Electric fencing
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2 Use for security or incarceration 3 Reference |
Use for Stock Control
According to (1), electric fences were used for stock control in the United States no later than 1936, and developed further in both the United States and New Zealand. New Zealand continues to be a pioneer in fencing technologies to this day.
Electric fences have improved significantly since the early days. Improvements include:
- Polyethylene insulators replacing porcelain beginning in the 1960s
- Improvements in electrical design of the charger (or fencer)
- Changes in laws. In some jurisdictions, mains fencers were unlawful until the 1950s or 1960s. In other areas, signage requirements and other restrictions limited usability.
- Introduction of HT fence in the 1970s in New Zeland and in the 1980s in the United States
Early mains-driven 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.
Later "low impedance" fence chargers use a different design. A capacitor is charged by a solid-state circuit, then released using a thyristor or similar solid-state component. Voltage is consisent 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.
Permanent electric fencing may be constructed using conventional HT fencing techniques, with plain steel wire serving as the conducting wire. The wire must be kept insulated from the earth. Typical methods for doing so involve the mounting of the fence wire on plastic or porcelain insulators; other techniques include using fence posts that are themselves insulators. In the U.S., permanent electric fence is most often run using soft steel wire, above or in front of a woven wire or barbed wire fence that provides a physical barrier.
Permanent electric fencing is popular in many agricultural areas, as construction of electric fences (using plain wire and lighter construction, as the fence does not need to physically restrain animals) is much cheaper and faster than conventional fences. Its disadvantages include the potential for the entire fence to be disabled due to a break in the conducting wire, power failure, or forced disconnection due to the risk of fires starting by dry grass touching the electrified wire. In practice, once animals have learned of the unpleasant consequences of touching the fence they tend to avoid it for considerable periods even when inactive.
Substandard conventional fencing can also be repaired quickly and cheaply by the addition of a single electric wire mounted as a "stand-off" using spring-loaded insulated wire mounts from the original fence.
Electric fencing is probably more popular, however, for the construction of temporary fencing, particularly to support the practice of strip grazing. Typically, a single strand of wire, or flexible plastic tape embedded with conducting wire, is mounted on espeically-designed posts designed to be pressed in by the fencer's feet. Within a few minutes a large area can be fenced off. Portable, battery-powered fence "energiser" units are made for this purpose.
Use for security or incarceration
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. They continue to be used in like fashion at 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.
Livestock-type electric fences are occasionally employed to discourage suicide attempts on tall structures, graffiti, and other petty crime.
