System Sensor - Smoke Detector General Applications Guide
Smoke Detectors APPLICATIONS GUIDE: SYSTEM SMOKE DETECTORS
The use of early warning fire and smoke detection systems results in significant reduction in fire deaths. The sooner a fire is detected, the better the out
come for saving lives. This document provides guidance for the proper operation of fire detection systems for those who apply, install, and maintain them.
Correct installation and maintenance of smoke detectors prevents unwanted nuisance alarms. Occupants can become desensitized when repeated
nuisance alarms occur. In worst case scenarios, technicians could disconnect alarms from the system to avoid the unnecessary disruption. Either situ-
ation negates a detector’s potential life saving benefit, making the proper operation of an early warning fire and smoke detection system indispensable.
NFPA Codes and Standards ..............................................................................
Standards That Apply
Building and Fire Codes .....................................................................................
Testing Laboratories ............................................................................................
Industry Publications ...........................................................................................
Manufacturer’s Publications ................................................................................
Ionization Smoke Detector Operation ................................................................
How Smoke Detectors Work ............................................................................
Photoelectric Smoke Detector Operation ..........................................................
Photoelectric Light Scattering Smoke Detector ................................................
Photoelectric Light Obscuration Smoke Detector .............................................
Smoke Detector Design Considerations
Considerations in Selecting Detectors ...............................................................
Smoke Detector Limitations
Typical System Layout ......................................................................................
Wiring Supervision ...............................................................................................
Class B Circuits ....................................................................................................
Class A Circuits ....................................................................................................
Wireless Circuits ...................................................................................................
General Zoning Guidelines
Fire Safety Functions ...........................................................................................
Smoke Detector Installation
Wireless Systems .................................................................................................
Installation Dos and Don’ts .................................................................................
Proper Detector Applications, Placement, and Spacing* ...........................
Where to Place Detectors ...................................................................................
Where Not To Place Detectors..........................................................................
Standards for Smoke Detectors .......................................................................
Detector Spacing ...............................................................................................
Detectors in Air Handling and Air Conditioning Systems ...............................
Detectors in Above-Ceiling Plenum Areas Including Those Utilized
as Part of the HVAC System .............................................................................
Testing, Maintenance, and Service of Detectors ........................................
Typical Inspection, Test, and Maintenance Practices .....................................
What to Do About Unwanted Alarms ...............................................................
Reasons for Unwanted Alarms .........................................................................
Maintain an Alarm Log ......................................................................................
Effects of Location or Environment ...................................................................
Inspect Detector for Dirt, Review Maintenance ..............................................
Effects of Other Systems on Alarm System .....................................................
Miscellaneous Causes of Unwanted Alarms
Responsibilities of Detector Owners and Installers ........................................
Where to Get Help if the Source of Unwanted Alarms Cannot be Found
Glossary of Terms ............................................................................................
Fire Alarm Log
19 SYSTEM SENSOR
Building and Fire Codes
International Code Council, Inc. (International Building Code/
International Fire Code)
BOCA, ICBO, and SBCCI formed an umbrella organization known as the
International Code Council (ICC), to combine their codes into a single set
of model building and fire codes. The ICC International Building Code and
International Fire Code were first published in 2000 and were adopted by
5360 Workman Mill Road
Whittier, CA 90601-2298
Testing laboratories test smoke detectors, control panels, and other compo
nents of fire alarm systems to verify conformance with NFPA requirements
and their own standards. Equipment that passes their tests is identified by
a label and/or listing.
Underwriters Laboratories, Inc. (UL)
333 Pfingsten Road
Northbrook, IL 60062
455 E. Trimble Road
San Jose, CA 95131
1285 Walt Whitman Road
Melville, NY 11747
12 Laboratory Drive, P.O. Box 13995
Research Triangle Park, NC
2600 N.W. Lake Road
Camas, WA 98607
UL publishes an annual directory of fire protection equipment which bears
the UL label. The following standards apply to smoke detectors.
UL 217: Single and Multiple Station Smoke Detectors
Smoke Detectors for Fire Protection Signaling Systems
UL 268A: Smoke Detectors for Duct Applications
Standard for Control Units for Fire Protective Signaling Systems
Factory Mutual Research (FM)
1151 Boston-Providence Turnpike, P.O. Box 9102
Norwood, MA 02062
FM publishes an annual report listing fire protection equipment which bears
NEMA Guide for Proper Use of Smoke Detectors in Duct Applications
NEMA Training Manual on Fire Alarm Systems
NEMA Guide for Proper Use of System Smoke Detectors
The manufacturers of the smoke detectors being used should be contacted
for any published information pertaining to their products.
Two basic types of smoke detectors are used today: ionization and photo-
electric. The sensing chambers of these detectors use different principles
of operation to sense the visible or invisible particles of combustion given
off in developing fires.
The purpose of this guide is to provide information concerning the proper
application of smoke detectors used in conjunction with fire alarm sys
tems. The guide outlines basic principles that should be considered in the
application of early warning fire and smoke detection devices. It presents
operating characteristics of detectors and environmental factors, which
may aid, delay, or prevent their operation.
This document presents information for fire protection, mechanical, and
electrical engineers; fire service personnel, fire alarm designers; and install
ers. A key element in the effectiveness of smoke detection systems is the
latest version of NFPA 72 for installation and testing of systems. Installation
must comply with all code requirements and directions from Authorities
Having Jurisdiction (AHJs). AHJ directives always take precedence over
other codes and exercise final authority over installations and maintenance
NFPA Codes and Standards
Standards That Apply
NFPA publishes standards for the proper application, installation, and main
tenance of automatic smoke detectors. The principal codes and standards,
which should be reviewed before specifying or installing automatic smoke
detectors are listed below.
National Fire Protection Association (NFPA)
Quincy, MA 02269-9101
NFPA publishes codes and standards concerning all phases of fire protec
tion. These apply to automatic smoke detectors.
NFPA 70: National Electrical Code
NFPA 72: National Fire Alarm Code and Signaling Code
NFPA 72 covers minimum performance, location, mounting, testing,
and maintenance requirements of automatic fire detectors. Many NFPA-
application specific standards are available and should be considered in
addition to NFPA 72, when applicable.
Standard for the Installation of Air Conditioning and Ventilating
Installation of Warm Air Heating and Air-differentials
Smoke Control Systems in Malls, Atria, and Large Areas
NFPA 90A and 92B provide information for the use of smoke detectors
in ducts of heating, ventilating, or air conditioning (HVAC) systems and
smoke control systems.
NFPA 101: Life Safety Code
NFPA 101 specifies the requirements for smoke detection in both new and
existing buildings depending on the type of occupancy. APPLICATIONS GUIDE: SYSTEM SMOKE DETECTORS
Ionization Smoke Detector Operation
A typical ionization chamber consists of two electrically charged plates and
a radioactive source (typically Americium 241) for ionizing the air between
the plates. (See Figure 1.) The radioactive source emits particles that col-
lide with the air molecules and dislodge their electrons. As the molecules
lose electrons, they become positively charged ions. As other molecules
gain electrons, they become negatively charged ions. Equal numbers of
positive and negative ions are created. The positively charged ions are
attracted to the negatively charged electrical plate, while the negatively
charged ions are attracted to the positively charged plate. (See Figure
2.) This creates a small ionization current that can be measured by elec
tronic circuitry connected to the plates (“normal” condition in the detector).
Particles of combustion are much larger than the ionized air molecules.
As particles of combustion enter an ionization chamber, ionized air mol
ecules collide and combine with them. (See Figure 3.) Some particles
become positively charged and some become negatively charged. As
these relatively large particles continue to combine with many other ions,
they become recombination centers, and the total number of ionized par
ticles in the chamber is reduced.
How Smoke Detectors Work
This reduction in the ionized particles results in a decrease in the cham
ber current that is sensed by electronic circuitry monitoring the chamber.
When the current is reduced by a predetermined amount, a threshold is
crossed and an “alarm” condition is established.
Changes in humidity and atmospheric pressure affect the chamber current
and create an effect similar to the effect of particles of combustion entering
the sensing chamber. To compensate for the possible effects of humidity
and pressure changes, the dual ionization chamber was developed and
has become commonplace in the smoke detector market.
