Burglar (or intrusion), fire, and safety alarms
are found in electronic form today. Sensors are connected to a control
unit via either a low-voltage hardwire or narrowband RF signal, which is
used to interact with a response device. The most common security
sensors indicate the opening of a door or window, or detect motion via
passive infrared (PIR). In new construction, systems are predominately
hardwired for economy, while in retrofits, wireless systems may be more
economical and certainly quicker to install. Some systems are dedicated
to one mission, others handle fire, intrusion, and safety
,simultaneously. Sophistication ranges from small, self-contained
noisemakers, to complicated, multi-zoned systems with color-coded
computer monitor outputs. Many of these concepts also apply to portable
alarms for protecting cars, trucks or other vehicles and their contents
(that is, "car alarms"). Burglar alarms are sometimes referred to as alarm systems.
Alarm connection and monitoring
The desired result of an alarm system is to cause an appropriate alarm
output and response when the sensors indicate the valid conditions for
triggering of the alarm. The ability of the panel to communicate back
to the Monitoring Center is crucial to the concept of monitoring, and it
is often overlooked or down played.
Depending upon the application, the alarm output may be local, remote,
or a combination. Local alarms do not include monitoring, though may
include indoor and/or outdoor sounders (such as motorized bell or
electronic siren) and lights (e.g. strobe light) which may be useful for
signaling an evacuation notice for people during fire alarms, or where
one hopes to scare off an amateur burglar quickly. However, with the
widespread use of alarm systems (especially in cars), false alarms are
very frequent and many urbanites tend to ignore alarms rather than
investigating, let alone contacting the necessary authorities. In short,
there may be no response at all. In rural areas (that is, where nobody
will hear the fire bell or burglar siren) lights or sounds may not make
much difference anyway, as the nearest responders could take so long to
get there that nothing can be done to avoid losses.
Remote alarm systems are used to connect the control unit to a
predetermined monitor of some sort, and they come in many different
configurations. High-end systems connect to a central station or
responder (Police/Fire/Medical) via a direct phone wire (or
tamper-resistant fiber optic cable), and the alarm monitoring includes
not only the sensors, but also the communication wire itself. While
direct phone circuits are still available in some areas from phone
companies, because of their high cost, they are becoming uncommon.
Direct connections are now most usually seen only in Federal, State, and
Local Government buildings, or on a school campus that has a dedicated
security, police, fire, or emergency medical department. More typical
systems incorporate a digital telephone dialer unit that will dial a
central station (or some other location) via the Public Switched
Telephone Network (PSTN) and raise the alarm, either with a synthesized
voice or increasingly via an encoded message string that the central
station decodes. These may connect to the regular phone system on the
system side of the demarcation point, but typically connect on the
customer side ahead of all phones within the monitored premises so that
the alarm system can seize the line by cutting-off any active calls and
call the monitoring company if needed. Encoders can be programmed to
indicate which specific sensor was triggered, and monitors can show the
physical location (or "zone") of the sensor on a list or even a map of
the protected premises, which can make the resulting response more
effective. For example, a water-flow alarm, coupled with a flame
detector in the same area is a more reliable indication of an actual
fire than just one or the other sensor indication by itself. Many alarm
panels are equipped with a backup dialer capability for use when the
primary PSTN circuit is not functioning. The redundant dialer may be
connected to a second phone line, or a specialized, encoded cellular
phone, radio, or internet interface device to bypass the PSTN entirely,
to thwart intentional tampering with the phone line(s). Just the fact
that someone tampered with the line could trigger a supervisory alarm
via the radio network, giving early warning of an imminent problem. In
some cases, a remote building may not have PSTN phone service, and the
cost of trenching and running a direct line may be prohibitive. It is
possible to use a wireless, cellular, or radio device as the primary
communication method. There is controversy within the alarm industry as
to the usage of the Internet as a primary signaling method, due to the
twin issues of the immediacy and urgency of an alarm signal, and the
lack of quality of service within the current design of the public
internet.
Monitored alarms and speaker phones allow for the central station to
speak with the homeowner and/or intruder. This may be beneficial to the
owner for medical emergencies. For actual break-ins, the speaker
phones allow the central station to urge the intruder to cease and
desist as response units have been dispatched.
The list of services to be monitored at a Central Station has expanded
over the past few years to include: Intrusion Alarm Monitoring; Fire
Alarm & Sprinkler Monitoring; Critical Condition Monitoring; Medical
Response Monitoring; Elevator Telephone Monitoring; Hold-Up or Panic
Alarm Monitoring; Duress Monitoring; Auto Dialer tests; Open & Close
Signal Tracking, or Supervision; Open & Close Reporting; Exception
Reports; and PIN or Passcode Management. Increasingly, the Central
Stations are making this information available directly to end users via
the internet and a secure log-on to view and create custom reports on
these events themselves.
