Direct Visual Signaling as a Means for Occupant Notification in Large Spaces
7. Test Results and Analysis
This section of the report presents the results of the Post-Test Survey with some discussion of the results for each question. In section 8, Discussion, many factors are discussed relatively along with possible causes and consequences.
As noted in the test descriptions, the test in Reading was the only one where the audible signals could be completely silenced. The system at the Wal*Mart in Kissimmee allowed the horns to be disabled separately from the strobes, but only after a single “chirp” of the audible signal. Figure 1 shows that where audible signals operate along with visible appliances, the audible signals are generally the first means of occupant notification. This is as expected due to the ability for audible signals to penetrate and fill a space. Because visible signaling relies upon a system of distributed point type “line-of-sight” appliances, the coverage volume will almost always be significantly less than audible signaling.
Figure 1 - First Alerted By
Interestingly, even though audible signals did not operate in the Reading Home Depot test, two participants indicated that they were first alerted by audible, not visible signals. Both indicated in their Post-Test Survey that the sound was very faint. It is possible they were alerted by the control panel audible indicator. However, the panel was located in a separate room within an enclosed vestibule and both participants were quite a distance from that room.
Participants were asked if they could see the flash of the strobes reflecting off of the floor, stock or other surface without actually seeing a strobe light directly. The results summarized in Figure 2 show that the Danvers system was the most effective system and the Reading system was the least effective system for indirect signaling.
Figure 2 - Indirect Signaling Effectiveness
The variables and methods for calculating strobe illumination are presented in detail in Section 13. The discussion in the Annex notes that designers differ on whether calculations are done for a simple square or for an overlapping pattern of circular coverage areas. In addition, for each assumed area, calculations can be done for the basic distance from the strobe or corrected for the angle at which the light ray strikes a surface. Calculations for all four possibilities are included for each of the test sites for nominal heights and spacings.
Although there are many other factors involved in the actual effects of the systems (versus a strobe in an imaginary square) the calculations show the Reading system provides the lowest levels of illumination. Based on the industry standard for performance based calculations, the systems for Danvers and Kissimmee were over-designed while the Reading system most closely met the minimum requirements of the code.
In addition to strobe intensity and the resulting level of illumination, the clearance between the top of the stock/storage and the strobe lights affected indirect signaling in both the Reading and Kissimmee stores. In these stores, there were many aisles where strobes were not located directly overhead. This light had to come over the racks/shelves from adjacent lines of strobes. See Section 13.1 for more detailed discussion. In Danvers, in addition to a higher calculated level of illumination, 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 Kissimmee versus Reading permitted greater penetration into aisles that did not have strobes directly over them. Figure 3 shows a typical warehouse store with strobe coverage providing both direct and indirect signaling to the occupants. Figure 4 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 4 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.
Figure 3 - Direct and Indirect Strobe Coverage in Racks
Figure 4 - Actual Strobe Penetration in Racks
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 those spaces, 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, summarized in Figure 5, show that the Kissimmee system was the most effective at direct signaling to occupants.
Figure 5 - Direct Signaling Effectiveness
The better outcome in the Kissimmee Wal*Mart is probably the result of a greater clearance between the top of stock/storage and the strobe lights than either of the two Home depot stores. As a result, occupants could see more of the ceiling and, hence, more strobes from most vantage points. This is discussed in more detail in Section 8. Also, there may still have been locations where strobes or their effects were not viewable – see Blind Spots.
Participants were asked if as they could see more than one strobe light flashing as they moved about, and if so, how many. Figures 6 through 8 show the number of strobes viewed by participants for each test location. Figure 9 sums the data for all three locations.
Figure 6 - Number of Strobes Directly Viewed – Reading
Figure 7 - Number of Strobes Directly Viewed – Danvers
Figure 8 - Number of Strobes Directly Viewed – Kissimmee
Figure 9 - Number of Strobes Directly Viewed – All Stores Combined
The data show that for these test conditions, all participants could see 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.
Despite there generally being three to six strobes directly visible as participants walked around the stores, there still were locations where they did not directly see a strobe or its indirect reflection. Participants were asked if there were there any locations where they could not see a strobe light or its reflection. The results are summarized for all locations in Figure 10.
Figure 10 - Blind Spots
At the Kissimmee location, 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. By most accepted standards, the system was over-designed with respect to strobe intensity for a given height and spacing. However, 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 when they were more than two or three aisles away. This is discussed in more detail in Section 8.
In Reading, the ceiling was an open plan type 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.
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 11 through Figure 13 and combined for all locations in Figure 14.
Figure 11 - Strobe Effectiveness - Reading
Figure 12 - Strobe Effectiveness - Danvers
Figure 13 - Strobe Effectiveness - Kissimmee
Figure 14 - Strobe Effectiveness - All Locations
In general, the systems were effective. However, clearly there were differences that made the Danvers system stand out as the most effective and the Reading system as the least effective system. The Danvers system used higher intensity strobes, on a reduced spacing and located over almost all aisles between racks. In Reading, the strobes locations were designed to be over aisles. Before completion, the rack layout changed resulting in most aisles not having a row of strobes directly overhead. In Kissimmee, most respondents felt the system was effective. Based on strobe intensity, ceiling height and strobe spacing, the system was over-designed. Nevertheless, when superimposed on aisle/stock layout, there were aisles where coverage was minimal or non-existent. This is discussed in more detail in Section 8.
For the Reading test, the three hearing impaired persons gave ratings of 1, 2 and 3 out of 5. In Danvers, both hearing impaired persons rated the system 4 out of 5. In Kissimmee the hearing impaired all rated the system at 3 or higher (3, 3, 4, 5 and 5). Thus, the hearing impaired persons rated the system effectiveness approximately the same as other participants.
Participants were asked if, in their opinion, strobe lights are an effective method for alerting deaf or hearing impaired persons. See Figure 15.
Figure 15 - Universal Effectiveness of Strobe Systems
There was general agreement among participants that strobe lights were an effective means for alerting the deaf and 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.
Although the sample size is too small to draw conclusions, it is interesting to note that in the Reading test, the two persons that felt that strobe light systems were not effective were both hearing impaired (one severe, one mild). The third person with a hearing impairment thought that the systems, in general, were effective.
In Danvers, the two hearing impaired persons thought that strobes were an effective alerting system. In Kissimmee, four of the five hearing impaired persons thought that strobes were an effective alerting system. The fifth hearing impaired person qualified their response by writing “in this case”, rather than making a general evaluation. When asked, they were non-committal with respect to general strobe effectiveness.