Understanding the characteristics of fluorescent and HID lamps makes it easier to recognize problems and simplifies corrective maintenance.
Lighting maintenance involves a series of required tasks to sustain the full economic value of a lighting system. Since lamps deteriorate with time, they must be replaced periodically, at the end of their useful life. Also, system components must be replaced when they fail. In addition, fixture enclosures should be cleaned to prevent needless loss of useful light output.
Symptoms and troubleshooting of lighting system problems
All discharge lighting systems offer a visual (and sometimes audible) clue when an incipient problem or a total failure exists. Usually, a problem affects only a few fixtures in a total lighting layout, so a practical technique is to observe which fixtures don't appear to be normal.
The four main defective states in the lamp's appearance are as follows:
* On and off cycling;
* Extra bright light output; and
* Low light output.
After the abnormal fixtures are identified, the next step is to determine which component (lamp, ballast, fixture, or electrical supply) is the cause. A convenient procedure is to select a normal and abnormal fixture and, with the electrical power off, interchange the lamps. During the interchange of lamps, inspect the fixtures and lamps, looking for a difference in lamp color, any broken lamp elements, burned or distorted bases, and defects in the fixture. If there are no major differences in appearance between the two lamps, the lighting system should be energized and the units observed after they have stabilized.
If the abnormal appearance has shifted to the system or fixture that originally appeared normal, the problem is in the lamp and not the ballast, fixture, or system power supply.
Generally, your troubleshooting task will not be that simple, since more than one factor frequently contributes to component failure. For example, the premature failure of an HID lamp may be caused by an incorrect tap on its ballast, in which case, the wiring must be corrected before the fixture is relamped.
Fluorescent lamps. The family of fluorescent lamps can be divided into the cathode Preheat type and those that require no cathode heating or starters. Rapid-start (RS), high output (HO), and 1500mA T12 lamps (along with the preheat type requiring use of a starter device) all have similar characteristics. They depend on the proper heating of cathodes to start and operate properly, and most of these lamp types can be classified under the heading of rapid-start lamps.
Instant-start and slimline lamps are two types that do not require the heating of cathodes, nor do they use starting aids. The ballast open circuit voltage of these lamps is about three times the normal lamp operating supply voltage, and these lamps will start and operate even when one cathode is completely deactivated (which would be the end of normal life). At that time, they usually show spiraling along the tube and occasional orange-colored flashes. Both rapid-start and instant-start types are shown in Fig. 1 (on page 65).
When a rapid-start lamp circuit is energized, a low-voltage transformer in the ballasts provides the cathode heating; at the same time, a higher voltage (called the open-circuit voltage) is applied across the lamp, or lamps, to begin arc conduction. Taking both voltage measurements is a good indication of proper ballast operation. Fig. 2 shows how to make voltage measurements on a two-lamp, series-type, rapid-start magnetic ballast. If heating is provided at only one cathode, either the lamp will fail to start or it will be slow in starting. Presence of heating voltage at only one cathode (or at neither) can be caused by any one of the following: Improper contact in the holder, a broken holder, an open contact in a base pin, a broken base pin, an open cathode, shorted leads in the base, corrosion or foreign material on base pins or holder contacts, a defective ballast, or improper wiring.
Heavy premature end-darkening (usually at only one end) is a good indication that one cathode is not being properly heated and, therefore, the heater circuit is incomplete. This is not the same as gray or brownish bands that occur about 2-in. from the lamp base, with the edge of each band on the side nearer to the base being sharper. This is caused by the cathode coating wearing out in use. While such bands may detract from the appearance of the lamp, moderately dark bands have no significance in regard to either the life or performance of the lamp.
It's very important that you replace failed lamps promptly to avoid ballast damage caused by rectification and overheating. If a good lamp is known to have burned at low brightness for some time, or shows indication of being near the end of life, it should also be replaced since its life may have been significantly reduced because of cathode damage.