A dual-chamber detector utilizes two ionization chambers; one is a sens
ing chamber, which is open to the outside air. (See Figure 4). The sensing
chamber is affected by particulate matter, humidity, and atmospheric pres
sure. The other is a reference chamber, which is partially closed to outside
air and is affected only by humidity and atmospheric pressure, because its
tiny openings block the entry of larger particulate matter including particles
of combustion. Electronic circuitry monitors both chambers and compares
their outputs. If the humidity or the atmospheric pressure changes, the out
puts of both chambers are affected equally and cancel each other. When
combustion particles enter the sensing chamber, its current decreases
while the current of the reference chamber remains unchanged. The result
ing current imbalance is detected by the electronic circuitry. (See Figure
5.) There are a number of conditions that can affect dual-chamber ioniza-
tion sensors such as dust, excessive humidity (condensation), significant
air currents, and tiny insects. All of these can be misread as particles of
combustion by the electronic circuitry monitoring the sensors.
Figure 1: Particle Radiation Pattern
Figure 2: Ion Distribution
Figure 3: Ion and Particles of Combustion Distribution
Figure 4: Dual Chamber
Figure 5: Dual Chamber with Particles of Combustion SYSTEM SENSOR
Photoelectric Smoke Detector Operation
Smoke produced by a fire affects the intensity of a light beam passing
through air. The smoke can block or obscure the beam. It can also cause
the light to scatter due to reflection off the smoke particles. Photoelectric
smoke detectors are designed to sense smoke by utilizing these effects
of smoke on light.
Photoelectric Light Scattering Smoke Detector
Most photoelectric smoke detectors are of the spot type and operate on
the light scattering principle. A light-emitting diode (LED) is beamed into
an area not normally “seen” by a photosensitive element, generally a pho-
todiode. (See Figure 6.) When smoke particles enter the light path, light
strikes the particles (Figure 7) and is reflected onto the photosensitive
device causing the detector to respond.
Photoelectric Light Obscuration Smoke Detector
Another type of photoelectric detector, the light obscuration detector,
employs a light source and a photosensitive receiving device, such as a
photodiode (see Figure 8). When smoke particles partially block the light
beam (Figure 9), the reduction in light reaching the photosensitive device
alters its output. The change in output is sensed by the detector’s circuitry,
and when the threshold is crossed, an alarm is initiated. Obscuration type
detectors are usually of the projected beam type where the light source
spans the area to be protected.
Light Sensitive Device
Figure 6: Light Scattering Detector
Light Sensitive Device
Figure 7: Light Scattering Detector with Smoke
Light Sensitive Device
Figure 8: Light Obscuration Detector
Light Sensitive Device
Figure 9: Light Obscuration Detector with Smoke
Smoke Detector Design Considerations
Although smoke detectors are based on simple concepts, certain design
considerations need to be observed. They should produce an alarm signal
when smoke is detected, but should minimize the impact of an unwanted
signal which can arise from a variety of causes. In an ionization detec
tor, dust and dirt can accumulate on the radioactive source and cause it
to become more sensitive. In a photoelectric detector, light from the light
source may be reflected off the walls of the sensing chamber and be seen
by the photosensitive device when no smoke is present. Insects, dirt, dry-
wall dust, and other forms of contamination can accumulate in the sensing
chamber and reflect light from the light source onto the photosensitive
Electrical transients and some kinds of radiated energy can affect the
circuitry of both ionization and photoelectric smoke detectors and be inter
preted by the electronic circuitry to be smoke, resulting in nuisance alarms.
The allowable sensitivity ranges for both types of detectors are established
by Underwriters Laboratories, Inc. (UL). Detector performance is verified
in fire tests. All smoke detectors are required to respond to the same test
fires regardless of their principle of operation.
Considerations in Selecting Detectors
The characteristics of an ionization detector make it more suitable for detec
tion of fast flaming fires that are characterized by combustion particles
in the 0.01 to 0.4 micron size range. Photoelectric smoke detectors are
better suited to detect slow smoldering fires that are characterized by par
ticulates in the 0.4 to 10.0 micron range. Each type of detector can detect
both types of fires, but their respective response times will vary, depend
ing on the type of fire.
It is often difficult to predict what size particulate matter will be produced
by a developing fire because the protected buildings normally contain a
variety of combustibles. The fact that different ignition sources can have
different effects on a given combustible further complicates the selection.
A lit cigarette, for example, will usually produce a slow smoldering fire if it
is dropped on a sofa or bed. However, if the cigarette happens to fall upon
a newspaper on top of a sofa or bed, the resulting fire may be better char
acterized by flames rather than by smoldering smoke.
The innumerable combustion profiles possible with various fire loads and
possible ignition sources make it difficult to select the type of detector best
suited for a particular application.
For more information, see NFPA 72-2010, paragraphs A.184.108.40.206 and
A.220.127.116.11, and Tables A.18.104.22.168, A.22.214.171.124(a), and A.126.96.36.199(b). APPLICATIONS GUIDE: SYSTEM SMOKE DETECTORS
Class B Circuits
Class B circuits differentiate between short circuits across the loop (alarm)
and open faults on the loop (trouble). Supervision of this circuit is accom
plished by passing a low current through the installation wiring and an
end-of-line resistor. The fire alarm control panel monitors the increases or
decreases in the supervisory current and sends an alarm or trouble con
dition, respectively. A single open in a Class B circuit disables all devices
electrically beyond the open.
FIRE ALARM CONTROL UNIT
END OF LINE
Figure 10: 2-Wire Detector Class B Circuit
Class A Circuits
Class A circuits also differentiate between short circuits across the loop
and open faults on the loop. Supervision is accomplished by monitoring the
level of current passing through the installation wiring and the end-of-line
resistor, which in a Class A circuit is an integral part of the fire alarm con
trol panel. Class A wiring must return to and be terminated in the control
panel. This technique requires that a minimum of four conductors terminate
at the panel. It also requires the fire alarm control panel to monitor Class A
circuits. The additional circuitry necessary for Class A supervision enables
the control panel to “condition” the initiating circuit to monitor the initiating
circuit from both ends when in a trouble mode due to an open fault on the
loop. This “conditioning” ensures that all devices are capable of responding
and reporting an alarm despite a single open circuit or non-simultaneous
single ground fault on a circuit conductor.
The compatibility considerations of smoke detectors that were detailed in
Class B circuits also apply to Class A circuits (Figure 11).
NFPA 72 requirements also dictate that alarm notification appliances
(including smoke detectors with built-in sounders) produce the 3-pulse tem
poral pattern fire alarm evacuation signal described in American National
Standards Institute (ANSI) S3.41 Audible Emergency Evacuation Signals.
Situations for Other Types of Detectors
In certain circumstances where standard smoke detectors are unsuitable,
special-purpose detectors, such as flame detectors, heat detectors, and
other detection devices may be used.
The application of these special types of detectors should be based on
an engineering survey and used in accordance with the manufacturer’s
Smoke Detector Limitations
Smoke detectors offer the earliest possible warning of fire. They have saved
thousands of lives. Special application rules can compensate for the limita
tions of smoke detectors. Smoke detectors may not provide early warning
of a fire developing on another level of a building. Detectors should be
located on every level of a building. Detectors may not sense a fire devel-
oping on the other side of a closed door. In areas where doors are usually
closed, detectors should be located on both sides of the door.
As already indicated, detectors have sensing limitations. Ionization detec-
tors are better at detecting fast, flaming fires than slow, smoldering fires.
Photoelectric smoke detectors sense smoldering fires better than flaming
fires. Because fires develop in different ways and are often unpredictable
in their growth, neither type of detector is always best. A given detec
tor may not always provide significant advance warning of fires when fire
protection practices are inadequate, nor when fires are caused by violent
explosions, escaping gas, improper storage of flammable liquids such as
cleaning solvents, etc.
Typical System Layout
The initiating circuits that connect smoke detectors to a control panel should
be supervised to detect and annunciate a fault (trouble) condition that could
interfere with the proper operation of the circuit.
Smoke detectors are generally categorized as either 2-wire or 4-wire detec
tors. Two-wire detectors derive their power from their connection to the fire
alarm control panel alarm initiating device circuit. Since they are dependent
on the initiating circuit, these 2-wire detectors must be tested and listed for
compatibility with the associated control panel, to ensure proper operation.
