Slight update 11/20/2006.
Yes, I am aware that in the USA technical types outside the automotive industry typically call a lightbulb a "lamp".
Please note that store brand lightbulbs with lumen light output figures and hour life expectancy figures like those of "Big Three" actually are "Big Three" lightbulbs.
"Big Three" "regular" (A19) lightbulbs will usually be:
100 watt ones with 750 hour average life and 1670-1750 lumens average output
75 watt ones with 750 hour average life and 1150-1210 lumens average output
60 watt ones with 1000 hour average life and 840-890 lumens average output
40 watt ones with 1000-1500 hours and 440-505 lumens average light output
Lightbulbs other than "big three" ones usually produce less light than "big three" ones.
a) Check the line voltage for excessive voltage. Call your power company if you determine for sure that your line voltage is excessive. Use longer life or 130 volt bulbs if the power company cannot or will not correct excessive line voltage.
b) Check for excessive expectations. If you have 12 750-hour bulbs each operated 6 hours a day, it is normal to burn out three of them a month.
c) Lightbulbs are junk such as "dollar store" bulbs. Get ones made by one of the "Big Three" lamp makers.
Lightbulbs in Recessed Ceiling Fixture Burn Out Too Quickly:
Check that the bulbs are of a type and wattage recommended by the fixture manufacturer. Heat builds up in these fixtures.
Lightbulbs in Small Enclosed Fixture Burn Out Too Quickly:
a) Check that you are not exceeding the maximum wattage recommended by the fixture manufacturer. Many fixtures are rated only for bulbs 60 watts or less.
b) Off-brand bulbs, especialy dollar store bulbs, may not be up to the job.
Lightbulbs in Ceiling Fixtures and Desk Lamps Burn Out Too Quickly:
a) Many of these fixtures are rated to use bulbs no more than 60 watts.
b) Off-brand bulbs such as dollar store bulbs may not be up to the job.
Lightbulbs Burn Out Prematurely and Have a White Smoky Appearance:
This means the bulb cracked and air got in and oxidized the filament.
a) This usually means the bulb was an off-brand piece of junk.
b) Water dripped on a hot bulb.
c) The bulb overheated by being the wrong kind/wattage for the fixture.
d) Something hits the bulbs and breaks them.
e) Bulbs heat up and then cold drafts hit the bulbs (unlikely)
f) Condensation on a bulb causes thermal stress when dry parts of the bulb get hot (not especially likely).
Lightbulbs Break During Use:
a) See just above.
Longlife Bulbs Don't Last as Long as Expected:
a) Bulb is a junkier off-brand one. Use longlife versions of "Big Three" bulbs, use traffic light bulbs especialy if made by the "Big Three", or use ones made in Poland (available in some hardware stores, less available in the USA nowadays).
Lightbulbs Sometimes Get Dim and/or Flicker and then Go Out:
a) Check for corroded contacts on the bulbs or in the socket.
b) Check for poor fit or mashed-down socket contact. You may be able to pry up the center contact in the socket (with power off!). Do not overtighten the bulbs.
c) Check for poor contact or wires screwed down too loosely in the fixture
or in the house wiring, especially if changing the bulb does not change
the behavior much.
d) Replace the socket or the fixture if necessary.
NOTE - Flickering with dimming must be fixed or avoided. There could be major heat production at the site of resistance due to poor contact. This is a possible fire hazard.
e) If the bulb starts doing this and then never works again after the next time it is turned off, then chances are it burned out with a "stable burnout arc" that bridged the gap in the filament. This does not happen often and when it does the filament usually broke after it warmed up, as opposed to during a cold start. A bulb with a "stable burnout arc" often, maybe usually produces a soft buzzing sound if operating on AC. This is not a fault of the socket nor the fixture. If the bulb has a visible break in the filament, whether or not the newly-broken ends of the filament show any signs of melting, then this explanation is even more likely.
Lightbulbs Sometimes Bet Brighter When Something Else is Turned On and also Burn Out at an Excessive Rate:
a)This usually means that you have a broken or poor neutral connection, usually in the main panel, or sometimes in a subpanel if you have any subpanels. Be sure that screws holding down wires in your fuseboxes / breaker boxes / panels are adequately tight. If you are not up to this fix or it fails to correct this situation, call an electrician. This is a dangerous condition that must be fixed urgently.