The lack of cathode heating voltage can result from a poor connection either between the lamp pins and the lampholder contacts, or between the ballast leads and the lampholder terminals. Poor contact at the lamp pins can be caused by improper lampholder spacing in the lengthwise direction, with the lamp end being held either too tightly or too loosely. For the commonly used 3/8-in.-thick twist-type lampholders, the back-to-back spacing of the lampholders should be 48-in., plus or minus 1/32 in.
If the spacing is too tight, the weight of the lamps will ride on the face of the base instead of the pins, and occasionally a pin will slide behind the clip instead of in front of it when the lamp is inserted into the lampholders. If the lampholders are too far apart, the lamp pins will ride on the edge of the lampholder rather than making contact along the full width of the metal piece.
Sometimes, the end plates and other supporting devices needed to keep proper spacing are removed when fixtures are installed in a row. This looseness at fixture ends allows the lampholders to bend away from the center of the fixture.
When installing lamps with exposed bipins, the positioning guide (an embossed mark on the metal end cap) should be just opposite the entrance slot of the lampholder when the tube is inserted. The tube is then turned 90 degrees. For HO and 1500mA lamps, the lamp boss, or cover, of the recessed double-contact ends should not be visible when the lamps are properly installed.
The 6- and 8-ft instant start and high-output lamps, which have their own specific maintenance considerations, will be discussed briefly. With instant-start lamps, the two-lamp ballast circuit can be either lead-lag or series sequence design. A lead-lag ballast operates lamps in a parallel circuit, meaning that if one lamp fails, the other should continue to operate properly. A series sequence ballast, on the other hand, operates lamps in series, meaning that if one lamp fails the other will fail or glow dimly.
Inoperative instant-start lamps should be replaced as soon as possible. In a two-lamp series magnetic circuit, when one lamp fails, the second lamp may continue operation at a lower brightness. This condition will reduce the second lamp's life and also cause a higher current to flow in the ballast. The increased current will cause a rise in ballast temperature, reducing its life and possibly initiating a smoke condition.
Flickering instant-start lamps that show heavy end blackening should be replaced, even if the lamps are lighted. This condition, known as lamp rectification, will cause a reduction in ballast life if it continues.
Fluorescent ballasts. A ballast that is thought to be defective should be replaced with one known to be operative. It's difficult to measure output ballast voltages in the luminaire because the primary circuit of the ballast is automatically disconnected when an instant-start lamp is removed. If a replacement ballast is wired to a fixture, make sure all leads have a good electrical connection, especially if the lampholders have "pressure lock" or "quick-wire" terminals rather than screw terminals.
If lamps in a fixture fail to light, improper wiring could also be the cause. Sometimes leads without the insulation removed are found. In some cases, one slot is used for two wires and, although there are two spring dips inside the socket, both wires are set under one.
Basically, it's good practice to examine the wiring of the ballast leads to make sure that it matches the diagram on the ballast label. And at the same time, make a visual inspection of the ballast casing. Leaking compound (except for a small amount at the lead holes, which is normal), cracking or brittle insulation, and discoloration on the can are all indications of ballast approaching, or having reached, end of life.
Even though a ballast starts a lamp, this does not necessarily mean that the ballast is functioning properly; often, many successive lamps appear to have short life in a given fixture before it's realized that the ballast is the problem.
Hybrid magnetic ballasts were developed as a bridge technology between magnetic and fully solid-state electronic ballasts for 4-ft lamps, providing energy savings at a lower initial cost than fully electronic ballasts. These magnetic ballasts have components that cut off power to lamp cathode heaters after the lamp has ignited. This results in an additional 2W saving per standard lamp. It's very important that maintenance personnel be aware of their use.
The electronic ballast provides a completely new dimension in performance compared with the electromagnetic ballast, since it has only 6W to 8W heat loss, while the old nonenergy saving electromagnetic ballast (outlawed under federal law) has 20W of heat loss.
