Electric fence

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The wires attached to the posts with black insulators are electrified. Ht-electric-fence.jpg
The wires attached to the posts with black insulators are electrified.
Charger for a plus-minus net fence Electric-fence-charger.jpg
Charger for a plus-minus net fence

An electric fence is a barrier that uses electric shocks to deter animals and people from crossing a boundary. The voltage of the shock may have effects ranging from discomfort to death. Most electric fences are used today for agricultural fencing and other forms of animal control, although they are also used to protect high-security areas such as military installations or prisons, where potentially lethal voltages may be used.

Contents

Design and function

Detail of an electric fence material made of synthetic cord with metal interwoven through it, attached to a steel fence post with a plastic insulator. This material is more visible than wire, but most often used for temporary fencing. ElectricCord.jpg
Detail of an electric fence material made of synthetic cord with metal interwoven through it, attached to a steel fence post with a plastic insulator. This material is more visible than wire, but most often used for temporary fencing.

Electric fences are designed to create an electrical circuit when touched by a person or animal. A component called a power energizer converts power into a brief high voltage pulse. One terminal of the power energizer releases an electrical pulse along a connected bare wire about once per second. Another terminal is connected to a metal rod implanted in the earth, called a ground or earth rod. A person or animal touching both the wire and the earth during a pulse will complete an electrical circuit and will conduct the pulse, causing an electric shock. The effects of the shock depend upon the voltage, the energy of the pulse, the degree of contact between the recipient and the fence and ground and the route of the current through the body; it can range from barely noticeable to uncomfortable, painful or even lethal.

Fence energizers

Early alternating current (AC) fence chargers used a transformer and a mechanically driven switch to generate the electrical pulses. The pulses were wide and the voltage unpredictable, with no-load peaks in excess of 10,000 volts and a rapid drop in voltage as the fence leakage increased. The switch mechanism was prone to failure. Later systems replaced the switch with a solid-state circuit, with an improvement in longevity but no change in pulse width or voltage control.

"Weed burner" fence chargers were popular for a time and featured a longer-duration output pulse that would destroy weeds touching the fence. These were responsible for many grass fires when used during dry weather. Although still available, they have declined in popularity.

Most modern fences emit pulses of high voltage at a given interval of time, and don't take into account whether there is an animal or person touching the conductive wires, except for the voltage multiplier based electric fence charger that stores high voltage potential and dumps its charges as soon as a conductive load (grounded animal/person) touches the wires.

Depending on the area to be fenced and remoteness of its location, fence energizers may be hooked into a permanent electrical circuit, they may be run by lead-acid or dry cell batteries, or a smaller battery kept charged by a solar panel. The power consumption of a fence in good condition is low, and so a lead-acid battery powering several hundred metres of fence may last for several weeks on a single charge. For shorter periods dry cell batteries may be used. Some energizers can be powered by more than one source.

Fencing materials

Smooth steel wire is the material most often used for electric fences, ranging from a fine thin wire used as a single line to thicker, high-tensile (HT) wire. Less often, woven wire or barbed wire fences can be electrified, though such practices create a more hazardous fence, particularly if a person or animal becomes caught by the fencing material (electrified barbed wire is unlawful in some areas). Synthetic webbing and rope-like fencing materials woven with fine conducting wires (usually of stainless steel) have become available over the last 15 to 20 years, and are particularly useful for areas requiring additional visibility or as temporary fencing.

The electrified fence itself must be kept insulated from the earth and from any materials that will conduct electricity and ignite or short out the fence. Fencing must therefore avoid vegetation, and cannot be attached directly to wood or metal posts. Typically, wooden or metal posts are driven into the ground and plastic or porcelain insulators are attached to them, or plastic posts are used. The conducting material is then attached to the posts.

Palisade fences

Residential Electric Palisade Fence in Johannesburg, South Africa. The top spikes are electrified while the bottom slats of the palisade are grounded. Electric-palisade-fence.jpg
Residential Electric Palisade Fence in Johannesburg, South Africa. The top spikes are electrified while the bottom slats of the palisade are grounded.

Electrified palisade fences are usually made from painted mild steel, galvanized steel, stainless steel or aluminium. Typically the fences are 2.4 metres (7 ft 10 in) and typically send high voltage electric pulses through the palisade at a frequency of 1 Hz (one pulse per second).

