Notification Appliances and Evacuation Signaling
The signaling vocabulary
NFPA 72 defines public mode (to alert occupants generally), private mode (staff only), and voice communication systems that deliver intelligible prerecorded or live messages. Audible appliances must produce a sound level at least 15 dBA above ambient, or 5 dBA above ambient if ambient exceeds 105 dBA, measured at 10 ft (or at the pillow for sleeping areas, where 75 dBA is the floor). Visible appliances follow the current-corrected candela table: 15 cd for small rooms, scaling with room size and mounting height.
Synchronization and intelligibility
Appliances on the same circuit must be synchronized so that audible signals are not perceived as a separate alarm and visual flashes do not exceed the flash rate limits (typically 1 to 3 flashes per second, with synchronized flash). Voice systems must meet intelligibility criteria - the message must be understandable, not merely audible, which drives speaker spacing, tap settings, and reverberation control.
Zoning and evacuation
Notification appliance circuits (NACs) are arranged so that a single fault does not silence the entire building. Selective evacuation uses voice or zoning to relocate occupants (e.g., defend-in-place floors in high-rise). Alarm silence must not disable subsequent alarms; resound is required.
Traps
- Applying the 15 dBA rule at the wrong reference distance.
- Forgetting the 75 dBA sleeping-area minimum, or the 520 Hz low-frequency requirement for sleeping areas in newer editions.
- Assuming audibility equals intelligibility for voice systems.
- Mixing candela ratings without applying the room-size correction.
Worked example
Ambient in a corridor is 60 dBA. Required alarm level = 60 + 15 = 75 dBA at 10 ft. If the measured appliance produces 72 dBA at 10 ft, it fails; add appliances or increase tap. For a 30 ft by 40 ft room, the 15 cd rating is insufficient - the larger room requires higher candela per the correction table.