Most contractors install strobe lights to comply with the National Fire Alarm and Signaling Code by simply using a combination audible-visible appliance everywhere one or the other is shown on the plans. It should be obvious that if you follow this method of “compliance,” you will either have an incorrect number of audible appliances or an incorrect number of visible appliances. Each of these appliances is governed by distinct rules.
The code tells us that there are two methods of visible signaling: direct and indirect. Both of these methods are designed to notify building occupants of an emergency condition by a direct viewing of the visible appliance or by illuminating the surrounding area. Visible notification appliances are required to be located throughout the building or area and must be of a type, size, intensity and number to ensure the intended audience, regardless of its orientation in the space or area, sees the strobe’s light.
Flashing strobe lights are given an effective intensity expressed in candelas (cd). This measurement can be compared to the light produced by a steady burning light. For example, a flashing strobe light with an intensity of 15 cd has the same apparent brightness as a 15-cd steady burning light source. Unlike the sound level measurements you can make with a sound level meter in the field, accurate field measurements of effective visible intensity is not practical.
The code mandates that strobe lights used for fire alarm signaling or meant to indicate evacuation of an area or building must be clear or nominal white and cannot exceed 1,000 cd. Lights used to indicate that the occupants need to seek information or instructions also must be clear, unless the building emergency plan and the authority having jurisdiction for the building mandate a different color. The 2010 edition of NFPA 72 also requires that strobes used for mass notification systems (MNS) be clear as well. (It should be noted here that some military organizations still require amber strobes for MNS visible signaling.)
The code requires all strobes that are in one viewing to be synchronized. However, there are exceptions to this requirement. For example, NFPA 72-2010, Section 220.127.116.11 states that the “strobe synchronization requirements of... [chapter 18] shall not apply where the visible notification appliances located inside the building are viewed from outside of the building.”
The annex clarifies this exception, stating: “It is not the intent to establish viewing and synchronization requirements for viewing locations outdoors. As an example, there is no need for floor No. 1 to be synchronized with floor No. 2 if there is no visible coupling as in an atrium.”
The purpose of synchronization is to ensure that the fire alarm system visible signals do not cause a photosensitive epileptic to initiate a seizure. The code advises us “that studies have shown that the effect of strobes on photosensitive epilepsy lessens with distance and viewing angle” provided that the composite flash rate is no greater than the flash rate produced by two strobes.
The wall mounting of visible signals is required to have the lens of the strobe mounted not less than 80 inches and not greater than 96 inches above the finished floor. Guidance also is given where a ceiling is too low to permit the required installation height at the minimum of 80 inches. In this case, the code dictates that the visible appliance must be mounted within 6 inches of the ceiling, and the room size covered by the strobe must also be reduced by twice the difference between the minimum mounting height of 80 inches and the actual lower installation height.
Of course, those familiar with the code know that there are tables providing the size of the room covered by a strobe or strobes with a specific candela rating. Once you have determined the size of the room, the table will indicate what effective intensity value the strobe must be listed to produce.
The code also allows ceiling-mounted strobes to be used for compliance and has a separate table providing the relationship of room size and ceiling height to determine the effective intensity of the strobes in that application.
Corridor spacing of visible signals can be applied if the corridor width is no more than 20 feet wide. In this specific case, the visible appliances are generally rated at a minimum of 15 cd with one appliance located within 15 feet from the end of the corridor and a clear separation between appliances of not more than 100 feet. If two strobes are in any field of view they must flash in synchronization, and any interruption in the clear viewing path will require additional strobes.
Some occupancies may have lighting that is brighter than normal, such as a retail situation where the strobe effective intensity may need to exceed the minimums prescribed by the code to ensure the occupants will be able to perceive the flashing of the appliances when they operate. When dealing with situations that may “stretch” the code’s ability to provide guidance, the wise contractor will work closely with the designer, supplier and fire official to ensure compliance.
MOORE, a licensed fire protection engineer, frequent speaker and an expert in the life safety field, is a co-editor of the current National Fire Alarm Code Handbook. Moore is a principal with Hughes Associates Inc. at the Warwick, R.I., office.