Palisade electric fences are used in most countries, particularly where there is little vegetation to short circuit the fence or where the costs of security personnel is high in relation to automated security equipment. The electric pulse is a strong deterrent for criminals, while the palisade fence is mechanically stronger than a typical steel cable electric fence, being able to withstand impact from wildlife, small falling trees and wildfires.

Due to the high levels of crime in South Africa, it is common for residential houses to have perimeter defences. The City of Johannesburg promotes the use of palisade fencing over opaque, usually brick, walls as criminals cannot hide as easily behind the fence. In the City of Johannesburg manual on safety one can read about best practices and maintenance of palisade fencing, such as not growing vegetation in front of palisades as this allows criminals to make an unseen breach. [1]

History

Electric fence in urban environment Electric fence 01.jpg
Electric fence in urban environment

First published in 1832, Chapter 7 of Domestic Manners of the Americans by Fanny Trollope describes an arrangement of wires connected with an electrical machine used to protect a display called "Dorfeuille's Hell" in the Western Museum of natural history in Cincinnati, [2] which she herself invented. [3] Published in 1870, Chapter 22 of Jules Verne's 20,000 Leagues Under the Sea , describes, "The Lightning Bolts of Captain Nemo" the use of electrification of a structure as a defensive weapon. Published in 1889, Mark Twain's novel A Connecticut Yankee in King Arthur's Court , uses an electric fence for defensive purposes.

David H. Wilson obtained United States Patent 343,939 in 1886, combining protection, an alarm bell, and telephone communications. He constructed an experimental 30-mile electric fence energized by a water wheel in Texas in 1888, but it was not successful. [4]

In 1905, the Russian army improvised electric fences during the Russo-Japanese War at Port Arthur. In 1915, during World War I, the German army installed the "Wire of Death", an electrified fences along the border between Belgium and the Netherlands [5] to prevent unauthorized movement of people across the border. The fences covered 300 kilometres and consisted of several strands of copper wire, backed with barbed wire, and energized to several thousand volts. An estimated 3,000 human fatalities were caused by the fence, as well as the destruction of livestock.

Electric fences were used to control livestock in the United States in the early 1930s[ citation needed ], and electric fencing technology developed in both the United States and New Zealand.

An early application of the electric fence for livestock control was developed in 19361937 by New Zealand inventor Bill Gallagher. Built from a car ignition trembler coil set, Gallagher used the device to keep his horse from scratching itself against his car. [6] Gallagher later started the Gallagher Group to improve and market the design. [7] In 1962, another New Zealand inventor, Doug Phillips, invented the non-shortable electric fence based on capacitor discharge. [8] [9] This significantly increased the range an electric fence could be used from a few hundred metres to 35 km (~20 miles), and reduced the cost of fencing by more than 80%. [10] The non-shortable electric fence was patented by Phillips and by 1964 was manufactured by Plastic Products, a New Zealand firm, under the name "Waikato Electric Fence." [10] This idea was to replace ceramic with plastic insulators. A variety of plastic insulators are now used on farms throughout the world today.

By 1939, public safety concerns in the United States prompted Underwriters' Laboratories to publish a bulletin on electric shock from electric fences, leading to the ANSI/UL standard No. 69 for electric fence controllers. [11]

In 1969 Robert B. Cox, a farmer in Adams County, Iowa, invented an improved electric fence bracket and was issued United States Patent No. 3,516,643 on June 23, 1970. This bracket improved electric fences by keeping the wire high enough above the ground and far enough away from the fence to permit grass and weeds growing beneath the wire to be mowed. The brackets attached to the posts by what may be called a "pivot bind" or "torsion-lock." The weight of the bracket, the attached insulator and the electric wire attached to the insulator bind the bracket to the post.

Electric fences have improved significantly over the years. Improvements include:

Uses

Agriculture

Permanent electric fencing is used in many agricultural areas, as construction of electric fences can be much cheaper and faster than conventional fences (it uses plain wire and much lighter construction, as the fence does not need to physically restrain animals). The risk of injury to livestock (particularly horses) is lower compared to fences made of barbed wire or certain types of woven wire with large openings that can entangle the feet.

