Understanding how fluorescent and HID lamps work makes problem solving and corrective maintenance easier.

If you work with arc discharge lamps, you know troubleshooting can be tricky. It never fails: Your ingenious idea that solved yesterday's problem doesn't work today. So how do you predict the unpredictable? You must get to know these fluorescent and high-intensity-discharge (HID) light sources inside and out to truly understand their behavior.

Arc discharge lamps produce light by an electric arc struck between two main electrodes or cathodes. With few exceptions, they must operate with a ballast, designed to provide the proper starting and operating voltage for the lamp. A ballast changes the voltage of power supply to what the lamp needs to start reliably (provide the open circuit voltage), regulate the current (ampere flow) to the lamp, and maintain the proper voltage to the lamp during operation. Usually, you know there's a problem with a discharge fixture when the lamp (or lamps) shows one the following conditions: nonstart, cycle on and off, extra bright light output, or low light output. Any one of these is reason enough to deenergize the fixture and take a closer look. To help you troubleshoot, we'll look at the fluorescent source first and then the HID source.

Fluorescent lamp characteristics. Fluorescent lamps can be divided into the rapid-start and instant-start type (Slimline lamps that do not have cathode heating).

Rapid-start (RS), high output (HO), and 1500mA T12 lamps all have similar characteristics. They depend on the proper heating of the cathodes to start up and operate properly.

Slimline lamps do not use cathode heating. They use the ballast open circuit voltage (which can be about three times the normal lamp operating supply voltage) to start operation.

When fluorescent lamps don't operate, the ballast may not be the culprit. Therefore, before replacing the ballast, examine all components.

• Change or check lamps to ensure proper operation. As you remove them, examine sockets to ensure proper and positive contact with lamp pins.

• If you used starters, check and replace each as necessary.

• Examine the ballast leads to make sure the connection matches the diagram on the ballast label.

• Examine and test the ballast. Leaking compound (except for a normal small amount at the lead holes), cracking or brittle insulation, or discoloration on the can may indicate the ballast (especially an electromagnetic type) is approaching, or has reached, the end of its life.

At the outset, we'll discuss two specific problems found in any type of fluorescent system. We call the first cycling. The National Electrical Code (NEC) requires most indoor ballasts to have a cutout device that protects them from overheating. If high ambient conditions cause the ballast to overheat, the thermal protection device's switch disconnects it from the power supply; if the temperature at the ballast is reduced, the cutout device closes, reactivating the ballast, thus restarting the lamps. If such a cycling condition persists, you must find the cause of the high temperature and correct it.

We call the second condition swirling, where the light appears to swirl or spiral inside the tube during operation. This normal occurrence can last for a few hours with some new lamps when first energized. However, a cold ambient temperature or low input voltage can also cause the problem.

Rapid-start (RS) systems. To measure the starting voltage of an RS ballast, connect a voltmeter between the highest reading Red lead and Blue lead with the lamp removed. See Fig. 2 (in the original article). Consider the following example: For an F 40 T12 two-lamp ballast, the minimum starting voltage is 256V. The minimum starting voltage for F96T12HO two lamp ballast is 466V. For a F96T12VHO ballast, it's 470V. All numbers are for minimum ambient temperature of 50DegrF. You should find these open circuit voltages on the ballast's label. (Note: Electronic ballasts generally provide starting voltages higher than their electromagnetic counterparts. You can also find their open circuit voltages on the ballast's label.)

To measure the filament voltage on a single lamp unit, read voltage between Red-Red and Blue-Blue leads. For two lamp units, read voltage between Red-Red, Blue-Blue, and Yellow-Yellow leads. For all 4-ft and longer RS lamps, the filament voltage should be between 3.5V and 4.5V. Sometimes with a two-lamp, RS series ballast only one lamp will light to full brilliance. Refer to Fig 2 again (in the original article). If the lamp between the Red and Yellow leads is lit and the other lamp is out, look for a pinched Yellow lead. If the lamp between the Red and Yellow leads does not light and the other one does, the cause is probably a short.