Four-wire detectors are powered from a separate pair of wires, and, like the
2-wire detector, apply an electrical short across the associated alarm initi-
ating device circuit to transmit an alarm (Figure 10). Because they do not
derive power from the alarm initiating device circuit, electrical compatibility
is predicated upon the operating parameters of the power supply to which
the detectors are connected, and not the initiating circuit. Supervision of
the power to 4-wire detectors is mandated through the use of an end-of-
line power supervision relay. When power is on, the relay contacts of the
end-of-line relay are closed and connected in series with the end-of-line
resistor beyond the last initiating device. Loss of power at any point in the
power supply circuit will cause the relay to de-energize and a trouble con-
dition to occur on the initiating circuit.
NOTE*: Refer to the fire alarm control panel manufacturer’s operating man
ual to determine the ability of a specific initiating circuit to react in a Class
B or Class A fashion.
*NFPA 72 2010 classifies initiating device circuits by Class. SYSTEM SENSOR
FIRE ALARM CONTROL UNIT
Figure 11: 2-Wire Detectors – Style D (Class A) Circuit
Wireless detectors and their internal transmitters use one or more inter
nal batteries as the source for their operating power and are UL listed.
Supervision of the internal battery power source is incorporated within the
smoke detector circuitry. If the battery power source depletes to the thresh
old specified by UL, the smoke detector will sound a local alert and initiate
a trouble signal once per hour for a minimum of seven days or until the bat
tery or batteries are replaced.
The wireless initiating devices are supervised for tamper and/or removal
by initiating a distinct trouble signal. Each wireless device also initiates
a test transmission every hour to verify the communication circuit. Any
device failing to communicate is identified on the control panel no less
than every four hours.
General Zoning Guidelines
The faster the source of an alarm can be pinpointed, the faster action can
be taken. Although formal rules for zoning are not given in fire protection
codes, an exception is the rule for wireless devices stating that each smoke
detector must be individually identified. It is always advisable to zone any
system that contains more than a small number of detectors. In addition to
the zoning requirements of NFPA 72-2010, Section 10.16.6, experienced
detector installers and system designers recommend the following:
• Establish at least one zone on every protected floor.
Zone natural subdivisions of a large building, such as separate wings
on a single floor.
Minimize the number of detectors in each zone. Fewer detectors on a
zone will speed up locating the fire and simplify troubleshooting.
Install duct detectors in different zones than open-area detectors for
troubleshooting and locating purposes.
Fire Safety Functions
Often smoke detectors are utilized to control ancillary equipment. Care
should be taken to ensure that detectors utilized in such a manner are
approved for their intended purpose. A few of the typical applications are
to control the flow of smoke in air handling and air conditioning systems,
• to release doors to contain smoke in a fire situation,
• to release locks to allow exit in a fire situation,
• to capture and recall elevators in a fire situation,
• to activate a suppression system.
Spacing and placement requirements for detectors used in releas
ing service may be different from detectors used in conventional open
area applications. 4-wire detectors are recommended in these situations
because the control panel and detectors used will affect the power require
ments. More than one detector relay on a circuit may not receive enough
power from the 2-wire circuit to operate during an alarm.
Smoke Detector Installation
Wiring Installation Guidelines
All fire alarm system installation wiring should be installed in compliance
with Article 760 of NFPA 70, the National Electrical Code (NEC), the manu-
facturer’s instructions, and the requirements of the AHJ.
Typical Wiring Techniques
The primary rule of installation wiring is to Follow the Manufacturer’s
Instructions. This rule cannot be overemphasized. The requirement for
electrical supervision of the installation wires and their connections to ini
tiating devices makes fire alarm system installation wiring very different
than general wiring.
A manufacturer’s installation wiring drawing routes wires and shows connec
tions in a certain manner to accommodate supervision requirements.
variance from the manufacturer’s drawings might cause a portion of a
circuit to be unsupervised
and, if an open or short circuit fault occurred,
it could prevent the circuit from being able to perform its intended function
without giving the required trouble indication.
The rules of supervision are not very complex. However, unless installers
are experienced in fire alarm system installations, they probably would not
be familiar with them.
Smoke detector manufacturers’ installation drawings will show how their
detectors should connect into a system. However, manufacturers’ draw
show how devices located on the same floor, but served by a
different riser (vertical wiring run), should be connected. The diagrams on
this and the following page should be considered typical initiating device
circuits utilizing smoke detectors. They are offered to illustrate examples of
proper and improper installation wiring and termination techniques. Since
there are always exceptions to typical installation drawings, experienced
installers use the primary rule of installation wiring: follow the manufactur
er’s instructions, and meet the local codes.
Figure 12 illustrates improper wiring of smoke detector A. This wiring
method is referred to as T-tapping. This common installation error is often
made in riser wiring, as well as single floor wiring. The smoke detector may
operate properly under alarm conditions. If it becomes disconnected from APPLICATIONS GUIDE: SYSTEM SMOKE DETECTORS
the installation wiring loop beyond the T-tap, however, the detector would
not function, and a “no trouble” condition would occur.
NOTE: T-tapping is allowed with most intelligent fire alarm systems. Refer
to manufacturer’s recommendations.
Figure 13 illustrates the correct installation wiring method for smoke detec
tors. None of the connections can be broken without opening the circuit,
causing loss of supervision, and triggering the fire alarm control panel to
Smoke detectors should be connected to supervised installation wiring
to ensure electrical supervision of the device. Removal of a detector from
its associated initiating circuit should cause the loop to open, resulting in
a trouble condition. The required termination at the smoke detector may
involve either screw terminals or wire pigtails. Regardless of the method,
removal of the smoke detector or a single installation wire must open the
initiating circuit and send a trouble signal to the control panel.
Screw termination of either side of the initiating circuit may require only
one or two screws. Figure 14 is an example of proper termination when
one screw terminal is used. Note that the installation conductor has been
cut before termination. This assures full supervision of the smoke detector.
Figure 15 shows common connection errors. In both examples, removal of
the smoke detector wire does not open the initiating circuit. The fire alarm
control panel will not recognize a trouble condition, and the detector that
has been deliberately or inadvertently disconnected will be disabled.
Smoke Detector “A”
Figure 12: Incorrect wiring method for a conventional fire panel
Figure 13: Correct wiring method for smoke detectors monitored by a conventional
fire alarm panel
Figure 14: Proper termination
Figure 15: Improper termination
Figure 16 is an example of properly connected smoke detectors provided
with pigtails. This method of termination supervises all wiring to the point
at which it connects to the detector.
Figure 16: Pigtail Connections – Correct Wiring Method
Figure 17 shows an incorrect pigtail connection. This is a form of T-tapping
discussed earlier. Note that the conductor between the wire nut (or splice)
and the detector is unsupervised, and could be cut or disconnected with-
out resulting in a trouble signal.
Figure 17: Pigtail Connections – Incorrect Wiring Method SYSTEM SENSOR
Wireless smoke detectors do not require any field wiring as the power
for the initiating devices is contained and incorporated within the device.
Removal of a wireless smoke detector initiates a distinct tamper or trouble
signal. Follow the instructions in the manufacturer’s installation manual for
Installation Dos and Don’ts
Do verify that 2-wire or addressable smoke detectors have been tested
and UL listed for compatibility with the equipment to which they are
connected. If necessary, contact the manufacturer for this information.
Do locate any end-of-line devices electrically at the end of the circuit,
beyond all initiating devices (not at the control unit, except in a Class
A installation). On Class A loops, the end-of-line device is built into the
panel circuit. An end-of-line device must not be used.
Do use caution when utilizing 2-wire detectors with integral relays,
because they may require more power than the initiating device circuit
can supply. This could result in the inability of the relay to control auxil-
iary equipment to which it is connected.
Do follow the manufacturer’s installation instructions to assure proper
communication between the smoke detector and the control panel when
using wireless detectors. Testing following a fire alarm system instal
lation or the addition to an existing system shall confirm the intended
sequence of operation under the most stringent conditions.
• Do observe polarity when required.
Do protect detectors against contamination during construction or
• Do follow the manufacturer’s installation instructions carefully.
Don’t T-tap smoke detectors or circuit conductors, except when specifi
cally permitted by the manufacturer as part of an intelligent/addressable
• Don’t loop uncut installation conductors around screw terminations.
Don’t exceed the maximum resistance permitted for the initiating device
Don’t exceed the allowable number of detectors specified by the equip
ment manufacturer on any system. Note: Longer than allowed initiating
device circuits or Signaling Line Circuit loops may cause a malfunction
of the detectors/or the system even though the prescribed number of
detectors has not been exceeded.