Its disadvantages include the potential for the entire fence to be disabled due to a break in the conducting wire, shorting out if the conducting wire contacts any non-electrified component that may make up the rest of the fence, power failure, or forced disconnection due to the risk of fires starting by dry vegetation touching an electrified wire. Other disadvantages can be lack of visibility and the potential to shock an unsuspecting human passer-by who might accidentally touch or brush the fence.

Many fences are made entirely of standard smooth or high-tensile wire, although high quality synthetic fencing materials are also beginning to be used as part of permanent fences, particularly when visibility of the fence is a concern.

Conventional agricultural fencing of any type may be strengthened by the addition of a single electric line mounted on insulators attached to the top or front of the fence. A similar wire mounted close to the ground may be used to prevent pigs from excavating beneath other fencing. Substandard conventional fencing can also be made temporarily usable until proper repairs are made by the addition of a single electric line set on a "stand-off" insulator.

A temporary electric fence of synthetic materials and plastic step-in posts set about 12 feet apart ElectricSynthetic.jpg
A temporary electric fence of synthetic materials and plastic step-in posts set about 12 feet apart

Electric materials are also used for the construction of temporary fencing, particularly to support the practice of managed intensive grazing (also known as rotational or "strip" grazing). It is also popular in some places for confining horses and pack animals overnight when trail riding, hunting, or at competitions such as endurance riding and competitive trail riding. Typically, one or more strands of wire, synthetic tape or cord are mounted on metal or plastic posts with stakes at the bottom, designed to be driven into the ground with the foot. For a hand-tightened temporary fence of electrified rope or web in a small area, these are usually spaced at no more than 12 to 15 feet (about four metres) to prevent the fencing material from sagging and touching the ground. Larger areas where tools are used to stretch wire may be able to set step-in posts at larger distances without risk that the fencing material will sag.

With temporary electric fencing, a large area can be fenced off in a short period. Temporary fencing that is intended to be left in place for several weeks or months may be given additional support by the use of steel T posts (which are quickly driven in with hand tools and unearthed with relative ease, using a leverage device), to help keep the fence upright, particularly at corners. Livestock owners using rotational grazing in set patterns that are similar from one year to the next, may permanently drive a few permanent wooden fence posts in strategic locations.

Portable fence energizers are made for temporary fencing, powered solely by batteries, or by a battery kept charged by a small solar panel. Rapid laying-out and removal of multiple-strand temporary electric fencing over a large area may be done using a set of reels mounted on a tractor or all-terrain vehicle.

For sheep, poultry, and other smaller animals, plastic electric netting may be mounted on insulating stakes this is also effective at keeping out some predators such as foxes.

In practice, once most animals have learned of the unpleasant consequences of touching the fence they tend to avoid it for considerable periods even when it is inactive. However, some animals learn to avoid the shock, either by running under the fence quickly between pulses, or by pushing other individuals through the fence. Animals with thick woolly coats (such as sheep or highland cattle) may learn to push through the fence themselves, using their coats as electrical insulation. Some animals also learn to recognize the slight clicking sound made by some electric fences and thus can sense when the fence is off.

Wild animals

Electric fencing to protect a walkway against wildlife. Pilanesberg Game Reserve, South Africa Pilanesberg-Electric fencing-001.jpg
Electric fencing to protect a walkway against wildlife. Pilanesberg Game Reserve, South Africa
A wolf jumping over an electric fence only touching the plus pole not getting an electric shock. Electric fence - wolf.png
A wolf jumping over an electric fence only touching the plus pole not getting an electric shock.

Electric fences are useful for controlling the movements of wild animals. Examples include deterring deer from entering private property, keeping animals off airport runways, keeping wild boar from raiding crops, and preventing geese from soiling areas used by people. Electric fencing has been extensively used in environmental situations reducing the conflict between elephants or other animals and humans in Africa and Asia. [13] However, electric fences are increasingly used in small livestock farming as a management tool to exclude animals, such as potential predators, that will tunnel under these fences to gain entrance to livestock camps. Electrified fences are also increasingly used on both domestic livestock and wildlife (game) farms in Africa as a management tool to exclude predators from entering or exiting a camp. A number of animal species, other than predators, may dig under these fences in order to cross that barrier. [14] [15]

The value of electrified fences is offset by lethal electrocution threats to a number of species including vulnerable and endangered species such as African Ground Pangolin (Smutsiatemminckii), Southern African Python (Pythonnatalensis), and a number of tortoise species. [16] [17] [18] [14] [15]

In South Africa it is estimated that more than 31,500 reptiles (predominantly tortoises) are killed on electrified fences annually. The estimate for African Ground Pangolins killed by electrocution is 377–1 028 annually. [14] [15] The impacts of electrified fences on other species, such as Monitor Lizard (Varanus spp), have not been quantified.

