Lighting Research Center Lighting Research Center
    Volume 7 Issue 4
June 2003    
ballast - A device required by electric-discharge light sources such as fluorescent or HID lamps to regulate voltage and current supplied to the lamp during start and throughout operation. ballast factor (BF) - The ratio of the light output of a fluorescent lamp or lamps operated on a ballast to the light output of the lamp(s) operated on a standard (reference) ballast. Ballast factor depends on both the ballast and the lamp type; a single ballast can have several ballast factors depending on lamp type. compact fluorescent lamp (CFL) - A family of single-ended fluorescent-discharge light sources with small-diameter [16-millimeter (5/8-inch) or less] tubes. electrodes - The structure that serves as the electric terminals at each end of electric discharge lamps. high-intensity discharge (HID) - An electric lamp that produces light directly from an arc discharge under high pressure. Metal halide, high-pressure sodium, and mercury vapor are types of HID lamps. luminaire - A complete lighting unit consisting of a lamp or lamps and the parts designed to distribute the light, to position and protect the lamp(s), and to connect the lamp(s) to the power supply. (Also referred to as fixture.) load shedding - The practice of turning off electrical devices during peak energy demand hours to reduce building energy use. rated lamp life - The number of hours at which half of a group of product samples fail. The rated life is a median value of life expectancy; any lamp or group of lamps may vary from the published rated life. Rated life is based on standard test conditions. rated light output - The sum of the initial rated lamp lumens of the lamp(s) that were supplied with the luminaire.
What is a ballast?

In a fluorescent lighting system, the ballast regulates the current to the lamps and provides sufficient voltage to start the lamps. Without a ballast to limit its current, a fluorescent lamp connected directly to a high voltage power source would rapidly and uncontrollably increase its current draw. Within a second the lamp would overheat and burn out. During lamp starting, the ballast must briefly supply high voltage to establish an arc between the two lamp electrodes. Once the arc is established, the ballast quickly reduces the voltage and regulates the electric current to produce a steady light output.

Maintaining an optimum electrode temperature is the key to long lamp life. Thus, some ballasts have a separate circuit that provides a low voltage to heat the lamp electrodes during lamp starting and typically during lamp operation (Hammer, 1995).

To achieve full rated light output and rated lamp life from a fluorescent lighting system, a ballast’s output characteristics must precisely match the electrical requirements of the lamps it operates. Traditionally, ballasts are designed to operate a specific number (usually one to four) and type of lamp (such as a four-foot T8 lamp) at a specific voltage (in North America either 120, 277, or 347 volts). Thus, to find a ballast compatible with a particular luminaire (light fixture), lamp type, lamp quantity, and line voltage must all be known. For more on ballast technology, see NLPIP’s Specifier Reports: Electronic Ballasts.


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