Facility owners view lighting maintenance as an absolutely necessary evil-i.e. expense-to help sustain the full economic value of their system. Facility managers and lighting-maintenance contractors sustain the full value of their time through lighting maintenance.
As soon as a new lighting system is energized, the light level starts to gradually decrease because of various aging characteristics. For that reason, a lighting design usually provides, initially, an illuminance (a footcandle level) above the minimum-specified level to compensate for the light reduction over time.
Light depreciation is defined by the term light loss factor (LLF). Expressed as a percentage, the LLF considers all the conditions contributing to the reduction of light over time. In addition to the gradual and constant reduction in a lamp's lumen output, a percentage of gradual light reduction also comes from the dirt buildup on the reflecting surfaces of a luminaire and on the surfaces of a room. The LLF should always be included in any calculations to determine the lighting requirements in a space.
Maintenance reduces lighting costs The initial foot-candle level of a lighting system decreases over time because of depreciation. For that reason, a well-thought-out program of regular maintenance can greatly reduce these light losses and pay for itself by saving on energy costs.
In a new installation, rather than having excessive initial illumination to allow for a 0.70 LLF, regular washing and group relamping is recommended. Such an approach changes the LLF to 0.95 (a 5% loss), thereby permitting the lighting system to use 25% fewer lamps and luminaires. This reduces electric energy use by 25%. Additional savings come from reduced air-conditioning loads and smaller electrical system components, such as transformers, panelboards, and feeder conductors.
The key to getting the greatest benefit from group relamping is to select the best time interval between relamping. While it is impossible to predict when each lamp in a lighting system will fail, a certain number of lamps will fail in a predictable manner. This is shown in the lamp's morality curve supplied by the lamp manufacturer.
Thus, group relamping follows the lamp's morality curve. Two systems can then be used to apply the morality curve.
System 1. Group relamping with no interim replacement of lamps is particularly suitable for large ceiling areas where the failure of a few lamps does not affect the lighting level and aesthetics significantly. The individual lamp burnouts are ignored until a predetermined number of lamps fail, indicating that a group relamping is due. This system is suitable to a fluorescent source and the group replacement should not exceed 70% of average lamp life to make the best use of lumen depreciation.
System 2. A more popular and aesthetically pleasing way is to use group relamping along with interim lamp replacements. Right after a group relamping, at 80% of average life, 20% of the best remaining lamps are stored as replacements for the early burn-out lamps until the next planned relamping.
The cleaning of fixtures is often scheduled at shorter intervals than relamping, and that interval depends on several factors, including the general cleanliness of the area, the fixture design, labor costs, lamp costs and energy costs. For air-conditioned offices, cleaning every year or two years may be sufficient. Non-air conditioned offices or schools may require cleaning at least once a year.
Each fixture type has its own luminaire dirt depreciation factor, such as a recessed troffer with a lens, a recessed troffer with a louver, a suspended totally indirect fixture, etc. Thus, the design of the fixture determines the amount of dust or dirt that is collected and retained on the fixture. The dirt depreciation factor of each fixture type can be derived from the IES handbook or other industry publications.
Because a fixture is made of various materials, you should know the best way of keeping them clean. The following applies to most luminaire finishes:
Plastics-Use destaticizer. Do not wipe dry after applying rinse solution.
Glass-Use nonabrasive cleaners.
Synthetic enamel-Use detergent. Do not use alcohol or abrasive cleaners.
Porcelain-Use nonabrasive cleaners.
Aluminum-Use soap and cleaners, and then rinse with clear water. Do not use strong alkaline cleaners or acid solution.
Designing and then maintaining any lighting system involves selecting lamps that offer the most light and longest life. However, often times, a tradeoff between maximum light output and average rated life has to be made. Recent product developments include a premium line of T8 lamps with a rated life of 30,000 hours at 12 hours/start and enhanced color rendering. Philips Lighting offers another line of T8 lamps with 95% lumen maintenance and 86CRI, along with a 24,000-hour rated life.
Anyone concerned with the proper maintenance of a lighting system should be familiar with lamp technology. Let's study some of these aspects. The measure of the conversion efficiency from electric energy to light energy is called the lamp's luminous efficacy, expressed in lumens emitted per watt (LPW) of power consumed. For incandescent source it is from 11 to 22 LPW. For fluorescent lamps, it is from 45 to 110 LPW. For the HID sources, it is from 40 to 127 LPW. If you want a benchmark, the theoretical maximum efficacy for a light source is about 683 lumens per watt.