Electronic ballasts can be wired from the ballast case to the lamp sockets in either rapid-start (they heat the electrodes or filaments at either end of the lamp as previously mentioned) or instant start configurations. You should be aware of the difference in performance between the two types. If instant start electronic ballasts are used, the life of a rapid-start lamp is derated from 20,000 hrs to 15,000 hrs, at three hrs per start.
Electronic ballasts are also available in various designs (called ballast factor) to provide greater- or less-than-normal light output. To supply greater than normal light output, some electronic ballasts simply drive the lamp at the normal rate of current and the lamp phosphors achieve higher activation, thus delivering more lumens because of the high frequency (20,000 Hz) operation. This concept does not adversely affect lamp life. However, some electronic high-output ballasts do substantially overdrive the lamp to achieve more lumens; this performance characteristic may void the lamp warranty.
HID lamps. The electric arc of an HID lamp is much shorter and more intense in photometric brightness than a fluorescent arc, and an HID lamp often is higher in wattage rating. This type of lamp has a specific warm-up time when first energized and may require a specific operating position - base-up, base-down, or base-horizontal.
Because the precise size of the arc tube and mixture of materials in the tube differ for each HID lamp type, each has its own operating and end-of-life characteristics, which relate to maintenance/troubleshooting needs. The mercury-vapor (MV) lamp has the lowest efficacy of any HID source; most of the MV lamps in the 100W to 1000W ratings have an average rated life of 24,000+ hrs. Because of this relatively long life, coupled with a slow reduction in lumen output, these lamps should be replaced well before reaching their average rated life. Normal end-of-life (EOL) is a non-start condition or very low light output caused by blackening of the art tube from electrode deterioration.
The metal-halide (M-H) lamp will drop in lumen output from 7% to 20% during the first 100 hrs of operation, depending upon lamp wattage. Operating conditions, such as lamp burning position and normal variations in supply voltage or ballast characteristics, can affect lamp color and light output. Using a universal burning M-H lamp instead of the original position- specific M-H lamp, when relamping a fixture, may seem like a good economic decision, because it allows stocking of fewer lamp types. However, this attempt at saving money can be short-lived because an M-H lamp designed for a particular operational position can provide 10 % to 15 % more light and last as much as 60% longer than the equivalent universal position lamps. Normally at its EOL, an M-H lamp won't start because the mix of materials in the arc tube changes, along with the electrical characteristics, so the ballast can no longer sustain an arc.
Both the 1000W and 400W lamps, when burned vertically, have a passive EOL. The 1000W and 400W lamps and all other wattages when burned in other positions can fail in a way that causes the outer bulb to shatter. Therefore, these lamps should be installed in an enclosed fixture that will contain the lamp glass should it shatter. A special shrouded arc tube line of M-H lamps has a glass cylinder surrounding the arc tube so that, should the arc tube rupture, its resulting glass fragments will be blocked from hitting the outer glass bulb and causing it to shatter. This type can be used in an open fixture, where increased fixture efficiency and lower maintenance costs are important (a vertically mounted lamp can be removed from the fixture using a lamp-changing pole). A safety line of M-H lamps are made in such a way that if the outer bulb is broken or shattered, power to the lamp is interrupted. However, the lamp's ability to self-extinguish does not protect it against the danger of breakage itself.
An indication of the high-pressure-sodium (HPS) lamp's normal EOL is on/off cycling, since the aging lamp requires a higher voltage for operation than the ballast can supply. This cycling sequence may be normal, but it's not desirable; the cycling can damage or destroy the starting circuit and/or ballast. This is one of the most serious maintenance problems in an HPS lighting system.
A failed lamp should be replaced immediately as this situation can also cause starting circuit damage.
In addition, an HPS lamp should not be removed from a lamp socket for more than a short period of time, unless the circuit is deenergized. The starting circuit portion of the ballast can be damaged, because the starting circuit will operate continuously with the lamp removed.
