at 240 volts, similar to Europe's 230 volts. It is the split-phase that allows use of 120 volts in the home.

[[File:Power Grid of Japan.svg|thumb|Japan's utility frequencies are {{nowrap|50 Hz}} and {{nowrap|60 Hz}}.]]

In the [[electricity sector in Japan]], the standard voltage is 100&nbsp;V, with both 50 and 60&nbsp;Hz AC frequencies being used. Parts of the country use 50&nbsp;Hz, while other parts use 60&nbsp;Hz.<ref name=":1">{{Cite news|url=http://www.japantimes.co.jp/news/2011/07/19/reference/japans-incompatible-power-grids/|title=Japan's incompatible power grids|last1=Gordenker|first1=Alice|date=2011-07-19|newspaper=The Japan Times Online|language=en-US|issn=0447-5763|access-date=2016-03-12}}</ref> This is a relic from the 1890s. Some local providers in [[Tokyo]] imported 50&nbsp;Hz German equipment, while the local power providers in [[Osaka]] brought in 60&nbsp;Hz generators from the United States. The grids grew until eventually the entire country was wired. Today the frequency is 50&nbsp;Hz in Eastern Japan (including Tokyo, [[Yokohama]], [[Tōhoku region|Tohoku]], and [[Hokkaido]]) and 60&nbsp;Hz in Western Japan (including [[Nagoya]], [[Osaka]], [[Kyoto]], [[Hiroshima]], [[Shikokuchūō|Shikoku]], and [[Kyushu]]).<ref>{{Cite web|url=http://www.japan-guide.com/e/e2225.html|title=Electricity in Japan|website=Japan-Guide.com|access-date=2016-03-12}}</ref>

Most household appliances are made to work on either frequency. The problem of incompatibility came into the public eye when the [[2011 Tōhoku earthquake and tsunami]] knocked out about a third of the east's capacity, and power in the west could not be fully shared with the east, since the country does not have a common frequency.<ref name=":1" />

There are four [[high-voltage direct current]] (HVDC) converter stations that move power across Japan's AC frequency border. [[Shin Shinano]] is a [[Back-to-back connection#Power transmission|back-to-back]] HVDC facility in [[Japan]] which forms one of four [[frequency changer]] stations that link Japan's western and eastern power grids. The other three are at [[Higashi-Shimizu Frequency Converter|Higashi-Shimizu]], [[Minami-Fukumitsu]] and [[Sakuma Dam#HVDC frequency converter|Sakuma Dam]]. Together they can move up to 1.2 GW of power east or west.<ref>{{Cite web|url=https://spectrum.ieee.org/energy/the-smarter-grid/why-japans-fragmented-grid-cant-cope|title=Why Japan's Fragmented Grid Can't Cope|website=Spectrum.IEEE.org|date=6 April 2011 |access-date=2016-03-12}}</ref>

==== 240 volt systems and 120 volt outlets ====
Most modern North American homes are wired to receive 240 volts from the transformer, and through the use of [[Split-phase electric power|split-phase electrical power]], can have both 120 volt receptacles and 240 volt receptacles. The 120 volts is typically used for lighting and most [[AC power plugs and sockets|wall outlets]]. The 240 volt circuits are typically used for appliances requiring high watt heat output such as ovens and heaters. They may also be used to supply an [[electric car]] charger.

== Modern distribution systems ==
Traditionally, the distribution systems would only operate as simple distribution lines where the electricity from the [[Electric power transmission|transmission networks]] would be shared among the customers. Today's distribution systems are heavily integrated with [[renewable energy]] generations at the distribution level of the power systems by the means of [[distributed generation]] resources, such as [[solar energy]] and [[Wind power|wind energy]].<ref>{{Cite journal|last1=Fathabad|first1=A. M.|last2=Cheng|first2=J.|last3=Pan|first3=K.|last4=Qiu|first4=F.|date=November 2020|title=Data-Driven Planning for Renewable Distributed Generation Integration|url=https://ieeexplore.ieee.org/document/9112707|journal=IEEE Transactions on Power Systems|volume=35|issue=6|pages=4357–4368|doi=10.1109/TPWRS.2020.3001235|bibcode=2020ITPSy..35.4357F|s2cid=225734643|issn=1558-0679}}</ref> As a result, distribution systems are becoming more independent from the transmission networks day-by-day. Balancing the supply-demand relationship at these modern distribution networks (sometimes referred to as [[microgrid]]s) is extremely challenging, and it requires the use of various technological and operational means to operate. Such tools include [[battery storage power station]], [[data analytics]], optimization tools, etc.

==See also==
{{Portal|Energy}}
{{div col|colwidth=25em}}
* [[Backfeeding]]
* [[Cost of electricity by source]]
* [[Distribution network operator]]
* [[Dynamic voltage restoration]]
* [[Electric utility]]
* [[Electricity distribution companies by country]]
* [[Electricity generation]]
* [[Electricity retailing]]
* [[Network protector]]
* [[Overhead power line]]
* [[Power distribution unit]]
* [[Power-system automation]] – IEEE standard to interconnect tele-protection and multiplexer devices of power utility companies
* [[Power system simulation]]
* [[Transmission system operator]]
* [[High-voltage transformer fire barriers]]
{{div col end}}

==References==
{{Reflist|30em}}

==External links==
{{Commons category}}
{{Wikiversity|Electrical Power Distribution}}
*[http://www.ieee.org/pes IEEE Power Engineering Society]
*[http://grouper.ieee.org/groups/td/dist/ IEEE Power Engineering Society Distribution Subcommittee]
*[http://www.oe.energy.gov U.S. Department of Energy Electric Distribution website]

{{Electricity generation}}
{{Authority control}}

[[Category:Electric power distribution| ]]
[[Category:Electrical engineering]]