Pangolins are bipedal, walking on their hind legs with their front legs and tail held off the ground, leaving their ventral surface unprotected. [15] This makes them particularly prone to accidental electrocution on electrified fences. [15] Depending on the electrified fence’s design and height of the lower lines, the pangolin's head or exposed belly come into contact with the electrified wire(s). The initial shock will causes the pangolin to adopt its defense of rolling into a ball, often resulting in it inadvertently wrapping itself around the electrified wire. [14] [15]

An animal wrapped around the electric wire receives successive shocks, which in the case of pangolins results in them curling into an ever tighter ball around the live wire. [15] The repeated electrical pulses ultimately kill the pangolin.  Those not killed outright usually succumb to exposure, dehydration or starvation. [15] Pangolins found dead on electric fences often have epidermal burns, these sometimes burning through the scales. Internal injuries may also be significant. Pangolins found alive while wrapped around an electrified wire may suffer debilitating neurological damage if they have experienced prolonged exposure to the electric current. These ultimately succumb even when released. [15]

Security

Non-lethal fence

Security electric fences are electric fences constructed using specialised equipment and built for perimeter security as opposed to animal management. Security electric fences consist of wires that carry pulses of electric current to provide a non-lethal shock to deter potential intruders. Tampering with the fence also results in an alarm that is logged by the security electric fence energiser, and can also trigger a siren, strobe, or notifications to a control room or directly to the owner via email or phone.

In practical terms, security electric fences are a type of perimeter intrusion detection sensor array that acts as a (or part of a) physical barrier, a psychological deterrent to potential intruders, and as part of a security alarm system.

Multi-zone security electric fence used alongside a physical barrier. Security Electric Fence.JPG
Multi-zone security electric fence used alongside a physical barrier.

Non-lethal electric fences are used by both private and government-sector bodies to prevent trespass. These include freight carriers, auto auctions, equipment rental companies, auto dealers, housing communities, commercial factories or warehouses, prisons, military bases, and government buildings. Many of these electric fences act as monitored security alarm systems in addition to causing an uncomfortable shock. Electrified palisade fences are used to protect isolated property and high security facilities, but also around some residential homes.

They can also be used inside a building, for example as a grid behind windows or skylights to prevent people from climbing through. They have even been used on yachts and on large ships to deter pirates.

Electric fences are occasionally employed to discourage suicide attempts on tall structures, and to reduce the incidence of graffiti and other petty crime.

Due to the high levels of crime in South Africa, it is common for residential houses to have perimeter defences. The City of Johannesburg promotes the use of palisade fencing rather than opaque, usually brick, walls as criminals cannot hide as easily behind the fence. In the City of Johannesburg manual on safety, one can read about best practices and maintenance of palisade fencing, such as not growing vegetation in front of palisades as this allows criminals to make an unseen breach. [19]

Types of security electric fences include:

Piggyback
A Piggyback electric fence is mounted off the back of an existing wire or mesh fence, adding another level of security to the existing perimeter barrier. The piggyback profile is fastened to existing fence posts (e.g. pillars of a palisade fence) using rivets or screws. These are the most commonly used security electric fences.
Wall top
Wall-top electric fences attach to the top of an existing perimeter barrier such as a masonry wall. These are the second most common type of security electric fences.
Stand alone
Stand-alone electric fences act as the sole perimeter barrier. This type is normally found as one of many layers of perimeter security around high security establishments, meaning that in order touch it, an intruder would have had to previously bypass at least one physical barrier.
Nazi concentration camps were surrounded by electric fences, like this one shown above at Auschwitz, which contained lethal voltages, continuously rather than in pulses Barbed wire near by the entrance of Auschwitz I.jpg
Nazi concentration camps were surrounded by electric fences, like this one shown above at Auschwitz, which contained lethal voltages, continuously rather than in pulses

Stun-lethal fence

A "stun-lethal" electric fence can be set to deliver a shock if touched once, and a fatal jolt if touched a second time.

