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Visual Alarms
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Visual alarm provisions “illuminated”
By Richard Rogers
The following article was adapted from the U.S. Access Board as written in the State of Oregon Building Codes Division January/February 2003 issue of “CODE LINK.”
Visual alarms required
The requirement for an emergency alarm system in new construction is established by other provisions of the Oregon Structural Specialty Code and installed in accordance with Article 10 of the Oregon Uniform Fire Code. OUFC Article 10 references NFPA 72 as the standard for fire alarm systems. OSSC Chapter 11 does not mandate an emergency alarm system; its scoping provision at Section 1108.4.10 simply requires that when emergency warning systems are provided, they shall include both audible and visual alarms that comply with Section 1109.14. Thus the requirement for an alarm system in a facility will trigger the Chapter 11 technical specifications for alarms.
Because it is not always possible to fix the occupancy of a room or space or anticipate its use by a person with a hearing impairment, visual alarms are required in every common use room or space in facilities equipped with an emergency alarm system. This is particularly important in those common use spaces where a person may be alone. OSSC Section 1109.14.2 stipulates that alarm systems required to be accessible shall provide visible signals in restrooms, in other general and common use areas, and in hallways and lobbies. Common use areas also include meeting and conference rooms, classrooms, cafeterias, filing and photocopy rooms, employee break rooms, dressing, examination, and treatment rooms, and similar spaces that are not used solely as employee work areas.
System designers and specifiers must be particularly attentive to signal coverage issues. Where audible alarms are installed in corridors and lobbies to serve adjacent common use rooms, individual visual alarm signal appliances must be installed in those rooms, because the warning provided by a visual signal, unlike that of a bell or other annunciation system, can only be observed within the space in which it is installed. Dressing and fitting rooms, for example, can be easily protected by an audible alarm outside the room or space. However, the customer or patient who has a hearing impairment will not be alerted unless the dressing room he/she is using is protected with a visual alarm within the space. In general, it is not sufficient to install visual signals only at audible alarm locations.
Visual alarms not required
OSSC Chapter 11 does not require that areas used only by employees as work areas be fully accessible. Thus, visual alarms are not required in individual employee offices and workstations. In addition, mechanical, electrical, and telephone closets; janitor’s closets, and similar non-occupiable spaces that are not common use areas or assigned work areas are not required to have visual alarms.
Technical provisions for visual alarms
The technical provisions of OSSC Section 1109.14 include minimum standards for the design and installation of single-station and building-wide visual alarm systems. They are based on research sponsored by the U.S. Access Board and other groups, principally Underwriters Laboratories. To be effective, a visual signal — or its reflection from adjacent walls and ceiling — must be of an intensity that will raise the overall light level sharply, but not so intense as to be unsafe for direct viewing at a specified mounting height. Technical criteria for visual alarm signal appliances are established in OSSC Section 1109.14.2 Visual Alarms.
In research sponsored by the U.S. Access Board, a high-intensity xenon strobe lamp was found to be the most effective in alerting persons with hearing impairments. White light was judged to be the most discernible; colored lamps (particularly red) were not effective even at extreme intensities. Ninety percent of the research subjects were alerted by a 75 candela (cd) signal mounted 50 feet away on the wall directly behind them, where the horizontal output of a strobe lamp is measured at 100 percent of its nominal rating. For this reason, 75 cd is a minimum performance criterion — not a lamp sizing or specifying standard — for all locations within the 50-foot radius of the covered area. Because most strobes are not point sources, light output falls off sharply to the sides. A lamp with a maximum output of 75 cd when measured at 0 degrees will not provide the required increase in illumination at a 45-degree angle. Lamp intensity is given in effective candela, measured in use at the source.
Like a camera flash, the strobe produces a short burst of high-intensity light. The repetition of this pulse at a regular interval is the flash rate. Pulse duration — the interval of the flash between signal build-up and decay — is limited so that the signal is not temporarily blinding. Testing indicated that flash rate cycles between one and three Hertz (flashes per second) successfully alerted subjects with hearing impairments. A 3-Hz rate appeared to be somewhat more effective. Lamps tested at 1/3 Hz were adjudged ineffective. Section 1109.14.2 thus requires flash rates within the 1 to 3-Hz range.
