Emergency Lighting

Emergency lighting (Fig. 34-3) provides power for critical lighting systems in the event of a general power failure, the failure of the building electrical system, an in-

Figure 34-3 Emergency lighting units.

terruption of current flow to the lighting unit, or even from the accidental operation of a switch control or circuit disconnect. It is customary for the battery-powered units to be hard-wired into the building's electrical system, so that the battery can be recharged by the building power. Emergency lighting requirements are set by ANSI/NFPA 101, The Life Safety Code. This code, published by the American National Standards Institute (ANSI) and the NFPA, defines the locations within specific types of structures that require emergency lighting, as well as specifying the level and duration of the lighting. The National Electrical Code (NEC), also known as NFPA 70, mandates system arrangements for emergency light and power circuits, including egress and exit lighting. The NEC also discusses power sources and system design. NFPA 90, Health Care Facilities, dictates special emergency light and power arrangements for these occupancies. Requirements set by OSHA are primarily safety oriented and cover exit and egress lighting. Other industry standards are published by IESNA and the Institute of Electrical and Electronics Engineers (IEEE) as ANSI/IEEE Standard 446, Recommended Practice for Emergency and Standby Power Systems. There may also be additional local code requirements.

The general goals of emergency lighting are to avoid distress or panic and to provide lighting for egress from the building. The level of lighting required is related to the level of normal illumination and to the degree of hazard. Exit areas must maintain 50 lux (5 footcandles or fc), and stairs are required to have 35 to 50 lux (3.55 fc) of lighting. Hazardous areas like machine rooms are set at 20 to 50 lux (2-5 fc), and other spaces at 10

lux (1 fc). No point along the path of egress should have less than 1 lux (0.1 fc). Maximum to minimum illuminances along the egress path must not exceed a ratio of 40 to 1. These are low levels, but are sufficient to permit orderly egress once the eyes have adjusted.

Most codes require 50 lux (5 fc) on nonilluminated exit signs, internally illuminated signs, or self-luminous signs with light-emitting diode (LED) displays. Some exit signs are equipped with a battery and controls. Others illuminate the area beneath the sign, which is especially helpful for finding the way to an exit in a smoky room. Some have a flasher and/or audible beeper. Nonelectrical, self-illuminating signs are considered to be part of the emergency lighting system.

Exit lighting is required at all exits, and at any aisles, corridors, passageways, ramps, and lobbies leading to an exit. General exit lighting and exit signs must be lit at all times the building is in use.

Because it takes the eyes up to five minutes to adjust to the drop from normal lighting levels, bright spotlight-type emergency lighting heads must be very carefully arranged to avoid disabling glare and distorting shadows. It is best to provide some lighting at floor level, preferably of a type that aids direction finding, as peo ple may crawl along the floor to stay below the level of smoke. Ceiling mounted emergency lighting sometimes fails to illuminate the floor in smoky areas, or may create a bright, fog-like condition. Some codes mandate adequate egress lighting at baseboard level.

Emergency lighting must remain at the minimum 10-lux (1 fc) level for at least 90 minutes. It can then be lowered to 6 lux (0.6 fc); higher levels may be required if quick evacuation is not possible.

The easiest way to provide emergency lighting is to put some of the existing light fixtures on a separate circuit designed for emergency lighting, one that is connected to a backup power source. You must make sure that if one fixture burns out, it will not leave an area in darkness. Use dual lamp lighting fixtures and fixtures with battery packs, and design overlapping lighting patterns.

Local emergency lighting arrangements include a rechargeable battery and charger and voltage sensing and switching equipment. For fluorescent lights, an electronic ballast is available that operates the lamp at high frequency and usually reduced output. Packaged units are available with integral incandescent lights. Equipment should be maintenance free for up to five years.

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