|Visual Signaling in Large Spaces|
Visual Signaling in Large Spaces
NEMA| Fire Protection Engineering
The Winter 2006 issue of this magazine focused on fire protection in warehouses, big box stores, and other similar occupancies. In the Viewpoint article "So What Do You Expect," Warren Stocker noted that occupant notification is not always required by model codes for many warehouse-type spaces. 1 However, when an alarm system is required or desired as part of an overall fire safety plan, it is challenging to find codes and standards that address the unique characteristics of storage occupancies. He noted that there is a "natural tendency to apply the general requirements, which in some cases may border on excess."
This article is the first of a two-part series on visual signaling in large spaces. In this first part, the need for research on strobe effectiveness in large-volume spaces is presented, and an engineering research project aimed at supplying information for a code committee is described. That project directly addressed the concerns expressed by Warren Stocker in his Viewpoint article. In Part 2, which will appear in the Fall 2006 issue, the strobe project results will be described. Key factors affecting system design and performance will also be discussed and illustrated.
The NFPA 72 Technical Committee on Notification Appliances recognized that the effectiveness of the existing prescriptive and the performance-based requirements for visible appliances had not been tested in large-volume spaces. The requirements for the installation and performance of visible signaling in NFPA 72, The National Fire Alarm Code, are based on occupants being alerted by indirect signaling effects. 2 That is, they are alerted by the illumination of their surroundings, not necessarily by direct viewing of the signaling appliance. The testing that led to those requirements was limited to classroom and office-type spaces. 3 The methodology was never tested in large, well-lit spaces such as warehouses, large "super stores," etc. Nevertheless, because strobes are being used for occupant notification in these spaces, the installation and performance requirements of NFPA 72 are being enforced despite the lack of any technical foundation. In some cases, authorities are imposing their own requirements such as not allowing ceiling-mounted appliances. The Annex of NFPA 72 states that there may be more efficient methods of visible signaling in large spaces such as warehouses and distribution centers.
NFPA 72 permits a performance based design approach that actually exceeds the prescriptive requirements for visible signaling. 2 Ad hoc testing in a large home supply store showed that such an approach may be effective, but not necessarily for the same reasons that it works in smaller spaces. The tests appeared to show that high ambient light levels resulted in little or no indirect signaling effect (reflected light) in some parts of the space. The signal-to-noise ratio produced by the operating strobes was low in many locations. However, with strobes located over the aisles or unobstructed by stock, direct signaling and some indirect signaling was achieved. From that testing, a working hypothesis was formed: In large spaces, occupant notification is achieved in a large part due to direct viewing of the strobes, and indirect signaling (reflected light) is minimized due to high ambient light levels. That is, because of the long viewing distances, it might be possible for an individual to see strobes in direct line-ofsight at a distance, while not seeing those closest. See Figure 1.
The Fire Protection Research Foundation was asked to consider sponsoring a project to address some of the committee's concerns. The project sought to identify key factors affecting occupant notification by strobes in large spaces and to determine if the existing performance-based requirements of NFPA 72 would be effective. A proposal for limited engineering research and testing was submitted to the Fire Protection Research Foundation. The proposal was accepted, and the project was funded.
The project was intentionally designed for quick but meaningful results. Work began in mid-July 2005 and needed to be substantially complete by the end of October 2005 to provide feedback to the code committee. The project did not seek to define all variables and parameters required for success. Instead, the strobe alerting systems were studied as a whole to determine if they were effective. Because of the limited scope and small sample sizes, it was not possible, nor intended, to determine why a particular system or component was effective in providing occupant notification. Nevertheless, engineering analysis of the tests resulted in identification of several variables that affect the success of the systems in alerting occupants. As a result, some information on suggested design and installation practices was added by the technical committee for inclusion in the Annex of the next (2007) edition of NFPA 72.
The Fire Protection Research Foundation was the principle sponsor for the project. They provided a grant to conduct tests, draft code language, and write a report. 4 Several companies and organizations provided targeted financial support to the Foundation. One of the goals of this project was to keep costs low and to actively involve several stakeholders. To achieve these goals, the help of organizations other than funding participants was solicited. Project partners were solicited to provide facilities for testing, technicians to assist in the tests, and other services, including an American Sign Language interpreter, necessary to move the project forward. Several project partners provided in-kind donations of time, facilities, and services.
The results show that it is possible to have effective occupant notification by strobes installed per the requirements of the performance-based section of NFPA 72. Occupant alerting is achieved by a combination of direct and indirect signaling. The tests highlighted additional factors that designers, installers, and owners should consider in order to increase the effectiveness of systems in large spaces. As a direct result of this project, the NFPA 72 Technical Committee on Notification Appliances drafted a Committee Comment revising the Annex text regarding visible signaling in large spaces to incorporate ideas and concepts found in the testing.
