Where do you begin? The technician inside all of us wants to immediately lay out the design on the plans, but that should not be your first move. Instead, determine what goals the owner and other relevant stakeholders want the building’s fire protection system to do. Once you understand those goals, determine how the features of your fire alarm system design can best meet those intentions, providing reliable detection and notification.


With your understanding of detector placement and the effect that has on false alarms and on operational reliability, focus on these concerns early in the design discussions.


NFPA 72 2013, defines false alarms as either “malicious alarm,” “unwanted alarm,” “nuisance alarm,” “unintentional alarm” or an “unknown alarm.” Nuisance or unknown alarms fit more appropriately those types of false alarms over which you have some control and most often describe what people think of as “false alarms.” Nuisance alarms include situations where no hazard exists. The code defines an unknown alarm as the “unwanted activation of an alarm initiating device or system output function where the cause has not been identified.”


Annex A of the code will guide you in the environmental conditions that influence smoke detectors. Depending on the type of smoke detector, environmental issues—such as air velocity near the detector (too much airflow at a smoke detector will inhibit its response), humidity, ambient temperatures and color of smoke—could affect the detector’s response or cause false actuations.


In addition, electrical and mechanical influences and the presence of aerosols or other particulate matter in the spaces where you plan to install smoke detection can affect the devices’ proper operation. 


You now have two types of detectors that you may use: photoelectric and multicriteria. Most major manufacturers have chosen to discontinue the ionization spot-type smoke detector, so it is not a viable choice. 


In difficult environmental situations, the multicriteria smoke detector may be the best choice. However, in each location within the building where you intend to install smoke detectors, you need to determine the appropriate type of device through an evaluation of the ambient conditions. This means you must understand the ambient and operational conditions in the space.


During a fire, will there be sufficient thermal energy to drive the smoke upward to the locations of the detectors or will the smoke layer stratify? Stratification commonly occurs when the higher temperature of the rising smoke begins to equalize with the space’s ambient temperature. When this occurs, the smoke plume stops rising and spreads horizontally without reaching the ceiling where the smoke detectors are located.


The type of fire you intend to detect can materially affect the likelihood of stratification. For example, if you plan to detect a smoldering fire, realize that such fires have little thermal energy to drive the smoke plume upward. You may need to consider using alternative detector types or locations.


Linear-projected beam-type smoke detectors installed above the height of any interference from people or objects in the space, but considerably lower than the ceiling height, may increase the likelihood of detection. Similarly, using wall-mounted spot-type smoke detectors may, in some cases, make detection of a lower energy fire more likely.


Some situations call for more sophisticated smoke detectors, such as an air-sampling type, that will draw any smoke into the detection device without the hindrance from temperature differentials. Consider all of the operational parameters of a particular space when choosing the type of smoke detector.


The prescriptive portion of the code requires smoke detectors on smooth ceilings to be placed a minimum of 30 feet on center (between detectors) and 15 feet from a sidewall. However, to ensure that no authority having jurisdiction tries to enforce these exact measurements, the annex explains that the requirement allows for a ±5 percent [±18 inches] of the 30-foot number to allow for construction issues that would affect the nominal spacing. 


Additionally, the code states, “for irregularly shaped areas, the spacing between detectors can be greater than the selected spacing, provided the maximum spacing from a detector to the farthest point of a sidewall or corner within its zone of protection is not greater than 0.7 times the selected spacing (0.7S).”


The code also has specific spacing requirements based on ceiling construction. Reduced spacing is required for joisted or beamed ceilings. As a final note, NFPA 72 2013 has removed the so-called “dead air space” limitation on spacing for smoke detectors. However, the requirement remains in place for heat detectors.


Once you have determined the system features and the detection and coverage type, address the location of manual fire alarm boxes and the location of audible and visible notification appliances.


Your design must provide one manual fire alarm box located within 5 feet of each exit door on each floor. You will need additional manual fire alarm boxes to ensure that the travel distance from any point on a given floor to the nearest manual fire alarm box does not exceed 200 feet, measured horizontally on that floor. If you have a grouped opening exit design in the building (e.g., as in a mall or a large auditorium), you must mount the manual fire alarm boxes on both sides of the grouped openings and within 5 feet of the each side of the grouped opening whenever the total width of the opening exceeds 40 feet in width.


To design the location for the audible and visible appliances, you need to know the requirements contained in the most recent International Building Code (IBC) as well as NFPA 72 2013. Both documents define the coverage for audible appliances in all occupiable areas. 


There are two modes of signaling: public and private. Commonly, you will use the public mode requirements. Private mode applies to occupancies such as hospitals.


Public mode audible signals must “have a sound level at least 15 dB above the average ambient sound level or 5 dB above the maximum sound level having a duration of at least 60 seconds, whichever is greater, measured 5 feet above the floor in the area required to be served by the system using the A-weighted scale (dBA).” Both NFPA 72 2013 and the IBC require installers to submit the design sound pressure levels produced by the notification appliances for the various coverage areas for the review, approval and for documentation and use during the acceptance testing of the fire alarm system. 


Occupancies with sleeping areas, such as apartments or dormitories, have two additional requirements. First, the audible notification appliance(s) must produce a sound level of at least 75 dB, measured at the pillow level in each sleeping area. Second, all audible signals in each sleeping area must operate at a frequency of 520 hertz.


I have only presented a few of the highlights of the code requirements here, so you must carefully determine all of the features of the system and ensure that your design meets all of the code requirements before submitting it for review and approval.


The 2015 IBC requires you to submit the following for review and approval prior to system installation (emphasis added): (1) a floor plan that indicates the use of all rooms; (2) locations of alarm-initiating devices; (3) locations of alarm notification appliances, including candela ratings for visible alarm notification appliances; (4) design minimum audibility level for occupant notification; (5) location of fire alarm control unit, transponders and notification power supplies; (6) annunciators; (7) power connection; (8) battery calculations; (9) conductor type and sizes; (10) voltage drop calculations; (11) manufacturers’ data sheets indicating model numbers and listing information for equipment, devices and materials; (12) details of ceiling height and construction; (13) the interface of fire safety control functions; and (14) classification of the supervising station.


The items covered in this article will require you to assume more responsibility when designing and installing each fire alarm system. Ultimately, you will need to know and understand multiple codes and have a strong background in fire alarm system operations and installations to do your job efficiently, correctly and profitably.


You know that designing a fire alarm system involves more than placing circles and squares on a drawing. Although it is imperative to follow the requirements of the applicable codes, you also need to tap into your experience with fire alarm system installations to ensure that each installation is reliable. Today, you generally have more responsibility to ensure the design and installation meets the owner’s goals. You also should know that, with the exception of Chapter 29, all of the chapters in NFPA 72 2013, National Fire Alarm and Signaling Code, apply to both the design and installation of fire alarm systems.