OCCUPANCY SENSORS & ELECTROIC FLUORESCENT BALLAST

Switching off fluorescent lamps whenever a room is unoccupied saves energy. Frequent switching of lamps, however, may shorten their operating life. In order to achieve the desired energy savings while maintaining acceptable lamp life, it becomes important to select the ballast type as a function of lamp switching rate.

NEMA recommends that the minimum lighting “on time” be no less than 15 minutes. This allows for energy savings when people are out of the room for extended periods of time, but does not shorten lamp life by cycling lamps every time someone steps out of the room momentarily. 

A product survey performed by the Lighting Research Center found that the vast majority of sensors would permit a minimum “on time” setting of 15 minutes and that many were adjustable to 20 and even 30 minutes. In the event that a given sensor is limited to less than 15 minutes, NEMA recommends setting the sensor to the longest time possible. If lamp life results at the 15-minute setting are unacceptable, then the time should be increased for those sensors with such flexibility. For the complete product survey, see “Specifier Reports—Occupancy Sensors: Motion-Sensing Devices for Lighting Controls,” National Lighting Product Information Program, Vol. 5, No. 1, October 1998.

Each of the three main types of ballasts has their own starting characteristic that can affect lamp life:
•Instant start ballasts are the most efficient and the most popular electronic ballast available today. They are recommended for applications with switching frequencies of less than five cycles per day or where energy savings is considered more important than lamp life. Instant starting can make a ballast very efficient, but it causes the electrodes of the lamp to degrade a little every time the lamp ignites compared with programmed start ballasts.

•Rapid start ballasts are not as efficient as instant start ballasts due to additional filament heating power supplied to the lamp, although this additional filament heating can produce longer lamp life in applications where lamp starting occurs less often than every three hours. Like the instant start ballast, they are recommended for applications with switching frequencies of less than five cycles per day. Rapid starting of lamps causes the electrodes of the lamp to degrade a little every time the lamp lights compared with programmed start ballasts. 

•Programmed start ballasts provide the best lamp ignition and longest lamp life. In a programmed start ballast, electrodes are preheated prior to ignition, resulting in almost no electrode degradation. This allows frequent starts without a significant loss of lamp life. Programmed start ballasts are recommended in applications with frequent starts where extended lamp life is a primary concern.

Recommendations:
•Use the longest practical minimum “ON” time setting for the occupancy sensor and other automatic cycling (15 minutes is recommended).
•Only use ballasts that meet ANSI requirements for lamp ignition.
•Use programmed start ballasts in areas that will result in a high number of switching cycles per day and where lamp life is a primary concern.