Understanding the characteristics of fluorescent and HID lamps makes it easier to recognize problems and simplifies corrective maintenance.
Lighting maintenance is easy; just replace burned-out lamps." Unfortunately, that's a common attitude of those holding the corporate purse strings. Of course there's more to lighting maintenance than lamp replacement, particularly with discharge lamp systems. In fact, all components of a lighting fixture come into play here. You need to clean fixture enclosures and reflectors to prevent needless loss of useful light output. Defective sockets and ballasts also need replacement.
But, how do you know what needs replacement? What symptoms should you look for? What troubleshooting techniques can you use? Read on to find out.
Symptoms of lighting system problems. All discharge lighting systems offer visual (and sometimes audible) clues when problems or failures exist. Usually, a problem affects only a few fixtures in a total lighting layout. Here's what you should look for in a lamp's appearance:
Extra-bright-light output; and
After identifying any abnormal fixtures, you have to find out which component (lamp, ballast, fixture, or electrical supply) is the cause. Select a normal and abnormal fixture. Then, with the electrical power off, interchange the lamps. While doing this, look for differences in lamp color, any broken lamp elements, burned or distorted bases, and defects in the fixture itself. If you don't find any major differences between the two, you should energize the lighting system. After they stabilize, observe the light fixtures.
If the "abnormal" appearance shifts to original "normal" system or fixture, the problem is in the lamp; not the ballast, fixture, or system power supply.
Generally, your troubleshooting task won't be this simple, since more than one factor usually contributes to component failure. For example, an incorrect tap on an HID ballast can cause the lamp to fail prematurely. Here, you should correct the wiring before you relamp the fixture.
Fluorescent lamps. When you energize a rapid-start lamp circuit, a low-voltage transformer in the ballast provides cathode heating. At the same time, you're applying a higher voltage (called the open-circuit voltage) across the lamp. This voltage beginsarc conduction in the lamp. Taking both voltage measurements is a good indication of proper ballast operation. Fig. 1, left, shows how to make voltage measurements on a two-lamp, series-type, rapid-start magnetic ballast. If only one cathode gets the heating voltage, the lamp will fail to start or it will be slow starting. This condition results from any one of the following:
Improper contact in the holder;
Open contact in a base pin;
Broken base pin;
Shorted leads in the base;
Corrosion or foreign material on base pins or holder contacts;
Defective ballast; or
Heavy premature end darkening (usually at only one end) is a good indication that one cathode is not properly heated and, therefore, the heater circuit is incomplete. This isn't 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 latter symptom occurs when the lamp's cathode coating wears out. While such bands may detract from the appearance of the lamp, moderately dark bands have no significance in regard to the life or performance of the lamp.
The lack of cathode heating voltage can result from a poor connection between lamp pins, lampholder contacts, or ballast leads and lampholder terminals. Poor contact at the pins can result from improper lampholder spacing in the lengthwise direction, with the lamp end held too tightly or too loosely. For 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. Occasionally, a pin will slide behind the clip instead of in front of it when you insert the lamp into the lampholders. If the lampholders are too far apart, the pins may ride on the edge of the lampholder rather than making contact along the full width of the metal piece.
Sometimes, installers remove end plates and other supporting devices (needed to keep proper spacing) when installing fixtures in a row. This looseness at the ends allows lampholders to bend away from the center.
When installing lamps with exposed bi-pins, make sure the positioning guide (an embossed mark on the metal end cap) is opposite the entrance slot of the lampholder before inserting. Then, turn the tube 90 degrees. If properly installed, HO and 1500mA lamps should have the boss or cover of their recessed double-contact ends out of sight.
With instant-start lamps, the two-lamp ballast circuit can be lead-lag or series sequence design. A lead-lag ballast operates lamps in a parallel circuit; meaning if one lamp fails, the other should continue to operate properly. On the other hand, a series sequence ballast operates lamps in series (meaning if one lamp fails the other will fail or glow dimly).
A note of caution: Hybrid magnetic ballasts (developed as a bridge technology between magnetic and fully solid-state electronic ballasts for 4-ft lamps) have components that cut off power to lamp cathode heaters after the lamp ignites, resulting in an additional 2W saving per standard lamp. As such, verify the type of ballast before measuring heating voltage.
You should replace inoperative instant-start lamps as soon as possible. When one lamp fails in a two-lamp series magnetic circuit, the second lamp may continue to operate, but at a lower brightness. This condition reduces the second lamp's life, causing a higher current to flow in the ballast. The increased current, in turn, causes a rise in ballast temperature, reducing ballast life and possibly initiating smoke.
You should replace flickering instant-start lamps that show heavy end blackening. This condition, known as lamp rectification, will cause a reduction in ballast life if it continues.
HID lamps. The electric arc of an HID lamp is much shorter and more intense in photometric brightness than a fluorescent arc. An HID lamp often is higher in wattage rating. This type of lamp has a specific warm-up time when you first energize it. It also may require a specific operating position; base-up, base-down, or base-horizontal.