12-foot-high "stun-lethal" fences have been in use for some time in many US state prisons, like those in Arizona. The Federal Department of Corrections added them in 2005 to two prisons in Coleman, Florida, and prisons in Tucson; Terre Haute, Indiana; Hazelton, West Virginia; Pine Knot, Kentucky; and Pollock, Louisiana. [20]

A "stun-lethal" fence may also consist of two fences; one set of wires forming a conventional pulsed DC non-lethal fence, the second set (interleaved with the first) forming a 6.6 kV AC lethal fence, energized when the DC fence detects an intruder. Alternatively it may consist of a single, AC or pulsed DC fence capable of running in "safe", "unsafe" or "lethal" modes by varying the DC pulse energy, AC/DC fence voltage and/or fence on-off duty cycle. [21]

Lethal fence

Electric fences designed to carry potentially lethal currents can be used for anti-personnel purposes.

In 1915, during World War I, the German occupiers of Belgium closed off the border with neutral Netherlands, using a 300 km electric fence running from Vaals to Scheldt. Germany also erected a similar fence to isolate thirteen Alsatian villages from Switzerland. [5]

Electric fences are used in the DMZ, which separates North Korea from South Korea Entrance of Joint Security Area, Korean Demilitarized Zone, DPRK side.jpg
Electric fences are used in the DMZ, which separates North Korea from South Korea

Electric fences were used to guard the concentration camps of Nazi Germany during World War II, where potentially lethal voltages and currents were employed, continuously rather than in pulses. Some prisoners used the electric barbed wire fence to commit suicide. [22]

During the Algerian War the French erected the electrified Morice Line.

Sections of the inner German border were lined with a 3 m (10 ft) high electric fence to deter potential defectors from East Germany. [23] Similarly, the Czechoslovak border was lined with high electric fence during Cold War to prevent emigration from Czechoslovakia.

Electric fences continue to be used in similar fashion at some high-security prisons and certain other installations to this day. Typically a nonelectric fence is constructed on either side of such an installation, or the deadly current is carried out of casual reach atop a wall.[ citation needed ]

North Korea uses electric fences to seal off parts of its border with South Korea.

Other uses

Recent innovations include electrical fence monitoring for intruder detection as opposed to providing an electric shock to discourage entry. It can be used in addition to or as substitute for a host of other fence monitoring systems.

Buried electric fences (also called "invisible fences" or "electronic fences") are sometimes used to contain dogs or livestock. The buried wire radiates a weak radio signal, which is detected by a collar worn by the animal. The collar emits a warning noise near the wire, but if this is ignored, produces a mild shock. Humans and other animals are unaware of the buried line. In a similar system, the collar uses GPS signals to determine proximity to a predetermined "virtual fence" without a physical installation.

Interference and unwanted effects

Poorly designed or badly maintained electric fences can create sufficient electromagnetic interference to cause problems for nearby telephone, radio, and television reception - and has been a particular problem for dial-up Internet users in some rural areas. [24] [25]

See also

Related Research Articles

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Ground (electricity) Reference point in an electrical circuit from which voltages are measure

In electrical engineering, ground or earth is the reference point in an electrical circuit from which voltages are measured, a common return path for electric current, or a direct physical connection to the earth.

Fence Freestanding structure preventing movement across a boundary

A fence is a structure that encloses an area, typically outdoors, and is usually constructed from posts that are connected by boards, wire, rails or netting. A fence differs from a wall in not having a solid foundation along its whole length.

Electrical phenomena are commonplace and unusual events that can be observed and that illuminate the principles of the physics of electricity and are explained by them. Electrical phenomena are a somewhat arbitrary division of electromagnetic phenomena.

An overhead line or overhead wire is an electrical cable that is used to transmit electrical energy to electric locomotives, trolleybuses or trams. It is known variously as:

Electrical injury Physiological reaction or injury caused by electric current

Electrical injury is a physiological reaction caused by electric current passing through the body. The injury depends on the density of the current, tissue resistance and duration of contact. Very small currents may be imperceptible or produce a light tingling sensation. A shock caused by low and otherwise harmless current could startle an individual and cause injury due to jerking away or falling. Stronger currents may cause some degree of discomfort or pain, while more intense currents may induce involuntary muscle contractions, preventing the person from breaking free of the source of electricity. Still larger currents result in tissue damage and may trigger ventricular fibrillation or cardiac arrest. Consequences of injury from electricity may include amputations, bone fractures and orthopedic and musculoskeletal injuries. If death results from an electric shock the cause of death is generally referred to as electrocution.