Lightbulbs Break or Pop Off Their Bases when they Burn Out:
a) Some offbrand bulbs and a few production runs of "Big Three" bulbs have been known to lack internal fuse wires. The current surge due to a "burnout arc" reaches hundreds of amps and makes the wiring in the bulb explode.
Lightbulbs Damage Dimmers or Electronic Switching Devices when they Burn Out:
a) This usually means marginal quality dimmer or switch that cannot withstand the current surge drawn by a burnout arc. Get a sturdier dimmer or switching device.
b) The lightbulbs lack internal fuse wires - change brand.
c) For adventurous hacking homebrewers, replace the triac or (or SCR, less likely) in the dimmer circuit with one having much higher current capability and trigger current no higher than that of the original. Use a triac or SCR with slightly higher trigger current requirement at your own risk, although this usually works.
Lightbulbs Burn Out Too Quickly Only In Certain Rooms/Fixtures:
a) Check for wrong kind or overwattage bulb in the fixture.
b) Check if you are putting junky off-brand lightbulbs such as dollar store bulbs in the fixtures in question.
c) Check for vibration from slamming doors, people dancing nearby,
children jumping or bouncing balls, etc. Vibration-resistant bulbs may be
d) This can happen with the "open neutral" problem discussed above.
Lightbulbs Seem Dim:
a) Check line voltage - if necessary, shift loads or upgrade the wiring. Call an electrician if you need a wiring upgrade that you can't do yourself. Call your utility if the problem is upstream from your electric meter.
b) Longlife bulbs are dimmer than standard-life, "Big Three" bulbs.
c) 130 volt bulbs are dimmer than 120 volt bulbs - typically by 22-25 percent, more if the life at 130 volts is longer than "standard" or the bulbs are also vibration-resistant or off-brand. Good ones are available at Lowes.
d) Lightbulbs with vibration resistant or shock resistant or rough service filament design are normally less efficient than standard lightbulbs.
e) Junky off-brand bulbs such as most dollar store bulbs are dimmer than "big three" lightbulbs.
Projector Bulb or Photoflood Bulb Burns Out Quickly:
a) Note that these usually have short lifetimes anywhere from 2 to 60 hours.
b) Bulb is misused - use only the proper bulb and only in equipment designed for the bulb and use the bulb/equipment only as directed. Some of these bulbs have mounting position requirements and/or cooling requirements.
a) Halogen bulbs have to be on long enough to reach their usual temperature in order for the chemicals in them to work properly. Do not use them for closet lights or refrigerator lights.
b) Some halogen bulbs do this when dimmed. Off-brand ones do this more than ones made by the "Big Three", ones made by reputable specialty halogen bulb makers, or ones made in Japan.
c) I have heard of lousy off-brand 300 watt halogen bulbs for torchiere lamps and similar off-brand 500 watt bulbs doing this even when run optimally. Try a "big three" bulb.
d) Bulb may have cracked due to contamination of the quartz - see below.
Halogen Bulb Cracks or Explodes or Burns Out With A Smoky Appearance:
a) Although halogen bulbs have a slight risk of doing this anyway, this usually
means that the bulb cracked from a stress due to contamination. The bulb
contains substantial pressure and the quartz bulb operates at lead-melting-hot
to red-hot temperatures. Any sort of salt, ash, or alkali can slowly leach into
the hot quartz and cause strains, causing the bulb to crack.
Avoid touching the bulb with bare skin! Clean any contaminated bulbs with a clean cloth or paper towel or tissue soaked with alcohol, then rinse with distilled water. When handling a halogen bulb, it is best to use the original packaging - your skin touches only the outside of the packaging and the bulb touches only the inside of the packaging.
None of this contamination stuff matters to halogen capsules inside glass outer bulbs, such as Philips "Halogena" and Sylvania "Capsylite" - unless the outer bulb breaks.
b) The bulb has subtle cracks from excessive or undue force applied to the base or connection points.
c) The bulb is off-quality and/or extremely old (I have seen one bought in a flea market do this). Suspect extreme age or low quality especially if the smoky appearance is black or dark gray as opposed to white or light gray. Darker smoke color usually indicates excessive age or low quality and lighter smoke color usually indicates cracking of the bulb.