Wiring and System Checkout
Check the detector loop wiring for grounds, short circuits, and open faults
before the system is placed into operation as required for all installation
wiring of fire alarm systems. Each detector should be tested in accordance
with the manufacturer’s instructions.
When using wireless detectors, verify the radio signal transmission strength
in accordance with the installation manual.
Proper Detector Applications, Placement, and
After all detectors have been installed, test the complete system to ensure
that no wiring faults exist, and that all parts of the system operate as
intended. A complete system checkout consists of testing each detector
at its installed location and following the panel manufacturer’s instructions
for system checkout. Also, refer to NFPA 72 for additional information.
Where to Place Detectors
Because detector placement is critical to early warning functions, smoke
detectors should be installed in all areas of the protected premises. Total
coverage as defined by NFPA 72 should include all rooms, halls, stor
age areas, basements, attics, lofts, and spaces above suspended ceilings
including plenum areas utilized as part of the HVAC system. In addition,
coverage should include all closets, elevator shafts, enclosed stairways,
dumbwaiter shafts, chutes, and other subdivisions and accessible spaces.
Installed fire detection systems that meet local codes or ordinances may
not be adequate for early warning of fire. Some codes or ordinances have
minimum objectives such as capturing elevators or preventing circula
tion of smoke through the HVAC systems instead of early detection of fire.
A user should weigh the costs against the benefits of installing a com
plete fire detection system when any detection system is being installed.
The location, quantity, and zoning of detectors should be determined by
desired objectives that meet the minimum requirements of all local codes
Total coverage, as defined in NFPA 72, is the complete fire detection sys
tem. In some of the specified areas of coverage (e.g., attics, closets, and
areas beneath open loading docks or platforms) a heat detector may be
more appropriate than a smoke detector. Careful consideration should be
given to the detector manufacturer’s instructions and the following recom-
mendations in this guide.
*The guidelines in this section of the guide are adapted from Standards pub
lished by the National Fire Protection Association, Quincy, Massachusetts,
USA. These standards include NFPA 72, National Fire Alarm Code and
Signaling Code; NFPA 70, National Electrical Code, Article 760; and NFPA
90A, Installation of Air Conditioning and Ventilating Systems.
In general, when only one detector is required in a room or space, the
detector should be placed as close to the center of the ceiling as possible.
Central location of the detector is best for sensing fires in any part of the
room. If a center location is not possible, the detector may be wall mounted
within 12 inches from the ceiling if the detector is listed for wall mounting.
(See Figure 18.) (NFPA 72-2010, Figure A.188.8.131.52.1.)
When air supply and/or air return ducts are present in a room or space,
the detector(s) should not be placed in the path of the air flow supply or
return (NFPA 72-2010).
Smoke tests are helpful in determining proper placement. Special atten-
tion should be given to smoke travel directions and velocity, since either
can affect detector performance.
Placement of detectors near air conditioning or incoming air vents can
cause excessive accumulation of dust and dirt on the detectors. This dirt
can cause detectors to malfunction and cause unwanted alarms. Detectors
should not be located closer than 3 feet from an air supply diffuser or an
air return vent. APPLICATIONS GUIDE: SYSTEM SMOKE DETECTORS
Spot type detectors in properly engineered systems may also be placed in
return air ducts, or in approved duct detector housings designed for this
application. Although duct detectors are not a substitute for open area
detectors, they can provide an effective method of initiating building con-
trol functions to prevent smoke from being transported from the fire area to
other parts of a building.
(See Duct Smoke Detector Applications Guide.)
Top of Detector Acceptab
wn are to the
closest edge of the detector
Figure 18: Wall Mounted Detector – Placement
Where Not To Place Detectors
See Table A.184.108.40.206 (a) in NFPA 72-2010.
Consider possible causes of detector malfunction and determine detec
tor placement accordingly to avoid unwanted alarms. Application specific
devices have been created to address specialized environments. Installing
these application specific devices according to manufacturer specifica
tions and AHJ guidelines will reduce unwanted alarms, for example:
• Excessively Dusty or Dirty Areas
Installing smoke detectors in excessively dusty or dirty environ
ments may require more maintenance than NFPA recommends. It
may be advantageous to install smoke detectors with a remote main
tenance signaling module, or to install heat detectors in place of
smoke detectors. NFPA 72-2010 220.127.116.11 provides detailed guide
lines for placement in kitchen environments.
Avoid using detectors outdoors, in open storage sheds, or other
open structures affected by dust, air currents, or excessive ranges
of humidity and temperature.
• Wet or Excessively Humid Areas
Avoid damp, wet, or excessively humid areas, including areas next
to bathrooms with showers. Refer to NFPA 72-2010 A.18.104.22.168(5) for
additional placement guidelines.
• Elevator Lobbies
Do not place above ashtrays in elevator lobbies.
• Extreme Temperatures
Avoid very cold or very hot environments, or unheated buildings or
rooms where temperatures can fall below or exceed the operating
temperature range of the detector. At temperatures above or below
the operating range* of the detector, its internal components may
not function properly.
• Areas with Combustion Particles
Avoid areas where particles of combustion are normally present,
such as in kitchens or other areas with ovens and burners; or in
garages, where particles of combustion are present from vehicle
exhausts. When a detector must be located in or adjacent to such
an area, a heat detector may be appropriate.
• Manufacturing Areas
Avoid manufacturing areas, battery rooms, or other areas where
substantial quantities of vapors, gases, or fumes may be present.
Strong vapors can make detectors overly sensitive or less sensitive
than normal. In very large concentrations, gases heavier than air,
such as carbon dioxide, may make detectors more sensitive, while
gases lighter than air, such as helium, may make them less sensi
tive. Aerosol particles may collect on detector chamber surfaces and
cause nuisance alarms.
• Fluorescent Light Fixtures
Avoid placement near fluorescent light fixtures. Electrical noise
generated by fluorescent light fixtures may cause unwanted alarms.
Install detectors at least 1 ft (0.3 m) away from such light fixtures.
*Manufacturers’ specifications should list acceptable temperature ranges.
Special Application Detectors
The guidelines in this document generally apply to standard open-area
smoke detectors. System Sensor has a number of advanced technol
ogy detectors that are optimized for specific environments and should
Laser technology smoke detectors are designed for use in areas that
require extremely early warning of fire. They are designed to detect the
earliest particles of combustion making them ideal for clean rooms, com
puter rooms or telecommunication center, or any area with mission critical
operations. Laser-based smoke detectors are ultra-sensitive to smoke, as
much as 100 times more sensitive than standard detectors — so care
and judgment of application is needed to prevent unwanted alarms. See
the System Sensor Laser Technology Smoke Detector Applications Guide.
Aspiration smoke detectors use a pipe system and fan to draw smoke
particulates back to the detection chamber. The pipe configuration, hole
placement and hole diameter are designed from algorithms which take
into consideration air flow, room size, sensitivity requirements (speed of
detection) and other parameters to determine the optimal set up. For the
FAAST series of aspiration detectors PipeIQ™ software has been created
for pipe design and is the only software approved for use with the FAAST
system. See the FAAST Comprehensive Instruction Manual for additional
information. SYSTEM SENSOR
Multi-Criteria detection contains multiple sensors that separately respond
to physical stimulus such as heat, smoke, or fire gases. An alarm signal
is determined through advanced algorithms based on input from these
sensors. System Sensor offers several types of multi-criteria detection
including and Acclimate detector which combines photo and heat signals
and an Advanced Multi-Criteria detector which combines four signals :
photo, thermal, carbon monoxide and infrared. The combination of sen
sors offers better immunity to nuisance alarms in challenging environments
with faster response times to real fires.
Combination Carbon Monoxide and Smoke detectors are also available.
Combining two functions into one device improves installation time and cost
as well as offering a more aesthetically pleasing final product. This device
type provides separate signals for each event. In this device the CO sen-
sor may or may not be used for determining the presence of smoke/fire
depending upon the type of device. For additional information regarding
Carbon Monoxide detection see the System Sensor System-Connected
Carbon Monoxide Detectors Application Guide.
Standards for Smoke Detectors
UL has standards for three types of smoke detectors: duct detectors, UL
268A; single and multiple station smoke alarms, UL 217; and systems
type detectors, UL 268. Detectors should be used as specified for the
Section 22.214.171.124.4 of the 2006 NFPA 101 Life Safety Code states that single
station smoke alarms shall sound an alarm only within an individual living
unit, suite of rooms, or similar area, and shall not actuate the building fire
alarm system unless otherwise permitted by the authority having jurisdiction.
