Fire Detection and Alarm Systems: A Brief Guide
Inspection, testing, and maintenance requirements for these systems are extensive and ultimately are likely to cost more than the original installation.
Fire detection systems are designed to
discover fires early in their development
when time will still be available
for the safe evacuation of occupants. Early
detection also plays a significant role in
protecting the safety of emergency
response personnel. Property loss can be
reduced and downtime for the operation
minimized through early detection
because control efforts are started while
the fire is still small. Most alarm systems
provide information to emergency
responders on the location of the fire,
speeding the process of fire control.
To be useful, detectors must be coupled
with alarms. Alarm systems provide notice
to at least the building occupants and usually
transmit a signal to a staffed monitoring
station either on or off site. In some
cases, alarms may go directly to the fire
department, although in most locations
this is no longer the typical approach.
These systems have numerous advantages
as discussed above. The one major
limitation is that they do nothing to contain
or control the fire. Suppression systems
such as automatic sprinklers act to
control the fire. They also provide notification
that they are operating, so they can fill
the role of a heat detection-based system if
connected to notification appliances
throughout the building. They will not,
however, operate as quickly as a smoke
detection system. This is why facilities
where rapid notice is essential, even when
equipped with sprinklers, still need detection
and alarm systems.
The most basic alarm system does not
include detection. It has manual pull stations
and sounds only a local alarm. This
level of system is not what is typically used;
it relies on an occupant to discover the fire,
which can cause a significant delay.
The more quickly you want to be notified
of the fire, the more costly the system
you must install. Speed of detection is
expensive. The slowest system to detect a
fire is a heat detector, which is also the
least expensive. An air-aspirating smoke
detection system provides the most rapid
indication of fire, but these systems are
five to 10 times as expensive.
Where to Start When Choosing a System
The type of fire detection and alarm system
used in your facility should be based upon
your fire safety objectives. These objectives flow from a risk assessment of your facility
and operation. Your tolerance for risk and
how much you can afford to lose is an
important part of this process.
The issues within a hospital are not
the same as they are in a warehouse. In a
facility where life safety is the major concern,
such as hospitals where patients may
not be able to evacuate on their own,
early warning is essential. Dormitories,
hotels, and other facilities where occupants
may be sleeping when a fire starts
also require that a system provide more
rapid notification.
In a warehouse, where the occupants
will be awake and aware and there will
most often be fewer of them, the alarmsystem often does not need to provide
notice as early. In a generally unoccupied
structure where life safety is not a major
issue, detection of a fire can be slower
without significantly increasing risk.
When selecting a system, you also
must consider the ongoing commitment
that will be required over the life of the
system. Inspection, testing, and maintenance
requirements for these systems are
extensive. Meeting these requirements
over the life of a system usually will cost
more than the original installation.
Initiating devices are elements of the
system that originate a signal. Manual
pull stations, detectors, and supervisory
devices are included in this group of
components.
A manual pull station (Figure 1) is
essentially just a switch that activates the
alarm system when operated by a building
occupant. Pull stations should be positioned
so they are easy for occupants to
find. They are typically located along
routes of travel that would be used while
exiting the building.
Detectors are available in a wide
variety of types. The major categories are
heat, smoke, and flame detectors. Within
each category are numerous additional
specific types. The discussion here will be
limited to those most commonly used for
building fire detection and alarm activation.
Several varieties of detectors, such as
flame detectors, are used primarily to activate
suppression systems.
Heat detectors are the most basic
detection devices. They are available in
several types. These types are divided into
two major categories; spot and line. Spot
detectors are single units installed in
single locations throughout the protected
area. Line detectors provide a continuous
detector throughout the area of coverage.
Spot detectors are more commonly used,
with line type detectors being reserved for
special situations.
Spot type heat detectors (Figure 2) are
most commonly fixed temperature, rateof-
rise, or combination. Fixed temperature,
as the name implies, operate at a specific
temperature. Rate-of-rise detectors
activate based on the speed of the rise in
temperature, not a fixed point. These
detectors are best suited to use in areas
that may get hot under normal conditions,
such as detectors in a warehouse that is
not temperature-controlled. Detectors mounted at the ceiling level may get quite
warm from the heat collected by the roof
during the day. This temperature rise
occurs gradually, though, and a rate-ofrise
detector compensates for it.
