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 The generic types of sensors normally applied for glass breakage detection
are discussed below. Volumetric sensors may also be used in the vicinity of
the interior window area to detect the intrusion rather than the actual
glass breakage.
a) Breakwire. This type of sensor generally functions in the same
manner as foil. The breakage of glass also causes the breakage of thin (No.
24-36 AWG), low tensile strength (maximum 4 pounds) wire, imbedded in window
mullions or overlaid on the glass itself, which interrupts a low-voltage
direct current running through the wire producing an alarm. Another form of
this type of sensor is the application of a "trip wire" on the inside of the
glass area. Movement or breakage of this wire causes an electrical current
to be completed or, in another form, to be interrupted, producing an alarm.
The simplicity of this sensor makes it highly reliable but also makes it
relatively easy to defeat by jumping or bridging the wire circuit and then
cutting through it. Several application and installation considerations
apply to breakwire sensors as well. Because an unsophisticated intruder can
defeat this sensor with relative ease, it is not found in most DoD
applications. It is suitable for relatively low level security applications
such as quarters, convenience stores, etc., but should be used in
conjunction with another sensor (preferably a volumetric motion detection
sensor) to assure intruder detection if defeated or bypassed. Generally, a
breakwire sensor is used in a different application for DoD facilities as
"wire trap" for building vent and duct protection. Interlaced in such an
opening, it provides effective protection and is not visible (and hence not
vulnerable) when used for window and other glass protection. This sensor
may be used also for protection of other portals. Its use in providing
protection for overhead doors, for example, is one application. Fourthly,
when specifying installation as a "trip wire," the wire normally used as the
trip wire should be substituted during daylight. The use of blackcoated
wire, deployed only at night, also increases effectiveness. Lastly, the
man-hours involved in properly positioning and installing a breakwire
sensor, depending upon the application, far exceed the cost of the sensor.
Careful consideration of the tradeoffs involved must be a part of the system
design process. Another type of sensor which costs more, but takes less
time to install, may be more cost-effective.
b) Vibration/Ultrasonic. This category is two separate sensors.
However, since their outward appearance is identical and they are used for
the same application, both vibration and ultrasonic glass breakage sensors
will be covered in this section. Both types are also known as "window
bugs;" vibration sensors are also called "shock" sensors. Vibration sensors
detect attempts to penetrate building perimeter barriers, such as a window,
by sensing with a piezoelectric crystal the intense vibrations which are
associated with intrusion attempts. Normally, the better quality sensors of
this type have a sensitivity adjustment with a relatively extensive range
(e.g., one complete turn potentiometer) and mechanical filtering. This
assures minimal nuisance alarms due to flexing of glass and other nuisance
sounds such as heavy construction or traffic. Ultrasonic penetration
detection sensors detect the sounds made by a forcible intrusion attempt
even if these sounds are barely audible or inaudible to the human ear. They
are "passive" receivers and should not be confused with active ultrasonic
motion sensors discussed elsewhere in this section. These sensors consist
of a
13.02-36
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