The basics of emergency illumination
The emergency egress illumination power sources have two distinct categories: battery-powered sources and an emergency generator.
Battery-powered sources must comply with NFPA 111. All of the battery-powered systems must comply with the UL 924 standard, which is consistent with NFPA codes and IBC.
The most common battery-powered lighting source is a self-contained emergency lighting unit, which incorporates lamps in combination with a battery source and charger within a single enclosure. These units are sometimes referred to as either “bug-eye” or “frog-eye” units within the trades. Figure 1 depicts a typical self-contained emergency lighting unit. The units are generally powered by sealed, maintenance-free, lead acid batteries. These batteries have proven to be highly reliable and under most conditions need replacement at 7-year intervals. The units are circuited from an unswitched circuit, which supplies the local general lighting and turns on when the voltage serving the local general lighting drops to 80% of nominal. Upon return of normal power, the units will remain on for a minimum of 15 minutes.
Functional testing of these units can be accomplished via multiple methods. They include: an integral test switch; remote infrared handheld device, which one simply aims at the unit; and a factory-installed integral electronic device that automatically initiates code required tests. The automatic feature must produce an audible alarm with flashing LED if a test failure occurs. One can presume that if the unit is not in alarm and the device is UL listed for self-testing, then the testing requirement is satisfied and sufficient for the AHJ. Witnessing the actual test is not required along with the documentation. The critical design concern for bug-eye placement is to maintain a minimum of 1.0 fc along the entire length and width of the designated pathway of egress.
Some of these self-contained units have sufficient power to accommodate exit lights and remote lamp heads, which can be located adjacent to legally required exterior pathways to provide required emergency illumination levels. Exit signs must comply with UL 924 for luminance and with the AHJ for sign color and lettering size. The requirements vary among jurisdictions, so it is prudent to check the specifics before specifying exit signs. The two standard types of internally illuminated exit signs either internally house a powered source of light or are self-luminous signs. The most common type internally houses a source of illumination, either LED or fluorescent lamps. Both of these lighting sources use sealed, maintenance-free nickel-cadmium batteries. Exit signs are unswitched and continuously illuminated. They will revert to their battery power when the normal power drops below 80% of rated voltage. All of the testing requirements are the same as self-contained emergency lighting units. Fluorescent exit signs are required to have two lamps by code, in case one fails. The fluorescent lamps have an expected rated lamp life of 20,000 hours. The LED sources use less energy than the fluorescent lamps and have an expected lamp life of 50,000 hours.
Self-luminous exit signs are either self-powered or energy-storage type. The self-powered luminous exit signs contain tritium gas and provide continuous luminance for a minimum of 10 years. The stored energy type of luminous exit signs uses a strontium oxide aluminate compound to store ambient light, releasing the stored energy when the ambient source is turned off. The estimated useful life is in excess of 20 years. Both of these sources are listed for use in hazardous locations because they do not require external power sources and pose no threat of ignition in a hazardous environment.
Should the aesthetics of a given space preclude the use of the “bug-eye” type of emergency egress lighting, the engineer can incorporate an emergency fluorescent power unit within the area lighting fixtures. The packaged unit is self-contained with a built-in battery, battery charger, and inverter. (See Figure 2 for a typical self-contained unit.) It can power a single fluorescent lamp within the area lighting fixture continuously at a rated initial output of 1100 lumens. The packaged unit must provide at least 60% lumen output after 1.5 hours. The unit must be connected to an unswitched circuit, which serves the area lighting fixture. The unswitched circuit is permitted to run in a common conduit with the normal power branch circuitry. All of the periodic functional testing requirements as outlined in NFPA 101 must be accommodated by each packaged unit. The self-contained units can be remote mounted from the lighting fixture served. These units are capable of illuminating several lamps contained in multiple light fixtures and have power capacities up to 250 W.
The most comprehensive battery-powered emergency egress system incorporates a lighting inverter system, which is UL 924 listed and can meet the 90-minute requirement. The larger scale inverters have built-in panelboards and serve the emergency egress lighting directly. Achieving adequate lighting levels is fairly straightforward (see Figure 3). Since inverters are used exclusively to serve emergency lighting, the circuitry of emergency lighting is segregated from normal power sources. The inverters can range up to 130 kVa in size. Because one typically is illuminating only about 0.15 W for the entire emergency egress lighting system, most inverters are 30 to 60 kVa. The batteries’ sizes are proportional to the kVa rating of the inverter.
Engineers should use caution when sizing the inverter because the amount of electrolyte contained in the batteries might require continuous ventilation of the space, since it exceeds 50 gal for unsprinklered or exceeds 100 gal for sprinklered buildings (NFPA 1: Fire Code Chapter 52). The purpose of this requirement is the possibility of excess hydrogen being generated during the recharging cycle. The environmental and location requirements are outlined in NFPA 111. The main requirement is that the inverter system must be installed in a room separate from the normal power service entrance over 1000 amp and greater than 150 V to ground. The room must be dedicated for the inverter; n storage is permitted within the dedicated space. Ultimately, the inverter location and space requirements must be approved by the AHJ.
A typical one-line is included for an inverter system referred to an emergency power supply system (EPSS) in NFPA 111 Appendix B (see Figure 4).
The emergency lighting inverter, when used in conjunction with selected luminaries, will provide more than adequate egress illumination. Figure 5 represents a computer-generated output of the expected photometric results based on the proper spacing of the designated emergency lighting fixtures for a medical college.