Many designers of the notification portion of a fire alarm or mass notification system (MNS) seem unaware of the specific performance requirements for each type of appliance as required or allowed by NFPA 72 2016, National Fire Alarm and Signaling Code. Because of the different performance requirements for each appliance, a system designer must understand the design process associated with each concept.
Notification appliance requirements appear in chapters 18 and 24. I have discussed the basics for audibility in previous articles.
In the field, the most difficult issue relates to understanding the performance objectives and performance design for visibility requirements and then knowing when to combine the two technologies: audible and visible.
Audibility requirements for all fire alarm systems have resided in the code for a long time, and very little has changed over the years. For public-mode audible notification, using the A-weighted scale (dBA) on a sound-pressure-level meter, the audible appliances must have a sound level at least 15 decibels (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. This measurement must be taken 5 feet above the floor in the area that requires the system. All sleeping accommodations now require the new low-frequency notification appliances.
The process of notification appliance design can begin with the audibility or visibility portion of the fire alarm, or mass notification system, but you should choose one or the other. Many designers make the common mistake of using combination audiovisual appliances for every location that has a horn (or speaker). They do this because they do not understand the different performance requirements of audibility and visibility.
Most qualified designers begin with the audible notification appliances to meet the sound-level requirements of the code. After they have ensured the adequacy of the alarm notification sound levels for the building, they then evaluate the visible-notification requirements. Generally, they meet this design aspect using strobes.
When using strobes for evacuation signaling, the appliances must use a clear or nominal white lens and cannot exceed 1,000 candela (cd) effective intensity. When the strobes signal occupants to seek information or instructions (as in a MNS), they must have clear, nominal white, or other color lenses as required by the emergency plan and the authority having jurisdiction. The code does require the marking of strobes with colored lenses with the listed effective intensity using the lens color installed. Spacing must meet the public-mode spacing requirements of Section 18.5 using the effective intensity as the basis for spacing. Section 18.5.5 contains specific mounting requirements. However, we won’t cover these requirements here because, in my opinion, the spacing of the visible appliances has greater importance.
The designer must space all visible notification appliances in accordance with the requirements found in Table 18.104.22.168.1(a) and Figure 22.214.171.124.1 for wall-mounted appliances, or Table 126.96.36.199.1(b) for ceiling-mounted appliances. The code requires the listing of visible appliances to specify either wall or ceiling mounting.
For wall-mounted appliances, the candela ratings directly relate to the room sizes shown in the table. The code does not dictate which appliance to use; the designer must make that decision. The code only requires the appliance to have the appropriate candela output for the installed space.
Although Table 188.8.131.52.1(a) has numerous selections for effective intensity, in reality the limiting factor will rest with the intensity of strobes available from the manufacturer of the control system. Once the room size and the necessary effective intensity is known, simply use the highest available intensity, at or above the table values, from the control manufacturer. If you choose wall-mounted strobes and have established the base design, you may now begin to evaluate their locations to determine where the combination audible/visible units might serve appropriately.
If you choose to use ceiling-mounted notification appliances as a performance alternative, know that the effectiveness of ceiling-mounted appliances does not depend on them being mounted on a surface. Therefore, the code permits you to suspend them below the ceiling, using proper electrical installation methods.
Appliances mounted parallel to the floor, whether on a ceiling or suspended, can sometimes significantly reduce installation costs and provide better coverage. The code provides alternative information for ceiling-mounted notification appliances when ceilings exceed 30 feet. In those instances, you must either choose to suspend ceiling-mounted visible notification appliances at or below 30 feet, install the ceiling-mounted appliances at the mounting height determined using the performance-based alternative of Section 184.108.40.206, or install wall-mounted visible notification appliances in accordance with Table 220.127.116.11.1(a).
As with the use of low-frequency audible appliances for sleeping areas, the code provides special requirements for strobes when used to awaken sleeping individuals. For this application, consult Table 18.104.22.168.2 to determine the minimum required intensity.
The code states: “Where the appliance is mounted less than 24 in. from the ceiling, it must have a minimum 177 cd effective rating because it might be in a smoke layer at the time it is called upon to operate. If the appliance is 24 in. or more from the ceiling, it is permitted to be rated 110 cd effective or more. Note that the requirement for increasing the intensity when mounted close to the ceiling applies only to strobes used in sleeping areas to awaken sleeping people. It is assumed that in nonsleeping situations, a strobe is not needed to alert someone if there is a developing smoke layer.”
When strobes are used in MNSs, they cannot have any “fire” marking but they may be marked with the word “alert” stamped or imprinted on the appliance.
In addition to strobes used as visible notification appliances, the code permits the use of “textual” and graphical visible notification appliances. Textual visible appliances may consist of scrolling message boards or flat screens. Static signs serve as one example of graphical visible notification appliances. Typically, MNSs will have the largest amount of textual visible appliances. Chapter 24 permits the use of textual and graphical visible notification appliances for primary or supplemental notification.
The code considers textual and graphical visible notification to serve as a means of primary notification where it provides the only method used to convey emergency mass notification information to the general public or specific individuals. Chapter 24 requires that, when audible notification is provided, the MNS must also provide visible notification information to serve the hearing impaired and for high-noise areas. Additionally, Chapter 24 permits the use of textual and graphical visible appliances for nonemergency purposes; however, the code requires emergency textual and graphical messages to override nonemergency textual and graphical messages.
Although this article does not provide an exhaustive account of the visibility and audibility requirements found in the code, it serves to make designers aware of the importance of understanding the underlying performance requirements and objectives of the two methods of alerting individuals in buildings.