Why is good contact of lamp pins with their sockets so important? When an RS lamp circuit first energizes, the low-voltage winding of the ballast provides the cathode heating. Shortly afterward, the open-circuit voltage is applied across the lamp, or lamps, to begin arc conduction.

If only one cathode heats up, either the lamp will fail to start or it will be slow in starting. Heavy premature end-darkening, usually at only one end, usually indicates one cathode is not getting proper heat. Therefore, the heater circuit is incomplete. This is not the same as gray or brownish bands occurring about 2 in. from the lamp base, with the edge of each band on the side nearer to the base being sharper. These bands occur when the cathode coating wears out. While they may detract from the appearance of the lamp, moderately dark bands have no significance concerning either life or performance of the lamp. However, you should replace failed RS lamps promptly to avoid any possible ballast damage.

The lack of cathode heating voltage can come from a poor connection, either between the lamp pins and lampholder contacts, or between the ballast leads and lampholder terminals. Improper lampholder spacing in the lengthwise direction, with the lamp end held either too tightly or too loosely, can also cause poor contact at the lamp pins. Measure the voltage at the socket terminals to determine if adequate heater voltage is present.

Sometimes, in high-humidity conditions, RS lamps may start slowly or not at all, even though the cathodes are heated properly. This can result from dirt on the lamps, which makes the silicon coating ineffective. It can also be due to a poor silicon coating. If a new installation experiences random starting under high-humidity conditions, low supply voltage or poor silicon coating on the lamps is generally the cause. If you see the same poor starting in high-humidity conditions on an older installation, the cause is usually an accumulation of dirt on the lamps. In this case, just wash the lamps.

Why is the RS lamp surface condition so important? For the lamp to start within the voltage range of the ballast, it is necessary to excite the gas fill inside the lamp by means of an external voltage; thus the excitation ionizes the gas fill. The capacitance between the lamp and reflector or fixture channel assists this external excitation. Of course, you must bond (connect) the metal fixture to the equipment grounding circuit, and operate the ballast from a grounded electrical system. Any dirt or moisture coating on the lamp can reduce this capacitance.

If you experience random starting of RS lamps, check the fixture for proper grounding. As previously stated, for completely reliable starting in RS circuits, you must have a starting aid, defined as an electrically grounded metal strip at least 1 in. wide extending the full length of the lamp. The lamp should be within half in. of the grounded metal strip for 40W lamps and smaller (three-fourths in. for T8 lamps) and 1 in. for higher output lamps.

The modified RS ballast, a version of the RS ballast, reduces power consumption by a few watts, and operates the lamps similar to the standard RS ballast. The only difference is: You reduce or eliminate filament heating, after the lamps ignite, so the 3V to 4V cathode heating voltage reading will not be measured during normal operation.

Slimline fluorescent systems. We use two types of electromagnetic ballast circuits for two-lamp operation of Slimline lamps. The Lead Lag Slimline circuit has two sections: one called the lead section, consisting of an inductive coil and capacitor in series with the lamp; the other section consists only of an inductive coil. The ballast operates one lamp independently. So if one lamp becomes inoperative, the other will still light. You measure starting voltage between Red and White and Blue and White leads.

A Series Sequence Slimline circuit is smaller and lighter than the lead lag type and operates two lamps in series, with the lamps starting in sequence. Measure starting voltage between Red and White leads. Insert lamp in Red and White position, then read the voltage between Blue and Black. For an F48T12 Slimline lamp, the starting voltage is 385V.

The open circuit voltage of a Slimline ballast is high enough to start a lamp with one lamp filament deactivated. The lamp will flicker and become very black at one end. If you don't replace the lamp, the ballast will overheat and fail. If one lamp of an electromagnetic Slimline ballast is operating and the other is not, the temperature of the coil will rise and possibly fail. This problem does not occur with an electronic Slimline ballast, which operates the lamps in parallel.