Incandescent lamps The percentage of input energy emitted as light by an incandescent lamp is relatively low. However, these lamps offers a number of features for specific applications. These features, or attributes, include ease of light beam control, small size and the ability to dim the lamp economically.
The rated wattage of an incandescent lamp is equal to its rated voltage times the current that flows through the filament. Incandescent lamps are designed to operate on a specificvoltage, for example, a 120 V rating. The higher the wattage rating, the higher the current drawn by the lamp, and as the wattage of an incandescent lamp goes up, the relative efficacy of the lamp also increases.
Understandably, an incandescent lamp has a relatively short operating life. General service lamps have a rated life of about 1000 hours. However, the tungsten halogen incandescent lamp type-with four, five and six times that rated life- are being widely specified.
The light output of an incandescent lamp is based on operating the lamp at its rated voltage. In many electrical systems, the actual voltage will vary from the rated voltage. This can be due to voltage drops within the power system and variations in the voltages supplied by the electric utility. Small deviations from the rated lamp voltage cause about 3% decrease in lumen output for every 1% decrease in voltage. However, with a 5% decrease in input voltage to the socket, the expected life of the incandescent lamp doubles. A voltage reduction is incorporated into the lighting design by inserting a boost/buck transformer in the lighting branch circuit.
So consider a planned voltage reduction at the socket of an incandescent to extend the time intervals between lamp changes.
As the operating hours of any incandescent lamp increase, the lamp filament gradually deteriorates due to the evaporation of the tungsten. The tungsten is then deposited on the inner surface of the bulb, causing a noticeable blackening of the lamp, especially near the base, if the lamp is operated in base up position. Output decreases over time because of deterioration of the filament and because of absorption of light by black tungsten deposits.
A special family of incandescent lamps greatly reduces the deterioration of the lamp filament and the darkening of the bulb. Known as tungsten-halogen (TH) lamps, they use halogen elements, such as bromine and iodine, to minimize bulb blackening by creating a cleaning cycle within the lamp. Because quartz glass is generally used to enclose the filament in these lamps, they are also called quartz lamps. Because these lamps operate at high temperatures, they must be mounted in a well-constructed fixture, having suitable temperature-rated sockets.
General Electric Lighting recently its HIR XL extra long life (6000-hour) halogen PAR lamp, available in 45-, 55- and 90-W spot and flood versions. The high efficiency of the IR lamp is achieved through a process involving thermal recovery. Capturing the infrared heat within the lamp capsule reduces the energy input required to maintain the filament at its optimum operating temperature and, therefore, increases its operating efficiency.
Electric discharge lamps The family of arc-discharge lamps includes fluorescent and high-intensity-discharge (HID) light sources. Discharge lamps operate in conjunction with a ballast, which provides the necessary open circuit voltage for starting the lamp, and also provides current regulation (ampere flow) for proper operation of the lamp.
Fluorescent lamp concerns. The rapid-start (RS) and high output (HO) family of fluorescent lamps (along with the preheat type that use a starter device) all operate in a similar way. Generally called rapid-start lamps, they rely on the proper heating of cathodes to start and operate.
Two other fluorescent lamp types, instant-start and slimline lamps, don't require the heating of cathodes, nor do they use starting aids. The open circuit voltage of their ballast is about three times the normal lamp operating supply voltage, and they will start and operate even when one cathode is completely depleted, which would be the end of normal life. At that time, they usually show spiraling along the tube and occasional orange-colored flashes. Any instant-start or slimline lamp that shows these conditions should be replaced promptly to avoid ballast damage caused by rectification and overheating.
Many maintenance projects today involve the changeout of aging ballasts to gain higher lighting system efficiency. Today's fluorescentballasts are available in three levels of performance, or grades.
Energy-efficient magnetic ballasts use components and high quality materials that reduce inherent internal losses, compared to pre-1994 magnetic ballasts.
Hybrid magnetic ballasts cut off power to the lamp cathode-heaters once turned on.