In addition to possible code noncompliance, the following deficiencies
would exist in a series of residential smoke detectors, connected in a sys-
Since the system is not supervised, vandals or others could disconnect
a detector or the entire system, leaving a building without protection.
The residents would be unaware of this serious life threatening condition.
Residential smoke alarms do not latch in alarm. In other words, the
smoke alarm self-resets. One smoke alarm in alarm will sound all the
smoke alarms connected together. It would be difficult to identify or
locate a specific smoke alarm that initially put the system into alarm
after the alarm condition was cleared.
System detectors latch in alarm. They do not reset until power is momen
tarily disconnected. This makes it convenient to identify the location of a
detector that caused the control panel to alarm. In addition, system detec-
tors are specifically designed to connect to a panel. Two-wire detectors
and addressable smoke detectors require a UL compatibility review to
verify that the detector and panel operate together. A typical life safety
fire protection system for an apartment complex would use system detec-
tors and manual fire alarm stations in the hallways and common areas of
the complex and residential single station type smoke detectors and heat
detectors in the individual apartments. The system detectors, manual sta-
tions and heat detectors would be connected to a control panel, sound
a general alarm and automatically notify the proper authorities that a fire
condition exists. Residential smoke detectors located in the apartments
would be interconnected only within the individual living quarters of each
apartment. These residential units would sound an alarm only in the apart-
ment where a fire started.
General Spacing Guidelines
Some fire protection codes specify detector spacing on a given center-to-
center distance between detectors under ideal conditions. These distances
are based on rooms with smooth ceilings having no physical obstructions
between the contents being protected and the detectors. Moreover, they
are based on a maximum ceiling height, and on the assumption that the
value and the combustible nature of the contents of the room to be pro
tected do not warrant greater protection or closer spacing.
If we assume a typical center distance spacing guideline is 30 ft (9.1 m),
how do we determine whether a given room or space can be protected
by a single detector? Figure 19 shows four detectors spaced horizontally
and vertically 30 ft (9.1 m) apart. Detectors B and D, however, are more
than 30 ft apart. Clearly, in this example detector spacing can exceed the
given 30-ft spacing and still comply with the code if any source of combus
tion is within 21 ft (6.4 m) of the horizontal projection of a detector, and if no
more than a 900 sq ft (82.8 sq m) area is being protected by one detector.
To determine which coverage patterns are permissible within the 30-ft spac
ing, start by tracing a circle with a radius of 21 feet. Any square or rectangle
that fits within the circumference of the circle may be protected by one
detector. (See Figure 20.)
In other words, if a diagonal through the center of the room is no greater
than the diameter of the circle, or 42.4 feet (12.8 meters), one detector can
be used under ideal conditions. Figure 21 shows how a length of hallway
can be protected by only two detectors under ideal conditions.
30 ft. (9.1 m)
21.2 ft. (6.4 m)
30 ft. (9.1 m)
Figure 19: Typical Detector Spacing
Special Spacing Problems
Code guidelines are based on ideal conditions that do not exist in the
majority of buildings. Detector installers usually have to deal with a variety
of problems, such as uneven ceilings or ceilings crossed by beams and
joists; storage racks and partitions that obstruct the path of smoke toward
detectors; air stratification due to uninsulated roofs, peaked or sloped
ceilings, or localized heating or cooling from heating, ventilating, and air
conditioning systems; and extensive variability in the value and combustion
characteristics of building contents. The following are suggested tech
niques for dealing with some of the special detector spacing problems:
Solid joist and beam construction.
Per NFPA 72-2010, solid joists
are to be considered equivalent to beams for smoke detector spac
ing guidelines. For ceiling heights of 12 ft (3.66 m) or lower and beam
depths of 1 ft (0.3 m) or less, smooth ceiling spacing running in the
direction parallel to the run of the beams is to be used, and half of the
smooth ceiling spacing is to be used in the direction perpendicular to APPLICATIONS GUIDE: SYSTEM SMOKE DETECTORS
Peaked or Sloped Ceilings.
Peaked or sloped ceilings can foster air
stratification. Codes may specify spacing detectors by using horizontal
spacing from the peak of the roof or ceiling. For instance, if the speci
fied distance from the peak is 3 ft (1 m), the distance is measured on the
base of the right triangle formed by a vertical line dropped from the peak
of the roof, with the roof incline as the hypotenuse. Additional detectors
are installed on the selected spacing, using the horizontal distance, not
the distance along the incline of the ceiling. (See Figure 22.)
Detector may be placed
anywhere in shaded area.
Detector may not be
placed in this area.
S = Detector Spacing
Figure 22: Detector Spacing Layout – Sloped Ceilings (peaked type)
Alternate Detector Mounting.
Mounting alternate detectors up to 3 ft (1
m) below the ceiling can increase detection of small or smoldering fires
when the possibility of air stratification exists. Figure 23 illustrates such
an installation. Specific designs for such an alternate detection should
be based upon an engineering survey.
Heating, Ventilating and Air Conditioning.
HVAC effects on air flow and
air stratification should be determined and considered when planning
detector placement. In rooms where forced-air ventilation is present,
detectors should not be located where air from supply diffusers could
dilute smoke before it reaches the detector. This may require additional
detectors, because placing detectors only near return air openings may
leave the balance of the area with inadequate protection, especially
when the HVAC system is not operating.
Detectors placed in an above-ceiling air handling space should not
be used as a substitute for open area protection, because smoke may
not be drawn into the air handling space when the ventilating system
is shut down. The detector will be less responsive to a fire condition in
the room below than a detector located on the ceiling of the room below
due to dilution and filtering of the air in the air handling space before the
smoke reaches a detector. (See the discussion of detector placement
in Section 4: Where To Place Detectors).
Two factors influence the spacing of detectors and the amount of pro
tection required in a specific room or area: the fire’s burn characteristics
and the value of the protected assets. Refer to NFPA 72-2010 17.7.6 for
more detailed information on spacing of detectors under special appli-
cations. Likewise, if the contents are especially valuable, for example,
sophisticated and expensive machinery or irreplaceable records, detec
tors should be placed closer together.
the run of the beams. Spot-type detectors may be located either on the
ceiling or on the bottom of the beams. For beam depths exceeding 1 ft
(0.3 m) or for ceiling heights exceeding 12 ft (3.66 m), detectors are to
be located on the ceiling in every beam pocket. If the beamed ceiling is
also sloped, use the spacing determined for flat beamed ceilings. Use
the average height over the slope as the ceiling height in such cases.
Note that, by definition in NFPA 72, ceilings are to be considered smooth
unless the beams or joists are more than 4 in (0.1 m) in depth.
High storage racks.
Multi-level storage racks present special prob
lems for early fire detection. Developing fires, especially smoldering
fires, on the lower levels of the racks may not be sensed rapidly by ceil
ing mounted detectors. Upward convection of smoke can be slowed or
blocked by goods stored on the upper levels of the racks. Multi-level fire
detection is required. Detectors should be installed on the ceiling above
each aisle and on intermediate levels of the racks adjacent to alternate
pallet sections, shown in NFPA 72-2010 A.126.96.36.199(a) and A.188.8.131.52(b).
A consultant’s judgment may be required for specific installations.
Partitions and many types of large, tall equipment standing
on the floor can block the flow of smoke toward detectors. Any parti
tion or similar obstruction that is less than 18 in (45 cm) from the ceiling
should be treated as a side wall dividing the area protected.
Air stratification in a room may keep air containing
smoke from reaching ceiling-mounted detectors. Three conditions are
known to accentuate air stratification: 1. when a layer of hot air exists
under a poorly insulated roof heated by the sun, cooler air will stratify
the hot air layer at the ceiling; 2. when a layer of cold air exists under a
poorly insulated roof cooled from the outside by cold air, the heated air
is cooled as it reaches the cold air layer; or 3. when an HVAC system
creates artificial hot or cold air layers in a room, the layers may affect
the flow of smoke to the detectors.
Uninsulated roofs present special placement prob
lems. Air movement toward ceiling detectors is not impeded when the
outside temperature is cool, but stratification can occur when outside
temperature is warm or hot, or when the roof is heated by the sun on
bright, sunny days. Although true thermal barriers are not present in
many installations, smoke tests should be run in factories or warehouses
with metal roofs on warm sunny days to determine whether such a ther
mal barrier exists.