Some recent developments in HPS lamp technology are aimed at eliminating the end-of-life cycling, especially in highway and street lighting.
One product has a positive end-of-life indicator, which allows a maintenance crew to identify and replace lamps near the end of their service life. Instead of the usual on-again, off-again cycling, this version of the HPS lamp goes out without any cycling. This lamp version is also suitable for parking lot and garage lighting.
Other improvements in the HPS lamp should be investigated, since lamp makers continue to offer performance improvements. The increased use of electronic ballasts should also improve HPS lamp maintenance in the future.
The low-pressure-sodium (LPS) lamp has a U-shaped arc tube and a two-pin bayonet base. With the highest efficacy of all HID light sources, its output is a monochromatic yellow, so that all colors appear as shades of gray, brown or black, except for yellow objects. During its burning hours, the lamp's wattage increases. For example, a 180W LPS lamp consumes 247W at 20,000 hrs; thus, any wattage or ampere reading taken on the branch circuit will reflect this change of status condition.
HID ballasts. During operation, an HID ballast produces considerable heat and usually receives some heat from the lamp it serves. For that reason, its insulation material can break down and cause a short in the core/coil windings. Commonly, heat-caused deterioration will result in ballast winding or capacitor failure, either open or shorted.
An HID ballast can be checked in a way similar to a fluorescent ballast using a multimeter. You should take specific safety precautions when working on an HPS ballast, since it has a high-voltage starting circuit to initiate arc conduction. This starter circuit should be disconnected, since its high voltage starting pulse can damage commonly used multimeters. The voltage on 50W through 400W lamps is at least 2500V; for the 1000W lamp, it's at least 3000V.
An open-circuit voltage and short-circuit current measurement can be made on an energized HID ballast. With the lamp removed, a true rms voltmeter is used to measure the voltage at the socket. The short circuit current test is shown in Fig 4. The ballast capacitor can be tested with a suitable analog-type ohmmeter set at a very high resistance scale. The branch circuit power should first be turned off, and the capacitor disconnected from the circuit. A screwdriver or other piece of metal placed across the terminals will discharge the capacitor. Meter test leads then can be attached or clamped to each terminal. If the ohmmeter measures a very high resistance, the capacitor is open and should be replaced. If the reading is zero or a very low resistance, the capacitor is shorted and should be replaced. If the reading is zero or a very low resistance initially and the resistance slowly increases, the capacitor is good.
Also, you should check the voltage at the point of connection to the electrical distribution system to ensure proper conditions. Operation in an over-voltage state increases wattage consumption and reduces the life of the ballast and lamp.
RELATED ARTICLE: DISCHARGE LAMP FUNDAMENTALS
The family of arc discharge lamps includes fluorescent and high-intensity-discharge (HID) light sources. Light is produced in these lamps by an electric arc struck between two main electrodes or cathodes, which usually are located at opposite ends of the lamp.
With few exceptions, discharge lamps must operate with a ballast specifically designed to provide the proper starting and operating voltage for the lamp. A ballast changes the electrical characteristics of the power supply to those needed by the specific lamp in order for it to start reliably. Specifically, a ballast provides the open-circuit voltage, regulates the current (ampere flow) to the lamp, and maintains the necessary operating voltage. Thus, a discharge lamp and ballast must be precisely matched. They also must be connected together properly and operated within specific limits. If any of these conditions are not met, the light source will not work properly.
RELATED ARTICLE: TERMS TO KNOW
Ballast Factor (BF): An indication of the relative light output from a given ballast and lamp combination. Ballast factor equals commercial ballast light output divided by reference ballast light output. For example, if the total lumen output from the lamps in a fixture is 6000 and the BF is 0.85, then the actual light output from the lamps will be 5100 lumens. This difference should be taken into account in maintained light-level calculations.
Illuminance: The density of luminous flux light falling on a surface. The unit is called footcandle when using the English measurement system (foot) and lux when using the International system (meter).