Alarm response
Depending upon the zone triggered, number and sequence of zones, time of
day, and other factors, the monitoring center can automatically
initiate various actions. They might be instructed to call an
ambulance, the fire department, or police department immediately, or to
first call the protected premises or property manager to try to
determine if the alarm is genuine. They could also start calling a list
of phone numbers provided by the customer to contact someone who would
then check on the protected premises. Some zones may trigger a call to
the local heating oil company to go check on the system, or a call to
the owner with details of which room may be getting flooded. Some alarm
systems are tied to video surveillance systems so that current video of
the intrusion area can be instantly displayed on a remote monitor, not
to mention recorded.
The first video home security system was patented (patent #3,482,037) on
December 2, 1969, to Marie Brown, an African American inventor. The
system used television surveillance.
Access control and bypass codes
To be useful, an intrusion alarm system is deactivated or reconfigured
when authorized personnel are present. Authorization may be indicated in
any number of ways, often with keys or codes used at the control panel
or a remote panel near an entry. High-security alarms may require
multiple codes, or a fingerprint, badge, hand-geometry, retinal scan,
encrypted response generator, and other means that are deemed
sufficiently secure for the purpose.
Failed authorizations should result in an alarm or at least a timed
lockout to prevent "experimenting" with possible codes. Some systems
can be configured to permit deactivation of individual sensors or
groups. Others can also be programmed to bypass or ignore individual
sensors (once or multiple times) and leave the remainder of the system
armed. This feature is useful for permitting a single door to be opened
and closed before the alarm is armed, or to permit a person to leave,
but not return. High-end systems allow multiple access codes, and may
even permit them to be used only once, or on particular days, or only in
combination with other users' codes. In any case, a remote monitoring
center should arrange an oral code to be provided by an authorized
person in case of false alarms, so the monitoring center can be assured
that a further alarm response is unnecessary. As with access codes,
there can also be a hierarchy of oral codes, say, for a furnace servicer
to enter the kitchen and basement sensor areas but not the silver vault
in the butler's pantry. There are also systems that permit a duress
code to be entered and silence the local alarm, but still trigger the
remote alarm to summon the police to a robbery.
Fire sensors can be "isolated," meaning that when triggered, they will
not trigger the main alarm network. This is important when smoke and
heat is intentionally produced. The owners of buildings can be fined
for generating false alarms that waste the time of emergency personnel.
False / no alarms
System reliability can be a problem when it causes nuisance alarms,
false alarms, or fails to alarm when called for. Nuisance alarms occur
when an unintended event evokes an alarm status by an otherwise properly
working alarm system. A false alarm also occurs when there is an alarm
system malfunction that results in an alarm state. In all three
circumstances, the source of the problem should be immediately found and
fixed, so that responders will not lose confidence in the alarm
reports. It is easier to know when there are false alarms, because the
system is designed to react to that condition. Failure alarms are more
troublesome because they usually require periodic testing to make sure
the sensors are working and that the correct signals are getting through
to the monitor. Some systems are designed to detect problems
internally, such as low or dead batteries, loose connections, phone
circuit trouble, etc. While earlier nuisance alarms could be set off by
small disturbances, like insects or pets, newer model alarms have
technology to measure the size/weight of the object causing the
disturbance, and thus are able to decide how serious the threat is,
which is especially useful in burglar alarms.
System connections: technical details
The trigger signal from each sensor is transmitted to one or more
control units, either through wires or wireless means (radio, line
carrier, infrared). Wired systems are convenient when sensors (such as
smoke detectors) require power to operate correctly; however, they may
be more costly to install. Entry-level wired systems utilize a Star
network topology, where the panel is at the center logically, and all
devices "home run" its wire back to the panel. More complex panels use a
Bus network topology where the wire basically is a data loop around the
perimeter of the facility, and has "drops" for the sensor devices which
must include a unique device identifier integrated into the sensor
device itself. Wired systems also have the advantage, if wired properly,
of detecting tampering with the wiring connections. Wireless systems,
on the other hand, often use battery-powered transmitters which are
easier to install, but may reduce the reliability of the system if the
sensors are not supervised, or the batteries maintained. Depending on
distance, construction materials, or both, one or more wireless
repeaters may be required to get the signal reliably back to the alarm
panel. Hybrid systems utilize both wired and wireless sensors to achieve
the benefits of both. Transmitters or sensors can also be connected
through the premises' electrical circuits to transmit coded signals to
the control unit (line carrier). The control unit usually has a
separate channel or zone for burglar and fire sensors, and better
systems have a separate zone for every different sensor, as well as
internal "trouble" indicators (mains power loss, low battery, wire
broken, etc).
Independent certification
Some insurance companies and local agencies require that alarm systems
be installed to code or be certified by an independent third party.
Independent certification ensures a system meets a level of criteria
above and beyond what a sales representative may offer. This insures
clients have a system that will be reliable when needed. Third-party
alarm certifying agencies include the local fire department, the
building department, Underwriters Laboratories (UL), National Fire
Protection Association (NFPA), and the National Fire & Burglar Alarm
Association (NFBAA). (The fire department and building department set
standards and inspect as needed. UL is the only certification entity.
Other associations set guidelines.)