a) Even with sensitivity adjusted to reject leaves, etc., a tight,
well maintained quality fence is required, as any significant disturbance in
the electrostatic field can cause an alarm. Since the field extends above
the fence, outrigger barbed wire/tape must also be taut.
Performance in deep snow or with ice or frost coating is
c) Wire tension maintenance devices have not met performance
standards in the past.
d) Metal fatigue of transmitter (field) and sensor (receiver) wires
and required repair should be considered.
e) Chain link fence fabric limits the electrostatic field. Mounting
on the inside of the fence reduces nuisance alarms, since no field change is
sensed until the fence fabric is deformed by either climbing or breaching.
Mounting on the outside of the fence will detect an intruder's approach (as
well as other nuisance stimuli). Mounting is generally only effective on
the inner fence of a dual fence perimeter. Stand-alone mounting enlarges
the detection zone width, but increases cost significantly.
f) Minimizing nuisance alarms requires removing all vegetation
within a 4-to 6-foot radius of the electrostatic field. Continuous and
extensive maintenance is a must to assure optimal sensor performance. The
cost of this maintenance may be a consideration, as may be the aesthetic
requirement for a denuded clear zone.
Gates and other fence openings should be protected by the
h) Application in areas with large bird populations should be
avoided since the sensor attracts birds as a roost, causing nuisance alarms.
i) Wind affects the sensor by causing deformation of the field or
fence fabric, and debris may penetrate the field resulting in nuisance
4.3.4 Taut Wire Sensors. This type of sensor may be used as a stand-alone
sensor and barrier, or may be attached to the outside of an exist- ing
fence. This sensor consists of a series of horizontal wires under tension
with 6-inch vertical separation, mounted through ring eyes or slider clips
on the exterior of the fence posts covering the entire fence area. These
wires are normally barbed to increase barrier effectiveness. Tension is
maintained by wire springs at both ends of the sensor zone. These wires are
mechanically clamped to a vertical wire, called a sensor wire, attached
within a special sensor post in the center of the sensor zone. This wire is
connected to transducers which convert mechanical motion to electronic