Agricultural fencing

In agriculture, fences are used to keep animals in or out of an area. They can be made from a wide variety of materials, depending on terrain, location and animals to be confined. Most agricultural fencing averages about 4 feet (1.2 m) high, and in some places, the height and construction of fences designed to hold livestock is mandated by law.

Electrical substation

A substation is a part of an electrical generation, transmission, and distribution system. Substations transform voltage from high to low, or the reverse, or perform any of several other important functions. Between the generating station and consumer, electric power may flow through several substations at different voltage levels. A substation may include transformers to change voltage levels between high transmission voltages and lower distribution voltages, or at the interconnection of two different transmission voltages.

Single-wire earth return

Single-wire earth return (SWER) or single-wire ground return is a single-wire transmission line which supplies single-phase electric power from an electrical grid to remote areas at low cost. Its distinguishing feature is that the earth is used as the return path for the current, to avoid the need for a second wire to act as a return path.

Transmission tower Structure used to support an overhead power line

A transmission tower or power tower is a tall structure, usually a steel lattice tower, used to support an overhead power line.

High voltage Electrical potential which is large enough to cause damage or injury

High voltage electricity refers to electrical potential large enough to cause injury or damage. In certain industries, high voltage refers to voltage above a certain threshold. Equipment and conductors that carry high voltage warrant special safety requirements and procedures.

Overhead power line Above-ground structure for bulk transfer and distribution of electricity

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Lineworker

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Mast radiator Radio antenna consisting of a vertical mast in which the mast structure is energized and functions as the antenna

A mast radiator is a radio mast or tower in which the metal structure itself is energized and functions as an antenna. This design, first used widely in the 1930s, is commonly used for transmitting antennas operating at low frequencies, in the LF and MF bands, in particular those used for AM radio broadcasting stations. The conductive steel mast is electrically connected to the transmitter. Its base is usually mounted on a nonconductive support to insulate it from the ground. A mast radiator is a form of monopole antenna.

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A pet fence or fenceless boundary is an electronic system designed to keep a pet or other domestic animal within a set of predefined boundaries without the use of a physical barrier. A mild electric shock is delivered by an electronic collar if its warning sound is ignored. The system was first invented and patented by Richard Peck in 1973.

Stray voltage Electrical potential between unexpected places

Stray voltage is the occurrence of electrical potential between two objects that ideally should not have any voltage difference between them. Small voltages often exist between two grounded objects in separate locations, due to normal current flow in the power system. Large voltages can appear on the enclosures of electrical equipment due to a fault in the electrical power system, such as a failure of insulation.

Pest-exclusion fence

A pest-exclusion fence is a barrier that is built to exclude certain types of animal pests from an enclosure. This may be to protect plants in horticulture, preserve grassland for grazing animals, separate species carrying diseases from livestock, prevent troublesome species entering roadways, or to protect endemic species in nature reserves. These fences are not necessarily traditional wire barriers, but may also include barriers of sound, or smell.

Hot stick

In the electric power distribution industry, a hot stick is an insulated pole, usually made of fiberglass, used by electric utility workers when engaged on live-line working on energized high-voltage electric power lines, to protect them from electric shock. Depending on the tool attached to the end of the hot stick, it is possible to test for voltage, tighten nuts and bolts, apply tie wires, open and close switches, replace fuses, lay insulating sleeves on wires, and perform various other tasks while not exposing the crew to a large risk of electric shock.

Live-line working

In electrical engineering, live-line working, also known as hotline maintenance, is the maintenance of electrical equipment, often operating at high voltage, while the equipment is energised. Although this is more hazardous for personnel than working on electrical equipment with the power off, live-line maintenance techniques are used in the electric power distribution industry to avoid the disruption and high economic costs of having to turn off power to customers to perform essential periodic maintenance on transmission lines and other equipment.

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  24. "Electric fence interference with Internet", NZ Government website
  25. "Five-Step Electric Fence Check". Ministry of Economic Development