Published In September 2000
Finding the best ballast/lamp combination is an integral part of providing fluorescent lighting that is both effective and energy efficient. Balancing initial cost with future energy consumption and making sure all areas receive proper illumination can become an involved planning process that includes examining how space is used. The expected life of the fluorescent lamps is the key initial economic factor, and that part of the equation clearly becomes financially significant once a building is fully operational. Increasing lamp life and decreasing maintenance costs associated with changing lamps are objectives that developers and building owners can relate to immediately. New and renovated buildings are being designed with an increasing number of frequently switched applications. Both on/off switching and variable dimming are being specified for offices, conference rooms, restrooms, storage areas, and lobbies. Using ballast technology that can extend the life of a fluorescent lamp over 50 percent in frequently switched applications can be an important tool as architects, engineers, and contractors work to provide superior design efficiency. Linear electronic ballasts are an innovative progression from existing technology brought about by the continuing need to reduce costs and save energy. Starting methods affect bulb life Fluorescent lamps frequently turned on and off have historically burned out much more quickly than identical lamps used in long-burn applications. The two most common methods of starting linear fluorescent lamps are instant start and rapid start, both of which are clearly defined by the American National Standards Institute (ANSI). Instant start ballasts apply a high starting voltage to the lamp, causing the arc (lamp current) to start without preheating the lamp’s filaments. Rapid start ballasts apply both a starting voltage and a low voltage to the filaments before the lamp starts, and then maintain filament voltage throughout operation. Preheat time is that time between the initial application of filament heating voltage and full lamp current ignition. The arc current that flows during preheat is known as “glow current,” and results from the application of significant lamp voltage during filament heating. It is typical of the rapid-start products on the market today. Excessive glow current indicates improper starting and causes emissive material from the filaments to sputter, which creates end blackening and degrades lamp life. The life of a linear fluorescent lamp is affected by the ratio of filament resistance at ignition (hot) to the filament resistance at room temperature (cold). That resistance hot/resistance cold (Rh/Rc) ratio is important because it relates to the proper heating of the lamp before starting. An Rh/Rc ratio above 4.25 indicates that the filaments are above the minimum temperature (700 degrees Kelvin) required for proper thermionic emission. In rapid starting, the typical Rh/Rc ratio is less than 4.0 and indicates insufficient heating of the filaments, which can cause lamp filament degradation. This is critical in frequently switched applications. The filaments in an instant start ballast system are not preheated, so the Rh/Rc ratio does not directly apply. However, filaments heat up from the lamp current after a high voltage strikes the lamp. During this fraction of a second, as the filaments are brought up to temperature, sputtering occurs. Because of the negative impact of glow current and the improper heating of the filaments (low Rh/Rc), lamp life and overall performance is surprisingly similar for both rapid-start ballasts for frequently switched applications and instant-start ballasts. Programmed start easier on lamps The problem has been addressed through the design of a more precise method of starting T8 (and now T5) lamps. This method involves controlling both open circuit voltage and start time. New “programmed” ballasts apply voltage to filaments for a given time, heating the filament while the starting voltage is held near zero. After the filaments have been heated to above 700 degrees Kelvin, starting voltage is applied and the lamp ignites. This starting sequence virtually eliminates glow current and properly heats the filaments above 25 Rh/Rc, which minimizes lamp degradation during starting. In applications where lamps are turned on and off frequently, such as areas using occupancy sensors, the programmed start ballast can increase lamp life by over 50 percent where lights come on and go off frequently (see accompanying chart). Linear electronic ballasts offer an excellent option that merits consideration for initial design, replacement, and retrofits projects. DIMINO is product director of linear electronic ballast products for MagneTek Lighting Products, Nashville, Tenn. He may be reached at (615) 316-5316 or by e-mail at firstname.lastname@example.org.