HID lamp characteristics. The electric arc of HID lamps is much shorter and has a higher photometric brightness than a fluorescent arc. The lamps are also often higher in wattage ratings, and they 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, each type has its own operating and end-of-life characteristics, which often directly relate to maintenance/ troubleshooting concerns. The family of HID sources includes:

Mercury-vapor lamp (MV). With the lowest efficacy of any of the HID sources, most of these lamps in the 100V 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, you should replace them well before reaching their average-rated life. Normal end-of-life (EOL) is a nonstart condition or very low light output caused by blackening of the arc tube from electrode deterioration.

Metal-halide lamp (MH). Operating conditions, such as lamp burning position and normal variations in supply voltage or ballast characteristics, can affect lamp color and light output of MH lamps. Normally, at its end-of-life, an MH lamp won't start because the mix of materials in the arc tube changes. Therefore, the ballast can no longer sustain an arc.

High-pressure-sodium lamp (HPS). Normal end of life indication is on/off cycling, since the aging lamp requires a higher voltage for operation than the ballast can supply. This cycling sequence is normal, but not desirable, since the cycling can damage or destroy the starting circuit and/or ballast. Thus, you should check a cycling HPS fixture without delay. Other conditions that cause cycling include: a loose electrical connection, faulty internal electrical connection to the lamp or at an outdoor installation, and severe fixture vibration, causing lamp voltage to rise above operating limits.

Low-pressure-sodium (LPS). This lamp, which features high efficacy but somewhat limited applications because of its monochromatic yellow color, uses a U shaped arc tube and a two-pin bayonet base. During its burning hours, the lamp's wattage increases. For example, at 20,000 hrs, a 180W lamp draws 247W, so any ampere reading will reflect this increase in wattage consumption because of aging. When troubleshooting, look at the lamp, ballast, capacitor, ignitor (if used), socket, and perhaps the entire fixture for indications of excessive heat.

Let's look at each fixture component separately:

Lamp. Any cracks or holes in a lamp's outer jacket usually indicate a defective lamp. The presence of some form of foreign coating or material on the inside of the lamp's outer jacket can also indicate trouble. This can appear as a light yellow (with an MH lamp) or a silver mirrored appearance (with an HPS lamp). Remember, an HPS lamp will have some normal silver appearance around the lamp base. But, if all, or most, of an HPS lamp's outer jacket has that coating, there's usually a defect. Check the metal parts making up the mount or support for the arc tube for separation or damage. Also observe if the arc tube is cracked or broken. Another procedure is to put an ohmmeter (at the highest setting) across the lamp's center base contact and the base's screw shell. There should be no continuity (it should be open). If the ohmmeter shows continuity, there's a short in the base or mount, and the lamp will not operate. If these procedures don't help you uncover the problem, you can confirm the lamp is defective by putting it in a fixture known to be operational.

Ballast. An HID ballast typically steps up the incoming line voltage to a higher voltage to start the lamp. Thus, a common problem with a multiple-tap ballast can be an incorrect tap connection for the line voltage available. A defective core and coil ballast will usually show signs of its condition, such as darkening of the case or cracked or crumbling insulation, which definitely indicates excessive current flow or excessive heat buildup. Because of its construction, an F-can or encapsulated ballast does not allow you to see the winding. However, these two types often give off a strong odor if they have burned or melted ballast windings. You can make voltage measurements on an HID ballast, similar to a fluorescent ballast, using a multimeter. When you replace a defective ballast, also consider replacing the capacitor, ignitor (if used), and lamp.

Capacitor. Generally, a capacitor fails by either shorting or opening. When a capacitor shorts, it can damage the ballast, lamp, and/or cause excessive current flow, which may cause a breaker to trip. This type of failure is more typical in the older oil-filled (metal oval can) capacitor type than with the dry metalized film (round plastic can) type or the non-PCB oil-filled capacitors, which typically have an integral interrupter. A metal can type capacitor may bulge when it fails, particularly if it doesn't have an integral interrupter.