Smoke detectors (Figure 3) are available
in a variety of types. Photoelectric
smoke detectors operate based upon light
scattering within the detection chamber
of the detector. Light is projected
through the chamber and will be scattered
if it strikes smoke. This light,
reflected off the smoke in the chamber, is
detected by a photocell.
Ionization smoke detectors (the most
common in home use) detect the particles
in smoke. As smoke passes through
the chamber, the particles are ionized.
These particles may then be detected by
charged plates in the detector. Smoke
detectors are also available in combination
with a heat detector (Figure 4). The
beam type detector (Figure 5) operates
when the beam is interrupted by obscuring
smoke between the laser emitter and
receiver. These detectors are most often
used in areas of large open spaces.
An air sampling detection system uses
tubing placed throughout the protected
area. The tubing has small holes spaced
out along the length of the tube and air is
constantly drawn into the unit, which can
detect extremely low levels of combustion
products.
Supervisory and Notification Devices
Numerous supervisory devices can be
connected to the fire alarm control panel.
For example, a tamper switch (Figure 6)
may be placed on water control valves for
automatic sprinkler systems. If this valve is
closed by an unauthorized person, the
tamper switch will send a supervisory
signal to the control panel, alerting your
people to the problem. Supervisory
devices are available for a wide variety of
applications.
Systems may be addressable or nonaddressable.
In the first type, all of the
detectors on the system have a unique digital
identifier. The fire alarm control panel
can communicate individually with each
device. In non-addressable systems, detectors
may be divided into zones based on
all of the detectors being on the same pair
of wires, but the control panel cannot
determine any information about an individual
detector.
Addressable systems offer several
advantages. The first is that a specific indication
of the location of an activation is
available during a fire. (Would you rather
know that a detection has occurred somewhere
in the west wing of your building or
that it has occurred in office number 103?
The latter is clearly more informative.)
This specific location capability is also
part of the second major advantage of
these systems: trouble signals can specifically
identify the component with a problem. If a single detector fails, for
example, an addressable system will provide
a trouble signal that indicates the
specific detector. In non-addressable systems,
the zone will be identified, but a
repair person will have to check each
detector in that zone to determine which
one is not working.
Notification appliances are the
audible, visual, and other devices located
throughout the facility that warn occupants
when the system has detected a
fire. Horns, strobes, combination units
(Figure 7), and bells are examples of
these devices. Fire alarm control panels
often have features available that allow
alarms to be activated in selected locations
within the facility based upon the
location of the detector that activates.
This feature can be used to permit staged
evacuations, for example.
Alarms should be supplemented with
communications devices that allow you to
provide specific information and instructions
to building occupants. People tend
not to always respond as they should when
a fire alarm sounds. An emergency voice
communications system can significantly
improve response of your occupants.
Outside Assistance
One of the best references for fire alarms
is NFPA 72 National Fire Alarm Code®. It
contains requirements for design, installation,
inspection, testing, and maintenance.
It may or may not be adopted as law in
your area. Check with your local fire
department to identify the specific local
requirements.
Your property insurance carrier also
may place requirements on you as a condition
for coverage or a specific premium
level.
Detection and alarm systems are an
important part of your overall fire protection
process. Discovering fires early contributes
to protecting building occupants,
limiting property damage, and minimizing
interruption of your operation.
References
1. “Industrial Fire Protection Handbook”
2nd edition, R. Craig Schroll, © CRC
Press 2002, ISBN: 1-58716-058-7.
2. NFPA 72 National Fire Alarm Code®
2007 Edition, © National Fire Protection
Association 2006.
3. “Installed Fire Protection: Alarm Systems,”
Occupational Health & Safety,
February 2003.
This article originally appeared in the December 2007 issue of Occupational Health & Safety.