Part 1 of this article appeared in the Summer 2006 edition and described the origin of a research project on strobe effectiveness in large-volume spaces. That article also described the project tests, test results and several key factors affecting system design and performance. In this final installment, additional test results are presented and the factors affecting system performance are discussed. Finally, guidelines for strobe system design and installation in large-volume spaces are presented.
The post-test surveys and interviews after-each test sought information on how participants were first alerted, whether they were aware of direct or indirect strobe signaling and if they encountered blind spots. They were also asked to rate the system's effectiveness (see Part 1).
Results showed that the participants were capable of being alerted by both direct and indirect signaling. The original test hypothesis was that, in large-volume spaces, direct signaling would be the predominant mode of occupant notification.
Indirect Strobe Effects
In Stores #1 and #3, there were many aisles where strobes were not located directly overhead. Thus, strobe light had to come over the racks/shelves from adjacent lines of strobes. In Store #2, strobes were located directly over almost all aisles. This resulted in greater indirect coverage on the surface of stock. Similarly, the greater clearance from the top of the stock to the strobes in Store #3 versus Store #1 permitted greater penetration into aisles that did not have strobes directly over them. Figure 1 shows a typical warehouse store with strobe coverage providing both direct and indirect signaling to the occupants. Figure 2 is the same diagram highlighted to show the surfaces where one of the strobes provides indirect signaling by illuminating the surface of the floor and the stock on the racks or shelves. The highlighted surfaces in Figure 2 show that as the clearance between the top of the storage and the strobe is decreased, or as strobe spacing is increased, light penetration to adjacent aisles is decreased.
Direct Signaling Effects
These types of stores have large volumes and long viewing paths. In many places, the aisles, racks and shelves focus the occupants' vision in a way similar to corridors in schools and offices. In corridors and hallways less than 20 ft (6 m) wide, the National Fire Alarm code permits the use of lower-intensity strobes at greater distances since occupants are likely to directly view at least one appliance as they transit the corridor. Presently, most authorities require strobe system design-in warehouses and superstores to be based on NFPA 72 room coverage requirements. They do not permit the use of corridor rules.
Participants were asked if they were able to actually see (directly view) one or more strobe lights flashing without intentionally looking up at the ceiling. The results show that the system in Store #3 was the most effective at direct signaling to occupants. Analysis shows that this was probably the result of a greater clearance between the top of stock/storage and the strobe lights than in Stores #1 and #2. As a result, occupants could see more of the ceiling and, hence, more strobes from most vantage points.
All participants reported that they could see (directly) at least one strobe as they walked through aisles. The majority of responses indicate that three to six strobes were generally visible as the participants moved about the space.
Nevertheless, as participants walked around the stores, there still were locations where they did not directly see a strobe or its indirect reflection. At Store #3, the greater clearance between stock and strobe lights increased the likelihood of direct signaling.
Nevertheless, this location also had the greatest number of reported blind spots where a strobe or its effect was not visible. Even with a larger strobe clearance, the aisle spacing versus the strobe spacing resulted in a single row of strobes for three to five aisles.
Participants found that the strobes did not penetrate sufficiently to provide reliable direct or indirect signaling when they were more than two or three aisles away. See Figure 2.
In Store #1, the ceiling was an open-plantype with all structural members and utilities exposed. The strobes were located below almost all obstructions except air-handling ductwork. There were several locations noted where ductwork blocked the strobes.
There was general agreement among participants that strobe lights were an effective means for alerting the hearing impaired. The survey results show that the experience of the participants on that day, with a particular system, affected their opinions. Their opinions on universal effectiveness correlated with their opinions of the particular systems they just experienced. Nevertheless, they gave higher effectiveness ratings when generalizing. Even though they may have seen faults with the system they just witnessed, they still felt that strobes were an effective method for alerting.
To better understand the possible causes of the results of this project, it is helpful to understand the existing performance based requirements of NFPA 72 and the light-distribution requirements of UL 1971.1 These are addressed in detail in the complete project report.2 However, in general, while strobe intensity and spacing are important, the test results indicate that strobe location with respect to aisle spacing and stock height may be more important in maximizing occupant notification.
The Store #2 system was generally rated higher because strobes were located over almost all aisles. The Store #3 system showed that good performance does not require strobes over every aisle. Similarly, the Store #1 test showed that where aisles are moved, resulting in not having strobes directly overhead, adequate performance is still possible.
When all three tests are reviewed and compared, several significant points emerge:
- Strobe lights are effective for both indirect and direct viewing even if not located directly over an aisle, provided there is sufficient penetration to the aisle.
- A design with strobe lights over every aisle is more effective than one where strobes serve several aisles.
- Aisles focused the occupant's vision and improved direct signaling effects.
In discussions with participants after completion of the post-test surveys, the potential for blind spots and marginalized coverage was discussed. All seemed to agree that designers should take steps to minimize blind spots and to anticipate rack and aisle changes (where possible). Many felt that blind spots were an inevitable result given the complex store layouts and the nature of visible signaling. Some discussion ensued as to whether blind spots constituted a "failure."
Many, but not all, participants felt that in the context of total protection, some blind spots or areas with marginal coverage would not be cause for concern. Several other conditions combine to protect store occupants for the short time that they might not see direct or indirect strobe signaling. First, audible signals would also provide alerting for hearing-able and many hearing impaired persons. Second, the occupants are alert and mobile. If they are anywhere near a fire, other senses (smell, sight, touch) will provide additional cues. If they are not near the fire, they are not yet threatened and their normal movement will bring them to an area where they will be alerted by a strobe if they have not already been alerted audibly or by other occupants' behavior. Third, if they are not near a fire, the large volume of the space (to absorb smoke and heat) combined with sprinkler protection separates them from the threat. Once alerted, code-compliant means of egress provides several safe ways out of the space. These occupancies differ from others, such as apartments, offices and healthcare facilities in that there are typically no dead-ends, they have good visibility across the space when in main aisles and many locations have more than two ways for an occupant to move.
This project did not attempt to answer specific scientific questions, such as what threshold level of illumination over what area was necessary to alert occupants. Instead, it sought to simply find out if strobe systems could effectively alert persons in certain large-volume spaces. Visible signaling by strobes is only one potential method of occupant notification. This project did not try to determine what methods (audible, voice, text, etc.) or combinations of methods are the most effective in obtaining the desired occupant response.
The results show that use of performance-based design methods for strobe alerting systems is viable in large warehouse and superstore-type occupancies. Both direct and indirect signaling contributed to occupant notification, but only where strobes could penetrate into the aisles. For a more in-depth analysis and discussion, readers are referred to the complete report.2
The tests identified variables that are important to the successful design and installation of strobe systems in these types of spaces. As a direct result, the NFPA 72 Technical Committee on Notification Appliances has modified and added Annex text to the 2007 edition to incorporate some of the findings of this project.
- UL 1971, Signaling Devices for the Hearing-Impaired, Underwriters Laboratories, Inc. Northbrook, IL, 2002.
- Schifiliti, R., Direct Visual Signaling as a Means for Occupant Notification in Large Spaces Research Project, Fire Protection Research Foundation, Quincy, MA, 2006.
Editor's Note About This Article
This is a continuing series of articles that is supported by the National Electrical Manufacturer's Association (NEMA), Signaling Protection and Communications Section, and is intended to provide fire alarm industry-related information to members of the fire protection engineering profession.