Building Operating Management

Fluorescent Lights and Controls Myths

By James Piper, P.E.   Lighting

OTHER PARTS OF THIS ARTICLEPt. 1: This PagePt. 2: Dimming System and Occupancy Sensor Myths
Today's lighting controls offer facility managers an effective means of reducing energy use while maintaining a highly productive and safe visual environment. These systems accomplish those goals simply by providing the right quantity of light where it is needed, when it is needed, and no more. Those who have implemented a comprehensive lighting control program have found that lighting energy use can be reduced on average by 50 percent in existing buildings, and by 35 percent in new construction when compared to traditional wall switches. Those reductions in lighting energy use are particularly significant when one considers that in a typical commercial facility, lighting is responsible for approximately 40 percent of the facility's total electricity use.

With such energy and financial benefits to lighting control systems, why don't all facility managers aggressively pursue all possible lighting control options? While such issues as installation costs and disruptions do contribute to the reluctance to install lighting control systems, much of the blame can be traced to longstanding misconceptions. Some are the result of misunderstandings about how the systems work. Others concern issues that may have been true at one time but that have been reduced or eliminated by advances in lighting components and lighting control technologies.

MYTH #1: A fluorescent light takes more energy to start than to operate

This misconception can be traced back to older generations of fluorescent lamps and ballasts that, when turned on, went through a starting cycle that included flickering for several seconds before reaching full operating brightness. Users came to believe that while these lamps were flickering, they were using high levels of energy — much higher than the lamps would use during normal operation.

It is true that while starting, fluorescent lamps do use more energy than while operating. But even in those older generations lamps, the higher level of energy use lasted for only a few seconds. Today's lamps and ballasts typically reach normal operating currents in less than 0.1 seconds. During the startup time, there is a brief jump in current that is several times greater than the lamp's normal operating current. The duration of this increased current, however, is so short that the energy used is about the equivalent of operating the lamp for five seconds.

The energy required to start a fluorescent lamp, compared to the energy saved by turning the lamp off when it is not required, is insignificant.

MYTH #2: Lighting controls shorten lamp life and so are bad investments

Every time a fluorescent lamp is started, the electrode's emitting surface of the lamp's cathode is slightly eroded. Eventually, when the emission material is sufficiently depleted, the lamp can no longer start and must be replaced. It is this process of electrode erosion that has led to the misconception that frequent cycling of fluorescent lamps will cause rapid aging and early lamp failure. By this line of thinking, to get the full value of the investment in the lamp, facility mangers should minimize the number of times that fluorescent lamps are switched on and off.

While it is true that the lamp's service life does decrease with more frequent cycling, the question is which is greater: the value of the lost lamp life or the energy saved during the periods of time when the lamp is turned off. Determining the answer requires an understanding of how manufacturers' rate lamp life, the impact of more frequent lamp cycling on lamp life and the cost of energy.

Fluorescent lamps have a rated service life that depends on a number of factors, including how many hours they operate each time they are turned on. For example, a lamp with a 20,000-hour-rated service life typically assumes that the lamp will operate for three hours per start. If they burn more than three hours every time they are started, their service life will increase. If the lamps burn less than three hours per start, the service life will decrease. How much of a decrease depends on the particular lamp and ballast combination.

For example, consider the impact that more frequent starts would have on lamp life in a typical office space. The lamps are rated for a 20,000 hour service life based on three hours of operation per start. Assume that the lamps operate uninterrupted for 10 hours per day. Using manufacturer data, the service life of the lamps would be extended to approximately 26,000 hours, or 2,600 work days.

Now assume that the same lamps are turned off once for approximately 10 minutes per hour. Lamp life would be reduced to approximately 13,000 hours, a significant reduction. Converting this to the equivalent number of work-days results in a service life of 1,560 days. Assuming an electricity rate of $0.12 per kWh, the energy savings by switching the lamps off ten minutes per hour would be approximately $12.48 over the life of the lamp. Even factoring in the labor cost to replace the lamps more frequently, the energy savings exceeds the cost of reduced lamp life. And this was for one lamp with a relatively short turn-off period. Consider the savings that can be achieved for large areas where lamp operating time can be reduced by 50 percent.

Actual savings will vary with the number of times they lamp is started, the type of lamp and ballast installed, and the cost of electricity. In the above example, the only electricity cost that was factored into the calculation was the use charge for electricity. In practice, most facilities also pay an electrical demand charge. If lamps are turned off during the time that the facility reaches its peak electrical use, additional savings will be realized.

How often a lamp should be turned off is primarily dependent upon the cost of electricity. As a rule of thumb, if the space is to be unoccupied for more than 10 minutes, it is beneficial to turn the lamp off. Facilities with particularly high electric rates will benefit from turning the lamp off for periods as short as three to five minutes.

The reduced service life that comes from more frequent starts will increase labor costs associated with lamp replacement. One effective way to hold down these labor costs is through the use of a group relamping program. Group relamping programs can reduce the average labor cost of a single lamp replacement by 90 percent or more. The cost of giving up some remaining service life of an operating lamp when compared to labor savings achieved is insignificant.

Continue Reading: Debunking Myths of Lighting Controls

Fluorescent Lights and Controls Myths

Dimming System and Occupancy Sensor Myths

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  posted on 4/11/2011   Article Use Policy