Because the 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. These, in turn, 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), you should replace these lamps well before they reach their average rated life. Normal end-of-life (EOL) is a non-start condition, or low-light output, caused by blackening of the arc tube from electrode deterioration.
The metal-halide (MH) 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 MH lamp (instead of the original position-specific lamp) when relamping a fixture may seem like a good economic decision. After all, you can stock fewer lamp types. However, this attempt at saving money is short-lived. An MH 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 lamp. Normally at its EOL, an MH lamp won't start because the mix of materials in the arc tube changes, along with the electrical characteristics. The ballast can no longer sustain an arc.
Both the 1000W and 400W lamps, when burned vertically, have a passive EOL. The 1000W, 400W lamps, and all other wattages, when burned in other positions, can fail in a way that causes the outer bulb to shatter. As such, you should install these lamps in an enclosed fixture that contains any glass, in case it shatters. A shrouded arc tube line of MH lamps has a glass cylinder surrounding the arc tube to block any resulting glass fragments from hitting the outer glass bulb and causing it to shatter. You can use this latter type in an open fixture, where increased fixture efficiency and lower-maintenance costs are important. (You can remove a vertically mounted lamp from the fixture using a lamp-changing pole.) If the outer bulb breaks or shatters in a safety line of MH lamps, power to the lamp interrupts. However, the lamp's ability to self-extinguish does not protect it against the danger of breakage.
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. 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. You should replace a failed lamp immediately, as this situation can also cause starting circuit damage.
You should not remove an HPS lamp from a lamp socket for more than a short period, unless you de-energize the circuit. You can damage the starting circuit portion of the ballast, because the starting circuit will operate continuously with the lamp removed.
Some recent developments in HPS lamp technology aim at eliminating the end-of-life cycling, especially in highway and street lighting. One product has a positive end-of-life indicator, allowing you 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.
The low-pressure-sodium (LPS) lamp has a U-shaped arc tube and a two-pin bayonet base. With the highest efficiency of HID light sources, its output is a monochromatic yellow, so all colors appear as shades of gray, brown, or black (except yellow objects). During its burning hours, the wattage increases. For example, a 180W LPS lamp consumes 247W at 20,000 hrs. Keep in mind any wattage or ampere reading taken on the branch circuit will reflect this change of status condition.
Ballast troubleshooting. Fluorescent ballasts. It's difficult to measure the output voltage of fluorescent ballasts in a luminaire because the primary circuit of the ballast is automatically disconnected when you remove an instant-start lamp. If you wire a replacement ballast 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 you'll find leads without the insulation. In some cases, you'll find two wires using one slot and, although there are two spring clips inside the socket, both wires are set under one.
It's good practice to examine the wiring of the ballast leads to make sure it matches the diagram on the ballast label. 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. Remember, though a ballast starts a lamp, this does not necessarily mean the ballast is functioning properly. Short lamp life can result from ballast problems.
HID ballasts. During operation, an HID ballast produces 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; open or shorted.
You can check an HID ballast using a multimeter, similar to checking a fluorescent ballast. You should take specific safety precautions when working on an HPS ballast. Make sure you disconnect this starter circuit, since its high-voltage starting pulse can damage some multimeters. The voltage on 50W through 400W lamps is at least 2500V; for the 1000W lamp, it's at least 3000V. You can make an open-circuit voltage and short-circuit current measurement on an energized HID ballast. Here's how: With the lamp removed, use a true rms voltmeter to measure the voltage at the socket.
You can test the ballast capacitor with a suitable analog-type ohmmeter, set at a high resistance scale. Turn off the branch circuit power first; then discharge the capacitor by placing a screwdriver or other piece of metal across its terminals. Now, you can disconnect the capacitor from the circuit. Connect the meter's test leads to each terminal. If the ohmmeter measures a high resistance, the capacitor is open and you should replace it. If the reading is zero or a low resistance, the capacitor is shorted. Again, you should replace it. If the reading is zero or a very low resistance initially and the resistance slowly increases, the capacitor is good.
Also, make sure to check the voltage at the point of connection to the electrical distribution system. Operation in an overvoltage state increases wattage consumption and reduces the life of the ballast and lamp.
The family of arc discharge lamps includes fluorescent and high-intensity-discharge (HID) light sources. An electric arc struck between two main electrodes or cathodes produces light in these lamps, which are usually at opposite ends of the lamp.
Discharge lamps usually 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 for it to start reliably. The ballasts provide the open-circuit voltage, regulate the current (ampere flow) to the lamp, and maintain the necessary operating voltage. Thus, you must match a discharge lamp and ballast. You also have to properly connect them, operating within specific limits. If you don't meet all of these conditions, the light source won't work properly.
You can divide the family of fluorescent lamps into the cathode preheat type and those requiring 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. You can classify most of these lamp types 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. These 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. The figure, below, shows both rapid-start and instant-start types.