Light Output and Color Problems
Burnout and Blinking and Strange Glow Problems
Fluorescent Bulbs Burn Out Too Quickly:
a) You have a bulb-ballast mismatch. Read the label on the ballast and read the markings on the bulb. Note that 34 and 35 watt bulbs may say F40T12(color code) and "energy saver" or something like this.
b) Ballast is low grade or defective. If the ballast states an amp figure and you have an AC ammeter, check for highly excessive current consumption as a sign of a bad ballast.
c) The lamp is started too many times - starting causes wear.
d) You have poor contact with the bulb due to corrosion. Usually twisting the bulb around will break through the corrosion, at least temporarily. If there is corrosion, (WITH POWER OFF!) scrub the corrosion off the socket contacts or the bulb pins with fine sandpaper. This may be tricky. In bad cases you need a new fixture.
e) Some "shop light" bulbs have life compromised by designing them for extra energy efficiency or low cost - use standard bulbs if they are compatible with the ballast in the fixture. Also, some cheaper "shop light" fixtures with cheap ballasts may be hard on the bulbs.
Bulbs Die In Pairs - Is It The Ballast or One Bulb or Both?:
Many ballasts operate the bulbs in pairs. The bulbs are in series. If one bulb goes bad, both go out or go dim.
In many cases, there is a "bleeder resistor" or other auxiliary component to apply the full ballast output voltage to just one bulb if both are not conducting - this helps the bulbs start. If one bulb is glowing very dimly and the other is completely out, it is possible for the dim bulb to be the bad one and the completely-out bulb to be the good one.
If the good bulb spends a lot of time in dim glow, it can be damaged - prolonged glowing with the filaments at elevated but less-than-full-normal temperature can be bad for them.
Since the "good" bulb is probably through most of its life and may have suffered if it spent a lot of time in dim glow, it is normally a good idea to replace both bulbs of the affected pair. In case you are curious as to which bulb is the bad one, there is usually noticeable darkening all over one end of the bad bulb. This means 2-3 inches of tubing are noticeably darkened. A mere spot or blotch or ring / band at one end or both ends is usually not a sign of end-of-life.
Bulb Is Out or Very Dim and Ends Glow Dim Orange:
Bulb is dead and a rapid start or trigger start ballast is making the filaments hot enough to visibly glow. Replace the bulb.
Bulb Blinks from once every few seconds to a few times a second:
Preheat bulb has died. Replace the bulb. Remove the bulb even if you cannot immediately replace it. This blinking is hard on the starter, and the ballast can get stressed if the starter gets stuck in a starting attempt. If the bulb has been blinking a long time, it is a good idea to replace the starter anyway even if it is not yet dead.
Bulb in Fixture With a Starter Only Glows at the Ends, usually off-color or orangish, sometimes each end glowing a different color:
The starter is bad and the bulb may be bad. If the bulb is brand new, a stuck starter can still do this. This is bad for the bulb. Remove the bulb until the starter can be replaced. Replace the bulb along with the starter unless the bulb is known good. A bad bulb can ruin a good starter after a while and a bad starter can ruin a good bulb after a while.
WARNING - Stuck starters can cause excessive current to flow through the ballast and overheat it. I have known a fire to start this way - in an elevator yet! Remove bulbs that glow only on the ends or blink or blink but with only end glow, even if you cannot immediately replace the bulb. This is whether the end glow is any shade of brightness of orange or yellow-orange or whitish orange-yellow or closer to the normal color of the light from the bulb or pinkish or orangish-pinkish.
Near-normal or pinkish-normal or pinkish-orangish-normal color end glow indicates an arc across the filament and an incandescent-orange (or yellow-orange or dim orange) color indicates lack of such an arc. Filaments that glow brightly yellow-orange to whitish-orange-yellow without arcing are almost certainly worn beyond usefulness. Heating will usually not be much different whether an arc forms or not. In any case, remove the bulb.