× 41 ft. = 410 sq. ft.
× 39 ft. = 585 sq. ft.
× 37 ft. = 740 sq. ft.
× 34 ft. = 850 sq. ft.
× 30 ft. = 900 sq. ft.
Figure 20: Detector Coverage Patterns
Figure 21: Detector Placement in Hallways SYSTEM SENSOR
3 ft. Minimum
Smoke Detectors at Ceiling
Smoke Detectors below Ceiling
Figure 23: High Ceiling Area
Detectors in Air Handling and Air Conditioning Systems
See the System Sensor
Duct Application Smoke Detectors Guide and NFPA
72, National Fire Alarm Code
for more specific information.
Detectors in Above-Ceiling Plenum Areas Including Those
Utilized as Part of the HVAC System
Detectors should be placed in plenum areas above the ceiling, in the open
areas below, and in the ducts. Plenum detectors are required to be listed or
tested and approved for the air velocities within the environment in which
they are to be installed. Duct detectors should be installed in the ducts.
Detectors placed in plenums MAY NOT be used as a substitute for open
area protection, because smoke may not be drawn into the plenum when
the ventilating system is shut down. When the system is operating, the
detector may be less responsive to a fire condition in the room below than
a detector located on the ceiling of the room below. This may be caused
by blockage, dilution, and air filtering prior to its arrival at the detector loca
tion in the plenum area.
Since the air circulating through the plenums is usually at higher veloci
ties than would be prevalent in the room below, detector spacing should
Also, the dilution of the smoke in plenum spaces is an important consider
ation when utilizing smoke detectors rated for higher velocities. Therefore,
plenum detectors should be utilized to detect fire in the plenum but should
never be utilized as a substitute for duct detectors and open area detectors.
Maintenance requirements of detectors exposed to unusual velocities are
generally increased due to the excessive dirt buildup and contamination
present in these environments.
Testing, Maintenance, and Service of Detectors
Smoke detectors are designed to be as maintenance free as possible; how
ever, dust, dirt, and other foreign matter can accumulate inside a detector’s
sensing elements and change its sensitivity. They can become either more
sensitive, which may cause unwanted alarms, or less sensitive, which could
reduce the amount of warning time given in case of a fire. Both are unde-
sirable; therefore, detectors should be tested periodically and maintained
at regular intervals. Always follow the manufacturer’s specific recom
mended practices for maintenance and testing. Also refer to Annex B of
NFPA 90A and NFPA 72-2010 Chapter 14.
Smoke detectors are sophisticated electronic devices that need periodic
testing and maintenance. To maintain the integrity of any fire alarm sys-
tem, it is important to have a qualified person periodically test the system.
Typical Inspection, Test, and Maintenance Practices
Detectors should be given a visual inspection at installation and at least
twice a year thereafter. This ensures that each detector remains in good
physical condition and that there are no changes that would affect detec-
tor performance, such as building modifications, occupancy hazards, and
Notify the proper authorities that the smoke detector is undergoing mainte
nance, and therefore the system will temporarily be out of service. NOTE:
Disable the zone or system undergoing maintenance to prevent unwanted
alarms and possible dispatch of the fire department.
Use a high powered vacuum cleaner and remove dust from the detector
by placing the nozzle as close as possible to the openings in the outside
housing. A nozzle with a brush attachment will assist in dust removal. The
sensing chamber on some detectors can be removed for more thorough
cleaning. Refer to the manufacturer’s recommended procedure for details.
Test each detector’s sensitivity per the manufacturer’s recommended pro-
cedure within one year after installation and every alternate year thereafter.
Test each detector functionally in place annually, as detailed in NFPA
72-2007 (Chapter 14).
If a detector’s sensitivity is within specifications, nothing further needs to
be done to the detector. If the detector’s sensitivity is outside specifications,
clean the detector and retest. If that does not place the sensitivity within
the unit specified range, then follow the manufacturer’s recommended
Restore zone or system at the completion of testing.
Notify the proper authorities that testing has been completed and the sys-
tem is again operational.
Refer to NFPA 72-2010, Section 14.4.1 for additional information.
Test each smoke detector to verify that it is within its listed and marked sen
sitivity range using either:
• A calibrated test method, or
• The manufacturer’s calibrated sensitivity test instrument, or
• Listed control equipment arranged for the purpose, or APPLICATIONS GUIDE: SYSTEM SMOKE DETECTORS
Other calibrated sensitivity test methods acceptable to the authority
Detectors with a sensitivity of 0.25 percent/ft obscuration or more outside
the listed and marked sensitivity range should be cleaned and recalibrated
Detectors listed as field adjustable may be either adjusted
within the listed and marked sensitivity range, cleaned and recalibrated,
Restore the zone or system at the completion of testing.
Notify all the persons contacted at the beginning of the test that testing has
been completed and the system is again operational.
Some individuals rely on an aerosol chemical spray (canned smoke) to test
the sensitivity of a detector. This can give unsatisfactory results since an
aerosol chemical spray does not accurately test detector sensitivity. NFPA
72-2010, Chapter 184.108.40.206.7 states, “The detector sensitivity shall not be
tested or measured using any device that administers an unmeasured con
centration of smoke or other aerosol into the detector.”
Canned smoke should only be used to verify smoke entry. The duration of
spray, distance between the detector and the aerosol container, angle of
discharge, and different environmental conditions can produce random
results. In addition, many aerosols leave an oily residue. Over a period of
time, this oily residue can attract dust or dirt, which can make a detector
more sensitive and result in nuisance alarms. Be sure to follow the manu-
facturer’s recommendation on test gas or canned aerosol testers.
What to Do About Unwanted Alarms
No detection system is impervious to unwanted alarms. Statistically, as the
system size and the total number of detectors increase, the total number
of nuisance alarms per year tends to increase. Historical experience in a
given installation or data on similar sized buildings with similar utilization
patterns can provide a basis for a rough indication of how many nuisance
alarms are probable during a 12-month span; however, no two installa
tions are identical.
In small- to moderate-sized detection systems protecting relatively combus
tion-free environments, like office buildings, more than one or two unwanted
alarms per year would be unusual. In more adverse environments, such
as laboratory or manufacturing facilities where combustion processes are
present, more frequent alarms can be anticipated. In very adverse environ
ments, one alarm per month might not be considered excessive.
After the first few months, which serve as a shakedown period, it should be
possible to arrive at some reasonable expectation for probable unwanted
alarms from the system. After that, any unexpected change in frequency
or distribution indicates a problem that should be investigated. The best
way to monitor alarm frequency and distribution is to maintain an alarm log.
Reasons for Unwanted Alarms
Unwanted alarms can result from a wide variety of causes, including:
Improper environments: detectors will not operate properly because
of temperature extremes; excessive dust, dirt, or humidity; excessive
air flow rates; or the normal presence of combustion particles in the air
streams surrounding the detectors;
Improper installation: detectors and their wiring are subject to inter
ference from induced currents and noise in adjacent wiring systems,
radio-frequency transmissions, and other types of electromagnetic
Inadequate maintenance: gradual dust and dirt accumulation on the
detector’s sensing chambers;
Seasonal effects: for example, the reactivation of a building heating sys
tem after an extended summer shutdown can cause alarms;
Building maintenance issues: for example, accidental triggering of a
detector’s magnetic test switch, or the introduction of plaster dust from
drywall repairs into a detector’s sensing chamber;
• Induced current effects from lightning storms;
Infestation from insects small enough to enter the detector’s sensing
Vandalism or mischievous acts: for example, dormitory pranks.
If an alarm occurs and a fire does not exist, the alarm should be silenced
by an authorized service technician following procedures recommended
by the NFPA. The problem unit must be located and the alarm system con-
trols reset, so that the effectiveness of the detection system is restored.
Be sure to check all the detectors in the zone or addressable device(s)
before deciding that it is a false alarm. If a fire does exist, more than one
detector may be in the alarm state, although no signs of fire may be evident
in the vicinity of the first activated detector. The fire could be overlooked.
Maintain an Alarm Log
The next step for all alarms should be a written report in an Alarm Log. A
typical Alarm Log is shown on page 19 of this guide. Such a log serves
immediate and long-term purposes.
The Alarm Log indicates which individuals responded to the alarm and
whether or not they took appropriate action.
Periodic review of the cumulative Alarm Log can help those responsible
for the detection system discern patterns in the reported alarms. Generally,
several months (or even years) of data may be necessary before patterns
begin to emerge.
