When the Power Goes Off - EPM Archives

Recent blackouts and storms have quickly and dramatically focused interest in improving occupant safety during power disruptions. Emergency lighting has received much attention, with questions being asked about the adequacy of prevailing codes and standards, as well as the ability of some types of equipment to meet the current requirements for light levels and time.

Current national minimum codes (NFPA 101 Means of Egress Sections 7.8 and 7.9 (2000) require emergency illumination over all the paths of egress, including corridors, stairways, ramps, passageways, and all changes in direction on egress routes for 90 minutes (min) following a loss of normal lighting power.

Initial light levels must not be lower than 1 footcandle (fc), and no point on these paths must be less than 0.1 fc (at the floor). To minimize the perceived dark areas along the egress path, the uniformity ratio (max to min) cannot exceed 40:1. Further, light levels must remain greater than 60% of the initial emergency light level for a 90-minute (min) period. The transfer to emergency equipment must occur automatically within 10 seconds (s) of failure of normal lighting power supply.

Emergency unit equipment, luminaires with emergency ballasts, and internally lighted exit signs must be visually inspected, and operational tests must be performed periodically (every 30 days for 30 s and full 90-min discharge annually) and records kept of inspections and test results. Self-testing and self-diagnostic battery operated emergency lighting equipment are exempt from the 30-s monthly test, but not annual tests, provided visual inspections are done at 30-day intervals.

Local codes may be more stringent and authorities having jurisdiction (AHJ) have the final word, but these codes cover the temporary loss of power or less than 90 min. The codes assume that organizations will provide some type of backup power for their emergency lighting systems and to power their critical operations, so that the battery-powered lighting will only be used for short periods of time or until backup systems are online.

Standby power capabilities can be provided by microturbines, together with an energy storage system, such as an uninterruptible power supply (UPS) or fuel cells. Microturbines and other generators can provide power for extended operating times (>90 min) of lighting systems, even though they are not designed for emergency standby applications that require dual fuel sources and 10-s start up time.

Usually when generators or microturbines provide standby capability, a load management controller senses failure of normal supply power and automatically switches from grid connection to stand-alone mode, restoring power to the facility. When grid power returns, the controller switches back.

Planning backup power for lighting begins by employing more efficient equipment to reduce lighting load. Lighting audits show that many incandescent exit signs have burned-out lamps. The replacement of older internally lighted exit signs represents one of the most cost-effective lighting system upgrades available. Since exit signs burn constantly, 8760 hours annually, investments in new exit lights are usually paid back in less than three years. And the reliability of the emergency lighting system is improved if more signs lighted all the time.

The Underwriters Laboratories Standard 924 controls all aspects of the design and manufacture of exit signs, including the size and stroke of the letters, the luminance (brightness) of the lighted faces, and much more.

A solid-state, electroluminescent exit sign design is available in green only, but consumes no more than 1/4 watt and is rated for 30-year life. There are also photo-luminescent products that require no power, which are charged by normal fluorescent light levels and then "glow-in-the-dark" to light pathway, exit signs, and doorways.

Unit equipment includes the familiar incandescent or halogen "heads" and a battery pack. UL924 requires that the exit sign must supply illumination automatically and maintain light output for a minimum of 90 minutes.

Central battery-type inverter systems must also be able to maintain the designated emergency lighting load for a minimum of 90 min and then must fully recharge within 72 hours (hr).

Emergency ballasts that contain batteries can provide the Life Safety Code and UL required 90 minutes of backup fluorescent lighting on the egress path during power outages. Emergency ballasts with 2- and 4-hr operating times meet codes that require longer time.

When used in an application, these emergency ballasts are installed in selected fluorescent luminaires in a facility and usually do not light all the lamps. Some models are available with self-testing/self-diagnostic capability that automatically performs the monthly and annual testing required by codes and warns users of problems.

Many manufacturing, warehousing, "big box" retail, and gymnasium facilities make use of high-intensity discharge (HID) luminaires. These luminaires usually have magnetic ballasts that cause the lamps to stop functioning when the input voltage sags or is disrupted for more than a few cycles. Then the lamps must cool down before they can re-strike, which can cause the affected systems to be off for up to 20 minutes.

The National Electric Code (NEC) requires a backup light source for HID systems that can provide an immediate source of light during re-strike time. The traditional way is to use "quartz re-strike" in selected luminaires, but this may not work well in practice due to the short life of these halogen sources.

Disruptions to facility operations have led many managers to consider replacing their HID systems with fluorescent systems, including high-ceiling applications. For others, the ability to ride-through minor disturbances may be an important factor to consider when evaluating the new electronic metal halide ballasts. Backup ballasts with batteries that keep metal halide arc-tubes burning for up to 2 minutes while backup generators come on line are another solution. High pressure sodium systems can provide nearly continuous light output by using double arc-tube standby lamps.

Test your understanding

1. The NEC requires a backup light source for HID lighting systems

a. True

b. False

2. To plan for a backup generator,

a. review code requirements and facility needs

b. plan to reduce those lighting loads that will be on the generator

c. both A and B

3. Emergency lighting must

a. provide an average of 1 fc at the floor

b. provide 72 hours of uninterrupted light

c. light all paths of egress for 90 min

4. HID systems can ride through minor power disruptions when they have

a. electronic ballasts

b. battery backup ballasts

c. either A or B

5. UL 924 covers

a. only exit signs

b. only unit equipment

c. all emergency lighting equipment, including inverter systems