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 Section 4:
TYPES OF SENSORS
4.1 Point Sensors - Interior. Point sensors are devices normally employed
to protect particular locations on the perimeter of a structure or
particular objects or locations within a structure. By definition, they are
not capable of providing detection capability within a volume of space,
although conversely, some volumetric sensors may be applied for point
protection. In many applications, point sensors are used by themselves to
provide a basic level of intrusion detection. For higher level security
requirements, they should be employed in conjunction with other sensors such
as volumetric sensors. Generally, point sensors can be bypassed by a
skillful and knowledgeable intruder.
4.1.1 Door and Window Protection. Doors and windows, the primary openings
in a structure's perimeter, are also the primary points of intrusion
attempts. Many times, protection of these obvious portals is the first
consideration of the security system designer. Most DoD and USN directives
which require IDS also require, as a minimum, that structure portals receive
IDS protection. Point sensors used for door and window protection use very
mature technologies and are highly reliable if installed properly. Because
they have been used for many years, however, potential intruders with a
relatively low level of sophistication are thoroughly familiar with them and
their weaknesses. For example, many types of point sensors providing
protection for doors and windows sense the interruption of a continuous flow
of low voltage electric current or electromagnetic field. If an intruder
can enter through the portal without causing an interruption in the
electrical circuit, such sensors will be bypassed successfully. Improper
placement in sensor installation often contributes to such a vulnerability.
Consequently, the design basis threat's capabilities, the level of security
required, and the principle of defense (protection) in depth should be
considered in the application of point sensors for portal protection.
4.1.1.1 Balanced Magnetic Switches. Also referred to as door "switches" or
"contacts" or "magnetic contacts," this sensor is the most commonly used
intrusion detection device. As a switch, this sensor incorporates
electrical contacts that make or break an electrical circuit as a result of
physical movement. The standard magnetic switch sensor consists of two
components, each housed separately. One of the housings contains the
contacts which will open or close in the presence of a magnetic field. The
other housing contains a magnet to provide the required magnetic field. The
magnet is mounted on the inside (protected area side) of the portal
component which moves (e.g., door, window), and the switch is mounted on the
inside frame (see Figure 15). When the magnet is removed from the vicinity
of the switch (e.g., door is opened), the switch activates (see Figure 16).
The amount of movement required is generally less than 2 inches. Certain
DoD and USN directives require as little as 1 inch maximum separation for
switch activation; others require switch activation prior to visual access
or the capability being achieved to physically access the sensor. Since the
switch contacts are held in their
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