In a worst case example, a pattern of repeated alarms or small fires in a
particular area may indicate a serious deficiency in safety practices that
should be promptly corrected. In less obvious cases, patterns are indicated
by repeated alarms in the same or adjacent zones with similar probable
causes, or repeated alarms in the same zone that occur at about the same
time of day, or time of year.
Effects of Location or Environment
Check for the effects of location and environment. Review the information in
this guide on
Where To Place Detectors and Where NOT to Place Detectors
to determine whether the detector’s location or its environment is poten
tially causing the unwanted alarms. Also, refer to the installation manual
for further information.
One often overlooked source of problems is the placement of detectors
where air streams carry smoke or chemical fumes from some areas of an
installation past detectors in other areas unrelated to the source of the con
taminants. Diagnosing these problems requires that air movements into the SYSTEM SENSOR
problem area, especially near the ceiling, be carefully checked to deter
mine their sources. Experienced HVAC engineers or contractors usually
have the training and specialized equipment (flow meters, etc.) to con
duct such a study. In very difficult cases, a full-scale smoke test may be
required to solve the problem.
Conversely, strong air streams near air inlet or supply ducts can also pre-
vent a detector from signaling an alarm when a fire is present by blowing
smoke away from the detector heads.
Inspect Detector for Dirt, Review Maintenance
If the Alarm Log indicates a fairly stable alarm rate for several months or
a year, and then there is a gradual increase in the frequency of unwanted
alarms, this is usually an indication that the detectors in the system should
NFPA standards require that all detectors be visually inspected twice a year.
Smoke detector manufacturers also recommend this schedule.
Clean the detectors at least once a year, or more frequently if environmental
conditions warrant it. See the section on
Detector Testing and Maintenance
in this guide for more details.
In cases where the probable cause of a number of alarms appears to be
dust or dirt on the detectors, detector maintenance schedules should be
reviewed to determine the dates when the detectors were last cleaned and
tested. If the detectors are due or overdue for maintenance, scheduling
and performing the recommended cleaning and testing should eliminate
If the problem resulted from a temporary overall increase in airborne dust
due to nearby construction, scheduling a one-time special cleaning for all
the detectors in the system should alleviate the problem. If the problem
is confined to one or two zones and is the result of higher dust levels in a
particular area, scheduling the detectors in those areas for more frequent
maintenance and cleaning may prevent the development of similar alarm
problems in the future.
Effects of Other Systems on Alarm System
In checking for the effects of other systems on the alarm system wiring, the
Alarm Log may be very valuable in helping to pinpoint relationships among
apparently causeless alarms. One important fact that can be obtained
from an Alarm Log is the beginning date for a rash of apparently cause
less alarms that may or may not be grouped around one particular zone.
The sudden onset of such a group of alarms may result when an addition
or change in the alarm system or in another electrical or electromechanical
system in the building affects the detectors or the alarm system circuitry.
Systems that can affect the alarm system include: other security systems;
walkie-talkies; mobile telephones; HVAC controls; elevator call systems;
remote control equipment (door closers, etc.); and even the installation
of microwave antenna. If the alarm pattern supports the possibility of
some kind of interference with a fairly definite initiation date, all equipment
changes made in the building immediately prior to or concurrent with the
beginning of the development of the alarm pattern should be reviewed. In
addition, the wiring layouts of the alarm system and any recent building
or system modifications should be compared to make sure that the spac-
ing and/or shielding required to protect the alarm system wiring from other
potentially interfering electrical systems was maintained.
Miscellaneous Causes of Unwanted Alarms
Isolated alarm causes such as a maintenance person accidentally trigger
ing an alarm by touching a detector with a magnetic screwdriver can be
ignored, except to periodically remind maintenance personnel to be care-
ful when working around detectors.
Steps also should be taken to protect detectors from dust whenever
maintenance requires sawing, sanding, drilling, or other dust-producing
operations in the vicinity of the detector heads, to prevent false alarms
due to the dust getting into the detector sensing chambers. In new con
struction applications drywall dust contamination affects all types of smoke
detectors. To help overcome this problem, it is strongly recommended
that installation of detector heads be delayed until all trades have com
pleted work and job site clean up has occurred in accordance with NFPA
If alarms occur whenever the heating system is turned on after an extended
shutdown, due to the accumulated dust burning off as the system compo-
nents heat, the detector system can be turned off for a short period. If the
system is being shut down for inspection, the work must be performed by a
qualified technician following procedures prescribed by the NFPA. Another
option would be scheduling the start-up of the heating system for an eve-
ning, weekend, or other off-hours period to minimize the effects of alarms
on regular daytime activities.
Not all unwanted alarms are caused by dirt, interference or other effects on
the detectors. If the control panel shows an alarm but no detectors in the
zone are indicating an alarm condition, the possibility of interference or a
failure of a control panel component should be investigated.
Responsibilities of Detector Owners and Installers
The owners of smoke detector-equipped fire alarm systems are respon
sible for maintaining the integrity of the detection system. This can be
Maintaining an Alarm Log and training appropriate personnel to prop
erly maintain the system as described above in the section titled,
To Do When Unwanted Alarms Occur.
Maintaining a Detector Maintenance Log that records inspection, test-
ing and cleaning data for each detector in the system. (Refer to Section
6 of this manual, Testing, Maintenance, and Service of Detectors for
information on recommended testing and maintenance intervals and
procedures, and a sample Detector Maintenance Log page.)
Maintaining a complete file of information on the alarm system in a
readily accessible location. This file should include specifications and
installation instructions for the detectors, control panel, and auxiliary
devices, wiring diagrams, wire location information, and the manufac
turer’s recommendations for isolating the detection system wiring from
other electrical wiring to prevent interference and unwanted alarms.
Making certain that maintenance personnel or contractors working on
the building’s electrical systems are given copies of the alarm system
wiring layout and locations so that potential interference from other wir
ing systems can be prevented by proper insulation and spacing during
Keeping accurate records of installation and modifications to all other
building electromechanical systems that could cause interference with
the alarm system so that problems can be promptly found and elimi
nated. Consider keeping records of schematic update schedules, wiring
layouts, and wiring location information. APPLICATIONS GUIDE: SYSTEM SMOKE DETECTORS
Making a record of all actions taken during the investigation of a series
of alarms, indicating a problem exists. If assistance must be sought from
the installer or manufacturer, there will be an indication of the tests that
have already been done by the owner’s personnel.
These services can be provided by qualified outside organizations.
The installers of smoke detector equipped alarm systems are responsible
for providing the owners with the necessary information and training so
that their personnel can maintain the integrity of the alarm system. These
responsibilities should include:
Providing copies of the specifications and installation instructions for
the detectors, control panel, and auxiliary devices; wiring diagrams and
wire location information; and the manufacturer’s recommendations for
isolating the detection system wiring from other electrical wiring to pre-
vent interference and unwanted alarms.
Verifying that the alarm system installation meets all applicable code
Completely testing a newly installed, expanded, or modified alarm sys-
tem to ensure that all components are working properly.
Providing troubleshooting assistance to the owners for a specified
break-in period after installation in case problems develop.
Helping the owner set up appropriate Detector Maintenance and Alarm
Logs for the system.
Providing initial instruction and training to the owner’s personnel or out-
side organization which will be monitoring and maintaining the system.
Providing troubleshooting assistance if nuisance alarm problems cannot
be solved satisfactorily by the owner’s personnel or outside organization.
Where to Get Help if the Source of Unwanted Alarms
Cannot be Found
In the event a series of unexplained unwanted alarms and/or a review of
the Alarm Log indicates that a problem situation exists, the owner should
conduct the initial investigation to find a solution. If the owner’s personnel
are unable to determine the cause for the alarms, the installer or represen-
tative of the manufacturer should be contacted to help isolate the problem.
Manufacturers can be contacted by phone for additional suggestions. If
factory assistance is needed, a factory engineer may be able to explain the
source of the problem with data from your Alarm Log, a complete descrip-
tion of your alarm system including detector model numbers, make and
model numbers of the control panel and other components, and a complete
summary of all aspects of the problem that have already been checked. SYSTEM SENSOR
End of Line Relay
Device used to supervise power (for four-wire smoke detectors) and
installed after the last device on the loop.
An unwanted alarm caused by non-smoke contaminants such as insects,
malfunctions, electrical transients or radio frequency interferences.
