a) To provide optimal performance, the fence must be of good quality
construction with tight tension maintained. This type of sensor requires
frequent fence maintenance.
b) Signs, barbed concertina tape, and other items attached to the
fence are a source of nuisance alarms for this sensor.
c) To provide tamper resistance and minimize environmental wear and
tear, the cable connecting the sensors should be installed in conduit.
d) This fence sensor is generally rated by DoD test agencies to
provide 0.90 probability of detection at 90 percent confidence level if of
modern, good quality manufacture with a single or multiple mass switch
instead of a simple mercury or other tilt switch, and with electronic
e) Line supervision and tamper protection on the cable connecting
the sensors as well as on the processing electronics should be specified.
f) This sensor should be mounted on the inside (protected area side)
of the fence.
g) This sensor requires extensive electronic adjustment to minimize
nuisance alarms due to wind and is generally rendered inoperable in wind
speeds in excess of 25 mph, depending upon direction.
4.3.2 Strain Sensitive Cable. This sensor consists of an electret cable
attached to the fence fabric and a signal processor mounted on the fence
fabric. Zone lengths can be up to 1000 feet, but are normally the DoD
standard length of 100 meters. A signal produced in response to deformation
in the transducer cable caused by movement or cutting of the fence or fabric
is transmitted to the processor. If the frequency and time duration of the
disturbance satisfy predetermined criteria, an alarm is generated. The
processor provides for adjustment in amplifier gain, the number of
disturbances required to generate an alarm, and alarm time. The sensor is
tamper self-protecting. Figure 26 depicts the cable composition in detail,
as well as a type installation. The cable is connected normally with
plastic ties to the fence. The following considerations for application and
installation apply for the strain sensitive cable.
a) This type of sensor also has an inherent audio assessment
capability. The electronics to provide an alarm station operator with that
capability is normally an extra cost option.
b) Different colored cable is available. The use of black colored
cable should be avoided in hot, sunny areas due to expansion/contraction of