Bulb is flickering, especially with "swirling" effect, and one end has orangish/pinkish glow and alos has blackening:
This is a common sympton of a bulb burning out when the ballast is an instant start ballast. The end of the bulb that has the failing electrode can get very hot, especially if it is a T8 (1 inch diameter) one on an electronic ballastand no other bulbs are working from the same ballast.
Bulb cracks near one end:
This usually comes from the heat associated with burnout of a T8 bulb on an instant start electronic ballast. This mainly occurs when only one bulb was working from that ballast.
Fluorescent lamp sockets (lampholders) melt:
This is usually from end-of-life of a bulb being run on an instant start ballast, especially a T8 bulb on an electronic ballast. The fixture manufacturer should have used lampholders that can take the heat of a bulb that is in the process of burning out.
Bulbs Go Off And On every few minutes to once or twice an hour:
A ballast with a thermal cutout switch is overheating. This may mean the ballast is bad, or the wrong bulbs are being used, or the ballast or the fixture is not mounted properly. Some "shop lights" need to be suspended from the ceiling or beam instead of being mounted flush against the ceiling or beam - flush mounting blocks a way for heat to escape the ballast. "Shop light" fixtures that must be suspended usually have chains and hooks included.
Fluorescent Lamp Does Not Start In The Dark:
Believe it or not, some starters depend on the photoelectric effect to work! Try replacing the starter with a different brand of starter. Ones containing a trace of radioactive material don't need light. Maybe put some glow-in-the-dark material near the starter!
Fluorescent Lamp Only Starts When Someone Touches or Brushes the Bulb or Subjects it to Static Electricity:
a) Many fluorescent fixtures require the fixture to be grounded and the bulb to be within 1/2 inch (12.7 millimeters) of a grounded reflector or other grounded sheet conductor. Maybe also required is that black wires are "hot" and white wires are "neutral" (May be different outside the USA). All this affects the electric field distribution within a bulb undergoing a starting attempt.
b) Very slight chance this is from one of the other usual suspect problems such as aging bulbs, bulb-ballast mismatch, low temperature, corroded socket contacts, bad or low quality ballast, low line voltage, etc.
Lamps Fail To Start in Cold Temperature:
a) Although this is not a common problem, rapid start and instant start fluorescent lamps are supposed to have special low temperature ballasts for starting in the cold. You're on your own with preheat ones, but try a different brand of starter.
b) If the proper low temperature capability ballast is being used, check for proper connections, grounding of the fixture and reflector or other sheet conductor within 1/2 inch of the bulb, bulb-ballast mismatch, low line voltage, corroded contacts, etc.
c) 34 watt ("energy saver F40") bulbs are not as good as true 40 watt ones.
Starting is Unreliable or Generally "Cranky":
a) This is probably from any of the other usual suspect problems such as aging bulbs, bulb-ballast mismatch, low temperature, corroded socket contacts, incorrectly wired ballast, fixture not grounded or lacking a grounded reflector, light-requiring starter, bad or low quality ballast, low line voltage, 34 watt or "energy saver" version of F40 bulb, etc.
b) dual-20-watt trigger start fixture with reduced output and/or
flickering - the ballast is of a marginal design. Try a different brand of
bulbs or get a different fixture.
c) Sometimes bulbs get a coating of dirt/dust that gets conductive in high humidity, and that screws up the electric field distribution within a bulb that is trying to start. This happens more in coastal areas. If this is the problem, then the answer is to clean the bulbs.
Light Output and Color Problems
Fluorescent Lamps Look Dim:
a) They're cold. In really cool or drafty cool environments, you may need
those plastic protective sleeves sometimes called "tube guards" around the
bulbs to build up heat. Allow a few minutes for them to warm up. Note that
these sleeves do not help starting in the cold. Sleeves are not recommended
if the bulbs are running only slightly cooler than optimum since that can
make the bulbs run substantially hotter than optimum.
