Spacing guidelines for visible notification appliances in the 2022 ed. of NFPA 72, National Fire Alarm and Signaling Code (NFPA 72), are limited to a device (lens) elevation of 30 ft above finished floor. There are many cases where a client may want, or a jurisdiction requires visible notification where the appliance elevation will exceed the max. elevation.
Shipping, warehousing, manufacturing, & athletic facilities are examples where tall roof elevations of 30 ft+ exist. While the above facilities don’t always require notification it may be desired by the tenant & sometimes it is required as part of an overall performance-based design (PBD). Using a PBD with strobe spacing can allow the designer to raise the strobe above 30 ft to avoid conflict with building operations.
According to NFPA 72, when calculating light levels to justify PBD device spacing, adequate light levels (0.0375 lumens/ft2) must be proven along each angle from 0-90 by 5-degree increments. We have seen fire alarm shop submittals where the contractor did not prove light levels between 40 & 90 degrees were met, thinking that light levels at those unoccupied angles don’t matter. It’s a reasonable observation, as no human stands 35 ft tall, so why deliver light levels at a 90-degree angle to a strobe at that height?
The NFPA 72 Annex material (§ A.18.5.5.9) explains that direct viewing of the strobe is not the desired intent. “This level of illumination has been shown to alert people by indirect viewing (e.g., reflected light) in a large variety of rooms with a wide range of ambient lighting conditions.” Meaning, one does not need to be looking directly at a strobe to be aware of its presence. Think of an emergency vehicle out of your direct line of sight. Have you ever been inside your home & witnessed an emergency vehicle drive by? While you may not hear the siren, you may see the strobe light reflected on surfaces inside your home. In this case, you are aware of the vehicle without ever seeing it. Similarly, a fire alarm strobe may be seen because its light bounces off a surface above the occupant’s head. As such all angles that a person can see in their field of vision are important for reflective light, not just those at head height.
In addition to breaking the prescriptive 30 ft barrier, using a PBD may allow the designer to use fewer devices in a space at a higher candela rating than normal. Keep in mind that larger candela devices require more current draw. Selecting the most efficient devices along with adjustments to height & spacing may allow the designer to provide a more power-efficient design. Reducing the amount of power needed will allow for less notification appliance booster panels required to support the design. While fewer devices means more savings, that savings may be erased if more booster panels are required to support the design.
What ways have you used PBD fire alarm designs?