Tests were conducted in three different warehouse type stores. 4 In summary, for each test, participants were solicited and asked to walk around the store. The fire alarm system was then activated. Pre-and post-test surveys were used to gather data.
For each site, information was gathered concerning the design and installation of the strobe alerting system. Ceiling heights were either obtained from plans or measured. The mounting height of strobes relative to the floor, the ceiling, and lighting fixtures was measured. Stock heights, aisle widths, and rack widths were measured in several locations. Ambient light measurements were taken throughout the stores. Lighting in each of the locations was provided by fluorescent lamps. Store #3 also had skylights. Lighting levels were on the order of 430 - 1900 lux (40-180 ft-candles). Information about each location, including ceiling heights, strobe heights, stock heights, aisle widths, and aisle spacings, are listed in the complete report. 4
At Store #1, the fire alarm system was designed and installed to permit the audible signal to be disabled separately from the visible signals. This allowed the strobe lights to be activated without any audible signal.
Strobes were located below the ceiling, at about the same level of the hanging fluorescent lights. The original design called for the strobes to be located over the aisles, between racks. Within each aisle, the strobes were spaced approximately 45 ft (14 m) to 48 ft (15 m). Rack spacing varied, with most 16 ft (4.9 m) on center and some as much as 30 ft (9.1 m) on center. Thus, strobe coverage might be 45 ft (14 m) x 16 ft (4.9 m) in order to provide a line of strobes in each rack aisle. However, after the system was installed, the rack layout was altered, resulting in many lines of strobes not falling directly over an aisle.
The strobes at this location are the multi-candela-type that is field-adjustable. After the test, it was found that at least one strobe was never changed from the nominal 15 cd eff. out-of-the-box setting.
The fire alarm system at Store #2 did not permit the audible signal to be disabled separately from the visible signals.
The strobes were located on a suspended acoustical tile ceiling at the same level of the building's fluorescent lights. The design and installation resulted in most strobes being located over the aisles, between racks. Within each aisle, the strobes are spaced approximately 48 ft (15 m). Rack spacing varied, with most 16 ft (4.9 m) on center and some as much as 30 ft (9.1 m) on center. Thus, strobe coverage might be as low as 48 ft (15 m) x 16 ft (4.9 m) in order to provide a line of strobes in each rack aisle.
The test at Store #3 coincided with the Report on Comments meetings of the NFPA 72 Technical Committees. The participants were all Technical Committee members and included almost all members of the Notification Appliances Committee.
The fire alarm system at this location was designed and installed to permit the audible signal to be disabled separately from the visible signals. However, the manner in which this was effected resulted in a single audible chirp when the system was activated. After that first chirp, the audible signals stopped and the strobes continued to operate.
The strobes at this location were located on the bottom of the bar joists supporting the ceiling/roof. The fluorescent light fixtures were approximately 8-12 in. (200-300 mm) below the bar joists. The design and installation resulted in strobes being located over most of the main aisle and circulation areas. However, not every merchandise aisle had a row of strobes overhead. Typically, the strobes were over the main aisles and over every third to fifth stock aisle.
There were 13 participants at the Store #1 test, 12 at Store #2, and 22 at Store #3. Most participants were from the fire protection and fire service communities. Several participants in each test had mild to severe hearing impairments. None were considered to be deaf.
For each location, participants were asked to rate the effectiveness of the fire alarm strobe light system. The results for the three test locations are shown in Figure 2 through Figure 4 and combined for all locations in Figure 5.
In general, the strobe systems were judged to be effective. However, clearly there were differences that made the Store #2 system stand out as the most effective and the Store #1 system as the least effective system. The Store #2 system used higher intensity strobes on a reduced spacing and located over almost all aisles between racks. In Store #1, the strobe locations were designed to be over aisles. However, before completion, the rack layout changed, resulting in most aisles not having a row of strobes directly overhead. It is also possible that many of the strobes in Store #1 were not set to the correct output intensity. In Store #3, most respondents felt the system was effective. Based on strobe intensity, ceiling height, and strobe spacing, the system in Store #3 was over-designed. Nevertheless, when superimposed on aisle/stock layout, there were aisles where coverage was minimal or nonexistent. These locations received poor evaluations.
In Part 2 of this article (Fall 2006), additional test results are presented and the factors affecting system performance are discussed. Finally, guidelines for system design and installation are presented.
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 industryrelated information to members of the fire protection engineering profession.