Electronic ballasts (EB) supply power to the lamps at 20,000 Hz, rather than the standard 60 Hz frequency, to more efficiently convert power into light. When selecting an electronic ballast, consider its crest factor. Crest factor is a numerical factor relating to the input power to an arc discharge lamp that has a direct effect on its useful life. In a sinusoidal current or voltage waveform, the crest factor is the ratio of the peak value to the root mean square (RMS) value. In other words, it is ratio of the peak to the average. In a pure sinusoidal waveform, the crest factor is 1.41. If the waveform is distorted, the ration of peak value to rms value is higher than 1.41 and the current or voltage waveform is non-sinusoidal. Lumen maintenance curves from lamp manufacturers are based on operating with a sine wave of lamp current having a crest factor of about 1.41. Any value higher than 1.41 causes the emission material coating on the cathodes to be depleted at a faster rate than normal. As a result, lumen output and the useful life of the lamp are reduced.
Electronic ballasts are available with either an instant-start or a rapid-start circuit, which presents a specific maintenance issue. Instant-start ballasts apply a higher voltage (more than 400 V) to "jump start" the lamp, causing it to light immediately. Rapid-start ballasts initiate the arc with a lower starting voltage (200 V through 300 V), causing the lamp to achieve full output within one second. Rapid-start lamps also apply a low voltage of 3.5 V to the electrodes during startup and during operation, warming the electrodes and increasing active power by 2 W through 4 W per lamp versus instant-start operation.
MagneTek recently introduced a line of AccuStart Programmed Start electronic ballasts designed for frequently switched applications or as a rapid-start alternate to the instant-start ballast design. Providing "soft-start" operation, this ballast design can actually increase lamp life more than 50% in applications where the fluorescent lights are turned on and off frequently, such as areas using occupancy sensors.
Consider compact fluorescent lamps as maintenance-friendly substitutes in incandescent downlight fixtures, because lamp-changing intervals can be stretched up to five times in many cases. Designed as direct replacement units, some CF models have the familiar incandescent shape-a 20 W version produces 90% of the lumens of a 75 W Softwhite incandescent lamp.
Another direct replacement lamp for an incandescent reflector lamp is the Genura from General Electric. This 23-W lamp has a rated life of 15,000 hours and provides 1100 initial lumens. It can replace a 65-W R30 lamp with a rated life 2000 hours that provides only 755 initial lumens. Housed near the base of the lamp, the electronics convert low frequency, 120-V power into a high frequency voltage that produces electron-ion plasma via an induction coil.
Great advances have been made in medium-wattage, quad type, compact fluorescent lamps. In one case, a 28-W, dimmable (down to 20% of rated lumens) quad lamp is a direct replacement for a 100-W incandescent lamp. This lamp operates on electronic photocells and times and is rated for use in recessed fixtures.
HID lamp characteristics The electric arc of HID lamps is much shorter and has a much higher photometric brightness than the arc of a fluorescent lamp. In addition, the HID lamps are available in numerous wattage ratings to satisfy a wide variety of applications.
In recent years, the metal halide (MH) lamp has increased its popularity, because it offers superior color rendition, compared to the high-pressure-sodium lamp. When installed in a vertical position, a MH lamp operates best. When it burns horizontally, the arc tends tobow upward. This, in turn, slightly reduces the vapor pressure in the arc. The net effect is a reduction in the initial lumen output and also poorer lumen maintenance throughout life. Lamp manufacturers offer a line of position-specific MH lamps with a bowed arc tube designed to operate horizontally.
Note: when relamping a fixture, never replace a burned-out position-specific MH lamp with a universal burning MH lamp because the life and output of the universal burning lamp will be much less than the position-specific MH lamp.
A shrouded arc tube MH lamp is best for simplifying lamp changeout and reducing maintenance costs in open bottom fixtures. The shrouded arc tube MH lamp has a glass cylinder surrounding the arc tube, which, in case of rupture, block any glass fragments from shattering the outer glass bulb.
The high-pressure-sodium (HPS) lamp, which is available in ratings from 35 W to 1000 W, has an efficacy ranging from 60 to 127 LPW. In many cases, the HPS provides an average rated life in excess of 24,000 hours, making it an ideal candidate when weighing lamp replacement costs and general maintenance costs. Usually, an HPS lamp indicates that it has reached its end of life by turning off. However, when the arc tube cools down some minutes later, the ballast/ignitor automatically restarts the lamp. If this on- and off-cycling continues, the ballast and other components could be damaged. Therefore, the fixture should be de-energized and the lamp removed as soon as possible. Both GE and Osram Sylvania offer a version of the HPS lamp that shuts down the lamp the first time this "drop out" occurs. Philips Lighting offers its Prompter technology. At the end of life, the lamp's color changes to bluish-white, providing a visual clue that it is time to remove the lamp.