Electric fence

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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 fencing and 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 still used.

[edit] 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-voltage pulse. One terminal of the power energizer releases a high-voltage^[clarify] 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 the wire and the earth simultaneously will complete an electrical circuit and will conduct the pulse, causing a painful electric shock. The effects of the electrical shock depend upon the voltage, the electrical current used, and the degree of contact between the animal and the fence or ground; it can range from barely noticeable to uncomfortable, painful or even lethal.

[edit] 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. Though still available, they have declined in popularity.

Modern "low impedance" fence chargers use a different design. A capacitor is charged by a solid-state circuit – upon contact with a grounded animal or person, the charge is then released using a thyristor or similar solid-state component. Voltage is consistent due to electronic output controls, within the limits of output power. Pulse width is much narrower, often about 10 microseconds. This design works for either battery or mains power sources.

Depending on the area to be fenced and remoteness of its location, fence energizers may be hooked into a permanent electrical circuit, may be run by large automobile batteries, or may be powered by a small battery kept charged by a solar panel. The power consumption of a fence in good condition is low, and so an automobile battery powering several hundred metres of fence may last for several weeks on a single charge.

[edit] Fencing materials

Smooth steel wire is the material most often used for electric fence, 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. Synthetic webbing and rope-like fencing materials woven with fine conducting wires (usually of stainless steel) have come onto the market in 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, burn, or short out the fence. Thus, fencing must avoid vegetation, and cannot be attached directly to wood or to metal posts. Typically, wood or metal posts are driven into the ground, plastic or porcelain insulators are attached to the posts, and fencing material is attached to the insulators. Other techniques include using fence posts made of non-conducting synthetic materials.

[edit] History

The concept of the electric fence was first described in Mark Twain's A Connecticut Yankee in King Arthur's Court, in 1889, as a defensive weapon. Electric fences were used to control livestock in the United States in the early 1930s, and electric fencing technology developed in both the United States and New Zealand.

An early application of the electric fence was developed in 1936–1937 by New Zealand inventor William "Bill" Gallagher Sr. Built from a car's ignition coil and a meccano set, Gallagher used the device to keep his horse from scratching itself against his car. Gallagher later started a company to improve and market the design.^{[citation needed]} In 1962, another New Zealand inventor, Doug Phillips, invented the non-shortable electric fence based on capacitor discharge. This significantly increased the range an electric fence could be used from a few hundred yards to 35 miles, and reduced the cost of fencing by more than 80%.^{[citation needed]} 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."^{[citation needed]} This idea was a major contributing factor enabling New Zealand to become a world leader in efficient pastoral farming.^{[citation needed]}This non-shortable design is the system that is now used on farms throughout the world today.

Doug Phillips went on to develop and patent other improvements to electric fence technology, but has rarely been recognised or credited for his contribution by the industry. The Gallagher firm and other manufacturers^{[attribution needed]} obtained the Phillips patent in 1975, 3 years before it expired.^{[citation needed]} Today the Gallagher Group of companies is still heavily involved in electric fencing for livestock control.

Electric fences have improved significantly since the early days. Improvements include:

• Polyethylene insulators replacing porcelain insulators, 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.

[edit] Uses

[edit] Agriculture

Main article: Agricultural fencing

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 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.

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.

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 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 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 recognise the slight clicking sound made by some electric fences and thus can sense when the fence is off.

[edit] Wild animals

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.

[edit] Security

[edit] Non-lethal fence

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.

[edit] Lethal fence

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

• During World War I, the German occupiers of Belgium closed off the border with neutral Netherlands, using an electric fence.
• Electric fences were infamously 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. ^[1]

• 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 border were lined with a 3 m (10 ft) high electric fence to stop potential defectors from East Germany.^[2]
• North Korea uses electric fences to seal off parts of its border with South Korea.^[3]

[edit] Other uses

Recent innovations have included the use of electricity to monitor fencing for intruder detection as 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 GPS signals to determine proximity to a predetermined "virtual fence", without the need for any physical installation at all.

[edit] 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.

[edit] See also

• Temporary fencing
• Agricultural fencing

[edit] References

[edit] External links

• Controlling deer
• Wildlife control at Canadian Airports
• Controlling geese
• Uses in Montana
• Use in prisons
• New Zealand Government advice on electric fence interference
• Telecom New Zealand advice on electric fence interferenceda:Elektrisk hegn

de:Elektrozaun nl:Schrikdraad