UFC 4-021-02NF
27 September 2006
change 1, 23 October 2006
5-4.4.2 Taut wire systems. Taut-wire fence-sensing systems use a series of parallel
wires under tension with a numerous micro-switches attached to it. The system is very
sensitive, but requires frequent maintenance. For more information on taut-wire systems
refer to The Design and Evaluation of Physical Protection Systems.
5-4.4.3 Coaxial strain-sensitive cable systems use a coaxial cable woven through
the fabric of the fence. The coaxial cable transmits an electric field. As the cable moves
due to strain on the fence fabric caused by climbing or cutting, changes in the electric
field are detected within the cable, and an alarm condition occurs.
Coaxial strain-sensing systems are readily available and are highly tunable to adjust for
field conditions due to weather and climate characteristics. Some coaxial cable systems
are susceptible to electromagnetic interference and radio frequency interference.
5-4.4.4 TDR Systems. Time Domain Reflectometry systems send an induced radio-
frequency (RF) signal down a cable attached to the fence fabric. Intruders climbing or
flexing a fence create a signal path flaw that can be converted to an alarm signal. When
the conductor cable is bent of flexed, a part of the signal returns to the origination point.
This reflected signal can be converted to an intrusion point by computing the time it
takes for the signal to travel to the intrusion point and return. The cable can be provided
in armored cable, which requires more than a bolt cutter to sever the sensing cable.
These systems require their own processor unit and can be configured in a closed loop,
such that if the cable is cut, a detection can be detected by the other return path.
5-4.4.5 Fiber-optic strain-sensitive cable systems are similar to the coaxial strain-
sensitive cable systems. The fiber-optic system uses a fiber-optic cable, rather than a
coaxial cable, woven through the fence fabric. Strain on the fence fabric causes micro-
bending of the fiber cable, which is monitored by the control panel and generates an
alarm condition. Figure 5-11 shows a typical fiber-optic fence detection illustration.
Fiber-optic strain-sensing systems are relatively newer detection systems but have a
strong following. The systems are readily available and are highly tunable to adjust for
field conditions due to weather and climate characteristics. The systems are impervious
to lightning, electromagnetic interference, radio frequency interference, or other
electronic signals and can be used over long distances.
5-4.4.6 Defeat Measures and False Positives. Possible defeat measures include
tunneling, jumping, or bridging across the fence system. Careful climbing at corner
posts may not generate sufficient vibration to generate an alarm condition.
Possible false positives can occur from debris, animals, and plants.
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