Upgrading Security - The Drinking Water - page 267

The main differences among alarm systems lie in the types of event detec-
tion devices used in different systems. Intrusion sensors, for example, con-
sist of two main categories: perimeter sensors and interior (space) sensors.
Perimeter intrusion sensors are typically applied on fences, doors, walls, win-
dows, etc. and are designed to detect intruders before they gain access to a
protected asset (e.g., perimeter intrusion sensors are used to detect intruders
attempting to enter through a door or window). In contrast, interior intrusion
sensors are designed to detect an intruder who has already accessed the pro-
tected asset (i.e., interior intrusion sensors are used to detect intruders when
they are already within a protected room or building). These two types of
detection devices can be complementary, and they are often used together to
enhance security for an asset. A typical intrusion alarm system, for example,
might employ a perimeter glass-break detector that protects against intrud-
ers accessing a room through a window, as well as an ultrasonic interior
sensor that detects intruders that have gotten into the room without using
t he w i ndow.
Fire detection and fire alarm systems consist of various combinations of fire
detection devices and fire alarm systems. These systems may detect fire,
heat, or smoke, or a combination of any of these. A typical fire alarm system
might consist only of heat sensors located throughout a facility that detect
high temperatures or a certain change in temperature over a fixed time
period, whereas a different system might be outfitted with both smoke and
heat detection devices.
When a sensor in an alarm system detects an event, it must communicate
an alarm signal. The two basic types of alarm communication systems are
hardwired and wireless . Hardwired systems rely on wire that is run from the
control panel to each of the detection devices and annunciators. Wireless
systems transmit signals from a transmitter to a receiver through the air—
primarily using radio or other waves. Hardwired systems are usually
lower cost, more reliable (they are not affected by terrain or environmen-
tal factors), and significantly easier to troubleshoot than wireless systems;
however, a major disadvantage of hardwired systems is that it may not be
possible to hardwire all locations (e.g., it may be difficult to hardwire remote
locations). In addition, running wires to their required locations can be both
time consuming and costly. The major advantage to using wireless systems
is that they can often be installed in areas where hardwired systems are not
feasible; however, wireless components can be much more expensive when
compared to hardwired systems. Also, in the past it has been difficult to
perform self-diagnostics on wireless systems to confirm that they are com-
municating properly with the controller. Currently, the majority of wire-
less systems incorporate supervising circuitry, which allows the subscriber
to recognize immediately any problem with the system (such as a broken
detection device or a low battery) or if a protected door or window has been
left open.