Rates that exceed five flashes per second may be disturbing to persons with photosensitivity, particularly those with certain forms of epilepsy. Information received during the development of these guidelines suggests that multiple unsynchronized visual signals within a single space may produce a composite flash rate that could trigger a photo convulsive response in such persons. For example, two strobes set at 3 Hz in a room could generate a combined flash rate of 6 Hz. Installations that may produce a composite rate in excess of 5 Hz should, therefore, be avoided by decreasing the number of fixtures and raising the intensity of the lamps they contain, by decreasing the flash rate of multiple lamps, or by synchronizing the flash rates of multiple fixtures. This is particularly important in educational occupancies because children are more frequently affected by photosensitivity than are adults.
Mounting provisions were developed from NFPA 72 signal criteria and UL smoke test findings. Strobes — whether projected from a wall or suspended from the ceiling — must be a minimum of six inches below the ceiling plane to avoid smoke obscuration in the event of a fire. To comply with provisions covering protruding objects, alarm devices must be at least 80 inches above the finished floor. To preclude installations that might be outside the field of view in high-ceilinged spaces such as atriums and warehouses, the guidelines require a strobe to be mounted at the lower of the two heights. However, photometric calculations of lamp intensity for mounting heights of 80 inches and of 96 inches show only nominal differences and can be practically considered to be equivalent. A single visual signal meeting Section 1109.14.2 specifications could be expected to serve a large room or length of corridor if optimally located on perimeter walls or suspended below the ceiling so that the signal can spread throughout the space, unobstructed by furnishings, equipment, or room geometry.
In multipurpose facilities where bleacher seating, athletic equipment, backdrops, and other movable elements may at times be deployed, or in warehouses, libraries, convention centers, and other building types where devices would not be visible when installed at specified heights, optimal signal placement may require considerable study and the development of alternative intensity and placement calculations as an equivalent facilitation.
Provisions governing the spacing of visual alarms in hallways and corridors will generally require one fixture every 100 feet. In lengthy corridors, such as in shopping malls and large buildings, it is recommended that appliance spacing be maximized within the limits of the technical provision to minimize the effect of a composite flash rate on persons with photosensitivity. It is further recommended that the placement of visual signals along a corridor alternate between opposing walls to minimize the number of signals in a field of view.
Criteria that affect the design of visual alarm systems
In general, it is recommended that visual alarm lamp intensity be maximized so as to require the minimum number of fixtures. Large, high-ceilinged spaces may best be served by suspended flash tubes of very high intensity. (Lamps up to 1000 candela are available for such applications.) Smaller rooms, with an area that can be circumscribed by a circle 50 feet in radius, can be covered by a single, centrally located visual alarm meeting ADAAG intensity specifications. For very small rooms, such as examination, toilet, and dressing rooms, a single strobe of lesser intensity may well be sufficient as an alternate method.
Alternate methods
The Section 1109.14.2 technical provisions apply to normative conditions. Signal intensity and placement in very small and very large rooms, and in spaces with high ceilings, irregular geometry, dark or non-reflective walls, or very high ambient lighting levels may best be determined by specialized consultants employing photometric calculation for system design rather than by a literal application of Section 1109.14.2 specifications. For these reasons, Section 1101.3 permits alternative designs that achieve substantially equivalent or greater accessibility.
Lamp intensity (like sound) decreases in inverse relation to the square of its distance from the viewer. Thus, by varying lamp intensity and spacing, system designers can tailor an installation to the physical conditions of the space being served. It is impossible to provide specific guidance for the design of non-standard installations based upon the photometric calculations necessary to demonstrate equivalent facilitation. Experienced electrical engineers or fire alarm consultants under performance specifications for coverage and illumination levels derived from the technical provisions of OSSC Section 1109.14.2, NFPA 72, and ambient conditions in the space should generally design such applications. For example, a 75 cd strobe at 50 feet raises the ambient light by 0.03 at 0 degrees in the horizontal plane. Equivalent design configurations should, therefore, result in approximately the same increase at all positions within the covered space.
Given concerns for economy (lower-candela lamps are less expensive to purchase and connect) and lamp standardization within a building (lower-candela lamps are more available and simplify inventorying), specifiers may be motivated to standardize on a minimum- candela fixture, achieving coverage in large rooms by close spacing of low-intensity lamps. This practice is strongly discouraged by the Building Codes Division as well as the U.S. Access Board. Where a single lamp can provide the necessary intensity and coverage, multiple lamps should not be installed because of their potential effect on persons with photosensitivity.
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