8.
a. Possible points of entry into buildings or enclosures can be wired.
Electrically charged strips of tinfoil or wire are used.
An action which
breaks the foil or wire interrupts the circuit and activates an alarm. Foil
stripping is often used on windowpanes. Doors and windows may be equipped
with magnetic or spring activated contacts. These sound an alarm when the
door or window is opened.
Protective wiring running through concealed
wooden dowels may be used on walls and ceilings.
b. Characteristics.
(1) Advantages.
These circuits consistently provide the most
trouble-free service; they cause few, if any, nuisance alarms.
Electric
circuits act as a psychological deterrent.
(2) Disadvantages.
These circuits are costly to install for many
entry points.
Their effectiveness may be defeated by the bridging of
circuits. Also, this system is not capable of detecting intruders staying
behind.
9.
Light Beams.
a. This is a photoelectric type of intrusion detection device.
It
derives its name from the use of a light sensitive cell and a projected
light source.
An infrared filter over the light source makes the beam
invisible to intruders.
This device is connected by wires to a control
station.
When an intruder crosses the beam, he breaks contact with the
photoelectric cell. This activates an alarm. A projected beam of invisible
light can be effective for approximately 500 feet (see Figure 4-1).
b. Characteristics.
(1) Advantages.
When properly used, this device affords effective,
reliable notice of intrusion.
It may also detect fire through smoke
interrupting the light beam.
(2) Disadvantages.
Use is limited to those sites where it is not
possible to bypass the beam by crawling under or climbing over it.
This
device requires some type of permanent installation, also.
Fog, dust and
rain in sufficient density, will interrupt the light beam and produce
nuisance alarms.
This system requires frequent inspections of light
producing components to detect deterioration. Also, the ground beneath the
light beam must be kept free of tall grass and weeds, drifting snow and
sand.
10.
Detection of Sound and Vibration.
a. Detection of Sound.
(1) This type of intrusion detection can be effectively used to
safeguard enclosed areas (volmetric), such as vaults, warehouses and similar
MP1002
4-6
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