Usually, you use a single capacitor in a ballast circuit. However, sometimes a fixture has a pair of capacitors in the ballast compartment, wired in parallel to increase the capacitance. Thus, a common error in the field is the incorrect wiring of dual capacitors, which should operate in parallel, as seen in Fig. 3, in the original article. Verify the rating marked on the capacitor matches that on the ballast. Then, verify the actual capacitance in microfarads (mf), by taking a measurement. If low light output is the problem, the third rating to check is the mf tolerance (5) percentage. Some capacitors have a tolerance of 510%. A 110% would lower the wattage at which the lamps are operating. In applications where assurance of light level is vital, capacitors with a tight tolerances, such as 51%, are used (e.g. sports lighting applications).

You can test a capacitor with an ohmmeter set at a high resistance scale, after you turn off the branch circuit power and disconnect the capacitor from the circuit. Of course, always safely discharge the capacitor first. If the ohmmeter measures a very high resistance, the capacitor is open, so replac e it. If the reading is zero or a low resistance, the capacitor has a short. You should also replace it. If the reading is zero, or if a very low resistance shows initially, and the resistance slowly increases, the capacitor is good.

Ignitor. Some HID lamps require a high-voltage pulse, provided by an external starting aid, to excite mercury ions in the arc tube initially. This ignitor circuit produces a sharp impulse typically with a rating of 2500V to 4000V. The ignitor emits this pulse for one millionth of a second, at a minimum of once per half cycle, until the lamp starts, and then shuts off when it senses a current draw. It will also pulse when a lamp fails or the socket is empty. You can partially base ignitor life on the number of times the unit pulses. For that reason, you should repair or deenergize a nonoperating fixture as soon as possible. An oscilloscope with a proper high-voltage probe can show the pulse. HPS and some newer MH lamps use an ignitor as part of the ballast. We'll discuss this factor, because it can cause a maintenance person to troubleshoot problems that don't exist.

Socket. Because of the higher voltages carried and heat generated in its vicinity, you should check the socket carefully for cracking of the ceramic housing or discoloring of the metal parts. One note of caution: Manufacturers configure many MH lamp sockets specifically for a particular type of lamp. Thus, they should not accept any other type of MH lamp. MH sockets can produce the following confusing situations:

Position-oriented mogul base (POMB) socket for MH lamps. This type of socket fits a horizontal-burning lamp only, which has a bowed arc tube. When operated with that orientation, it offers increased light output. The lamp's screw-shell base has a protruding pin, about 1/4 in. in length. This configuration allows you to install the lamp in a POMB socket that has a slot to accept the pin, thereby ensuring proper burning position. For easy identification, the POMB socket is yellow. Thus, a POMB lamp will not screw all the way into a standard mogul base socket. Do not attempt to cut, grind, or bend the pin to make the lamp fit. Altering the pin may allow the lamp to descend further into the socket.

Extended husk medium-base socket. You can use this type of socket for a special MH lamp in an open fixture; one without a cover lens. Generally, you must use an MH lamp in a fixture with a cover lens. This requirement is a safety measure in the rare event a lamp ruptures or explodes at the end of its life. Over the past 10 years, lamp manufacturers introduced medium-base MH lamps that have additional protection inside the bulb, so you can use them in an open fixture. To ensure proper lamp usage, the open fixture has a special socket that accepts only an open-rated MH lamp. This socket is longer, and the opening is narrower than a standard socket.

Open-rated mogul base socket. In the last five years, certain MH lamps have required the use of a higher voltage rated socket we call open-rated. An open-rated, mogul-base MH lamp has an extended center contact on its base to fit into the cavity of a corresponding open-rated socket. For easy identification, this pulse-start socket, rated to withstand the up to 4000V pulse (open voltage) needed to start to start the lamp, is pink.

