- Electric fence
An electric fence is a barrier that uses
electric shocks to deter animals or people from crossing a boundary. The voltage of the shock may have effects ranging from uncomfortable, to painful or even lethal. Most electric fencing is used today for agricultural fencingand other forms of animal control purposes, though it is frequently used to enhance security of sensitive areas, and there exist places where lethal voltages are used.
Design and function
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 voltagepulse. One terminal of the power energizer releases a high-voltage This idea was to move from ceramic to using a plastic insulator at the post. A variety of plastic insulators are now used on farms throughout the world today.
In 1969 a farmer, Robert B. Cox, in Adams County, Iowa, near Corning, Iowa invented an improved electric fence bracket and was issued a 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 that may grow beneath the wire to be cut down by a mower. 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 since the early days. Improvements include:
Polyethyleneinsulators replacing porcelaininsulators, beginning in the 1960s. Polyethylene is much cheaper than porcelain and is not as breakable.
* Improvements in electrical design of the fence energizer, often referred to as a "charger" (USA) or "fencer" (UK).
* Changes in laws. In some jurisdictions, certain types of electrical outputs for fences were unlawful until the 1950s or 1960s. In other areas, signage requirements and other restrictions limited usability.
* Introduction of high tensile (HT) steel fence wire in the 1970s in New Zealand and in the 1980s in the United States
* Introduction of synthetic webbing and rope-like fencing materials woven with fine conducting wires.
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 wireor 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 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, though 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.
agricultural fencingof 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. 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 ridingand 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 by 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 3 meters) 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 of time. 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 pounded 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 wood 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
tractoror all-terrain vehicle.
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
sheepor Highland cattle) may learn to push through the fence themselves, using their coats as electrical insulation. Some animals also learn to recognise the slight clicking sound made by some electric fences and thus can sense when the fence is off.
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
boarfrom 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.
Non-lethal electric fences are used to prevent trespass by both private and government-sector bodies. These include housing communities, commercial factories or warehouses, prisons, military bases, and government buildings. Livestock-type electric fences are occasionally employed to discourage suicide attempts on tall structures, and to reduce the incidence of graffiti and other petty crime.
Electric fences designed to carry potentially lethal currents can be used for anti-personnel purposes.
World War I, the German occupiers of Belgiumclosed off the border with neutral Netherlands, using an electric fence.
*Electric fences were infamously used to guard the
concentration camps of Nazi Germanyduring 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. [ [http://www.ushmm.org/outreach/auschwtz.htm Auschwitz] (from the website of the United States Holocaust Memorial Museum)]
*They 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.
*Sections of the
inner German borderwere lined with a 3 m (10 ft) high electric fence to stop potential defectors from East Germany. ["E. Germany Builds Electric Fence", "The Times", 28 March 1984]
North Koreauses electric fences to seal off parts of its border with South Korea. [ [http://www.pbs.org/frontlineworld/stories/northkorea/thestory.html "North Korea: Suspicious Minds."] "Frontline", January, 2003. Web site accessed November 15, 2007]
Recent innovations have included the use of electricity to monitor fencing for
intruder detectionas opposed to providing an electric shock to discourage entry.
Buried electric fences (also called "invisible fences") are sometimes used to contain
dogs or livestock. The buried wire radiates a weak radio signal, which is detected by a special collar worn by the animal. The collar emits a warning noise in proximity to the wire, but if this is ignored, it produces a mild shock. Humans and other animals are unaware of the buried line. In a similar system, the collar uses GPSsignals to determine proximity to a predetermined "virtual fence", without the need for any physical installation at all.
Interference and unwanted effects
Electric fences have the theoretical potential to radiate a significant amount of energy, acting like an antenna or aerial. Poorly maintained electric fences (with insufficient grounding or bad design) can interfere with, and significantly degrade, the performance of nearby telephone and data connections, along with radio and television reception.
* [http://www.attra.org/attra-pub/deercontrol.html Controlling deer]
* [http://www.tc.gc.ca/civilaviation/AerodromeAirNav/Standards/WildlifeControl/tp11500/sectionf/SectionF3.htm#Electric Wildlife control at Canadian Airports]
* [http://www.wildlife.alaska.gov/index.cfm?adfg=birds.goose Controlling geese]
* [http://www.montana.edu/wwwpb/pubs/mt200010.html Uses in Montana]
* [http://www.doc.nv.gov/hdsp/general.php Use in prisons]
* [http://www.med.govt.nz/templates/MultipageDocumentPage____9733.aspx New Zealand Government advice on electric fence interference]
* [http://www.telecom.co.nz/content/0,8748,203702-203226,00.html Telecom New Zealand advice on electric fence interference]
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