A chemical reaction between oxygen and a combustible material where
rapid oxidation results in the release of heat, light, flame and/or smoke.
A device that detects the infrared, ultraviolet, or visible radiation produced
by a fire.
Four-wire Smoke Detector
A smoke detector which initiates an alarm condition on two separate wires
(initiating loop) apart from the two power leads.
A device that detects abnormally high temperature or rate-of-tempera
A circuit which transmits an alarm signal initiated manually or automatically
(such as a fire alarm box, smoke, heat, or flame sensing device, sprinkler
waterflow alarm switch or similar device or equipment) to a control panel
or any similar device which, when activated, causes an alarm to be indi
cated or retransmitted. An Initiating Device Circuit (Loop) is a circuit that
connects automatic or manual signal initiating devices where the signal
received does not identify the individual device operated.
Any manually operated or automatically operated equipment which, when
activated, initiates an alarm through an alarm signaling device.
Intelligent (Analog, Smart) System Smoke Detector
A system smoke detector capable of communicating information about
smoke conditions at its location to a control unit. This type of detector typi-
cally communicates a unique identification (address) along with an analog
(data) signal, which indicates the level of smoke at its location.
Ionization Smoke Detector
An ionization smoke detector has a small amount of radioactive material
that ionizes the air in the sensing chamber, thus rendering it conductive and
permitting a current to flow between two charged electrodes. This gives
the sensing chamber an effective electrical conductance. When particles
of combustion enter the ionization area, they decrease the conductance of
the air by attaching themselves to the ions, causing a reduction in mobil
ity. When the conductance is less than a predetermined level, the detector
responds in a fire alarm condition.
Device responds with an alarm or trouble indication even if the current value
no longer exceeds the alarm threshold or trouble threshold.
The action of light being reflected and/or refracted. Light scattering by
smoke particles is used as detection technology in photoelectric smoke
Glossary of Terms
Addressable System Smoke Detector
System smoke detectors, which, in addition to providing alarm and trou
ble indications to a control unit, are capable of communicating a unique
Air Sampling-type Detector
A sampling-type detector consists of piping or tubing distribution from the
detector unit to the area(s) to be protected. An air pump draws air from
the protected area back to the detector through the air sampling ports
and piping or tubing. At the detector, the air is analyzed for fire particulate.
Alarm (Signal) Notification Appliance
An electromechanical appliance that converts energy into audible or vis
ible signal for perception as an alarm signal.
A signal indicating an emergency requiring immediate action, such as an
alarm for fire from a manual box, a waterflow alarm, or an alarm from an
automatic fire alarm system, or other emergency signal.
Alarm Verification Feature
A feature of automatic fire detection and alarm systems to reduce unwanted
alarms, wherein automatic fire detectors must report alarm conditions for
a minimum period of time or confirm alarm conditions within a given time
period, after being reset, to be accepted as a valid alarm initiation signal.
A visible and/or audible indication of system status.
Automatic Fire Alarm System
A system of controls, initiating devices and alarm signals in which all or
some of the initiating circuits are activated by automatic devices such as
Class A Circuit (Loop)
An arrangement of supervised initiating device, signaling line, or indicating
appliance circuits that prevents a single open or ground on the installation
wiring of these circuits from causing loss of the system’s intended function.
Class B Circuit (Loop)
An arrangement of supervised initiating device, signaling line, or indicat
ing appliance circuits, which does not prevent a single open or ground on
the installation wiring of these circuits from causing loss of the system’s
Combination Smoke Detector
A smoke detector that combines two or more smoke or fire sensing
The recommended maximum distance between adjacent detectors or the
area that a detector is designated to protect.
The capability of a detector to automatically adjust its alarm sensitivity to
compensate for any changes over time in the factory settings for smoke
and/or fire detection. In analog systems, this may be done by the panel.
End of Line
A device such as a resistor or diode placed at the end of a Class B wire
loop to maintain supervision. APPLICATIONS GUIDE: SYSTEM SMOKE DETECTORS
Remote Maintenance Condition
The NFPA 72 listed method of providing annunciation at the control unit that
the smoke detector is outside of its listed sensitivity.
A device that detects the visible or invisible particles of combustion.
A method of “smoothing out” a detector’s interpretation of sudden, short-
duration increases in obscuration within the sensing chamber, intended to
reduce the occurrence of nuisance alarms.
Spot (Point) Detector
A device whose detecting element is concentrated at a particular location.
Typical examples are bimetallic detectors, fusible alloy detectors, certain
pneumatic rate-of-rise detectors, most smoke detectors and thermoelec
An effect that occurs when air containing smoke particles or gaseous
combustion products is heated by smoldering or burning material and,
becoming less dense than the surrounding cooler air, rises until it reaches a
level at which there is no longer a difference in temperature between it and
the surrounding air. Stratification can also be caused by forced ventilation.
The ability of a fire alarm control unit (FACU) to detect a fault condition in
the installation wiring, which would prevent normal operation of the fire
When a fixed temperature device operates, the temperature of the sur
rounding air will always be higher than the operating temperature of the
device itself. This difference between the operating temperature of the
device and the actual air temperature is commonly spoken of as ther
mal lag, and is proportional to the rate at which the temperature is rising.
Per NFPA 72, 1999 edition: “All fire detection devices that receive their
power from the initiating device circuit or use a signaling line circuit of a
fire alarm control unit shall be listed for use with the control unit.”
Two-Wire Smoke Detector
A smoke detector which initiates an alarm condition on the same two wires
that also supply power to the detector.
Any false alarm or nuisance alarm.
Wireless Smoke Detector
A smoke detector which uses internal battery power to supply both the
smoke detector and integral radio frequency transmitter. The internal power
source is supervised and degradation of the power source is communi
cated to the control panel. On stimuli, the detector transmits a radio signal
to a repeater or Fire Alarm Control Panel (FACP) that in turn generates a
signal or status condition.
Equipment or materials included in a list published by an organization
(e.g., Underwriters Laboratories) acceptable to the local Authority Having
Jurisdiction (AHJ). The listing organization conducts product/material
evaluations, maintains periodic inspections, and states whether or not
the product/material meets appropriate standards for use in a specific
Note: The means for identifying listed equipment may vary for each orga-
nization. Some organizations do not recognize equipment as listed unless
it is also labeled.
Method of providing annunciation of a smoke detector that it is out of its
listed sensitivity range.
National Fire Protection Association (NFPA)
An organization that administers the development and publication of codes,
standards, and other materials concerning all phases of fire safety.
An unwanted alarm caused by insipient fire sources not imminently threat-
ening to life or property such as cooking and fireplaces, or misinterpretation
of particulate matter from non-fire sources such as moisture or accumula-
tion of dust as a true fire signature.
A reduction in the atmospheric transparency caused by smoke, usually
expressed in percent per foot.
Particles of Combustion
Substances (products that either remain at the site of fire such as ash, or
scatter as volatile products) resulting from the chemical process of a fire.
Photoelectric Smoke Detector
A photoelectric light scattering smoke detector uses a light source and a
photosensitive sensor to determine when smoke particles enter the light
path. (See illustrations on page 4.)
Projected Beam Smoke Detector
A projected beam detector responds to changes in the amount of light
transmitted between a light source and a photosensitive sensor. Smoke
particles reduce the amount of light reaching the sensor by scattering and
absorbing some of the light, indicating smoke detection in the light path.
Rate-of-Rise Heat Detector
A device that responds when the temperature rises at a rate exceeding a
set rate, usually 15° per minute.
Reflected Beam Smoke Detector
In a reflected beam smoke detector, both the light source and the photo
sensitive sensor are mounted in the same location, with a reflector mounted
at the opposite end of the coverage area. Rays from the light source are
returned to the light sensor by the reflector. When smoke particles enter
the reflected light path, some of the light is scattered onto the sensor, caus
ing the detector to respond. An advantage vs. the projected beam smoke
detector is improved installation and maintenance time/costs through the
confinement of all wiring and electronics to one end. The reflector requires
no electronics or wiring. SYSTEM SENSOR
Inspection and Test Date:
Inspection and Test Performed By:
Check one Location
Action Taken By
(Bldg, Zone, Det #)
Name and Date
Fire Alarm Log APPLICATIONS GUIDE: SYSTEM SMOKE DETECTORS
©2016 System Sensor. The company reserves the right to change specifications at any time.
SPAG9102 • 4/16