What is the purpose of an open-rated mogul base socket and an open-rated lamp? Recently, MH lamp manufacturers extended "pulse-start" technology, already used in metal-halide lamps 150W and smaller, to other portions of their product line. Superior to the older pinched arc tube design, a pulse-start lamp offers impressive improvements. It can operate at lower ambient temperatures; about 10DegrF lower than the pinched body lamps (140DegrF instead of 130DegrF), higher lamp efficacy (up to 110 lumens per watt), improved lumen maintenance (up to 80%), and consistent lamp-to-lamp color (within 100 degrees K). Smaller than a pinched arc tube, a pulse-start lamp's arc tube uses an ignitor to create a high-voltage starting pulse, as does an HPS lamp. While both types of lamps use a mogul base, a pulse-start lamp's socket (and ballast) is incompatible with a pinched tube MH lamp's socket (and ballast), and visa-versa. (See Fig.4, in the original article.)

Heat. Excessive temperatures can cause lamps, ballasts, capacitors, and ignitors to fail prematurely. This condition can also affect light output. In particular, high-wattage, compact fixtures using reduced jacket MH lamps can have elevated temperatures. Good indicators of excessive heat include: continual short life of components, cover lens with a discolored appearance, defective ballast with charred windings, internal arc tube that looks black, and bulged or bluish outer bulb of the lamp. Placing construction material around an HID fixture may cause excessive temperature, or create a condition that restricts air flow and traps heat.

Test measurements. When troubleshooting, check the line voltage at the fixture, not at the panel, to account for any voltage drop. In most cases, the supply voltage should be within 10% of the rating for best operation. Some reactor ballasts require the voltage to be 55% for proper operation. If the line voltage exceeds the recommended range, erratic lamp and ballast operation may result. Identify and correct the external cause of the low or high voltage by running a larger gauge wire (if you find voltage drop to be the problem), changing taps on a distribution transformer, or balancing/adjusting the loads on multiphase power systems. Next, check the open circuit voltage (OCV). OCV is the voltage potential across the lamp socket and is higher than normal lamp operating voltage. This is because you need a high peak-to-peak voltage to strike an arc for starting the lamp. Read the OCV of an energized HID ballast by removing the lamp and placing one alligator clip of a true rms voltmeter on the lamp socket's center contact, and the other alligator clip on the screw shell of the socket. Using a meter other than a true-RMS reading tester can lead you incorrectly identify a ballast problem. If the OCV reading for the specific ballast is not within range, check to see if the ballast is receiving the correct line (input) voltage, and for a good connection of the correct ballast tap. If both are correct, the problem is usually a defective/marginal ballast or capacitor or an incorrect ballast/lamp combination.

Make sure you take specific safety precautions when working on an HPS ballast or some MH units, since they use a high-voltage starting circuit (the ignitor) to initiate arc conduction. Disconnect this starter circuit, since its high-voltage starting pulse can damage a multimeter. The voltage on 50W through 400W lamps is at least 2500V, and for the 1000W lamps at least 3000V. Therefore, determine if an ignitor is part of the circuit, and disconnect it before taking an OCV reading. A short-circuit current measurement gives you the highest current the lamp could see. It can also indicate if the ballast/capacitor combination is correct (or if the ballast/capacitor is defective).

To do the SCCM test, deenergize the fixture. Then, use a jumper lead with alligator clips at each end to create a dead short on the secondary side of the ballast; directly at the socket (the ballast acts as a choke and limits the current). One end of the of the jumper clips to the center contact; the other end clips to the screw shell. For maximum safety, use an HLR fuse holder with a 10A GLR fuse in the test wire, because the ballast could have an internal short. Although the fuse could blow, it usually doesn't. But for the few times it might, this precaution could save you from serious injury. While making sure your ammeter is not close to the ballast's magnetic field, since it could distort readings, clip the ammeter around the jumper wire. Next, energize the circuit, and take the reading. If the reading doesn't fall within the specified range, check the ballast input voltage and ballast wiring. If both are correct, the problem is probably a defective/marginal ballast or capacitor or the incorrect ballast/lamp combination.