In drafty and in slightly cool environments, use enclosed fixtures instead of open fixtures. Enclosed fixtures normally have adequate heat buildup.
a1) The 34 watt energy saving ones seem especially vulnerable to cold.
b) Broad spectrum ones (deluxes, etc.) normally produce a little less light than standard ones and "triphosphor" ones.
c) You have a junky off-brand or cheap "residential grade" ballast. I recommend ones made by General Electric, Magnetik, Universal, Robertson, or Valmont or the like, or made in North America or Europe or Japan for "iron" rapid start ballasts. Many magnetic or "iron" ballasts for 4-foot bulbs, especially for two or more 4-foot bulbs, come in "residential grade" and "commercial grade". The cheaper "residential grade" ballasts sometimes deliver reduced power.
d) You have 25 watt "shop lights" which are not as bright as the 40 watt ones.
e) Aging bulbs have worn phosphors. Try new bulbs if the bulbs have given you lots of hours of service.
f) Check for bulb-ballast mismatch.
g) See below for problems related to specific lamps.
20 Watt Fluorescents in Fixtures With Starters Look a Little Dim:
These fixtures normally have the common 14-15-20 watt ballasts, which usually deliver about 16 watts to 20 watt lamps. There is not much that can be done about this. Do not kludge up something to deliver increased voltage to the fixture since that will make the ballast overheat.
Dual-20-Watt Fixture Without Starter has Low or Irregular Light Output:
The usual dual-20-watt "trigger start" ballast is of a marginal design. Try a different brand of bulbs or a fixture of a different design.
20 Watt Lights of America Fixture Looks a Little Dim:
The semi-proprietary Lights of America ballast in this fixture delivers a non-optimum current waveform. Not much can be done about this.
20 Watt Fixture of Another Type Looks a Little Dim:
The ballast may be a 15-20 watt type. Not much can be done about this.
14 Watt Fluorescents Look A Little Dim:
14 watt fluorescents are less efficient than 15 and 20 watt ones. There is nothing to do about this except replace the fixture with something more efficient such as 15 to 20 watt or 13 watt "twintube"/PL compact fluorescent or one with the F17T8 bulb.
4-foot T8 (1-inch diameter) 32 Watt Low Mercury Lamps are Getting Dim:
I have seen this happen and it seems to happen more badly in cool and
drafty environments. I suspect some reason having to do with higher
"electron temperature" (kinetic energy of free electrons) that results
from reduced mercury vapor content. This may cause mercury ions to embed in
phosphor particles or cause 184.9 nm shortwave UV to damage the phosphor.
Use these bulbs in enclosed fixtures. In severely cool/drafty environments, it can pay to use those clear plastic protective sleeves sometimes called "tube guards" to build up heat. Watch for the tubes to brighten and then dim again during warmup - this indicates overheating which means you just want enclosed fixtures instead of the sleeves.
Fluorescent Bulbs Look Different In Color:
Fluorescent bulbs come in all sorts of colors. Be sure to match the color code or color type or color temperature. Note that minor differences from one brand or batch to another are normal. There are five major common color temperatures - 3000 (warm white), 3500 ("a whiter warm white"), 4100 (cool white), 5000 (an icy cold pure white), and 6500-6800 ("daylight" or icy cold bluish white).
How Colors Look Under a New Fluorescent Bulb is Different From Old Bulb:
Even if the basic color or color temperature is matched, the spectral character can be different. Most white fluorescent bulbs are in three spectral classes:
Halophosphate - standard cool white and warm white, lousy color rendering
Broad Spectrum - Improved halophosphate with reduced light output
Triphosphor - usually full light output and better color rendering with brighter rendering of colors. Color rendering index is usually 82-86 but there is a related class with color rendering index in the mid-upper 70's.
Compact Fluorescent looks dim and/or off-color:
1) The bulb is of a junky brand.
2) The bulb is being operated at non-ideal temperature, such as excessive cold or excessive heat. These easily overheat in small enclosed fixtures and in recessed ceiling fixtures.
For more compact fluorescent info, please go to my Compact Fluorescent Top Page.
The lamp warms up and as soon as it is warmed up, or maybe several minutes after warming up, it goes out. Maybe it takes about 3 minutes to relight, but in fixtures available from home centers it usually relights in closer to 1 minute with often a little dim orange-yellow glow at times before it relights:
This is usually "end of life cycling". That is a characteristic of the bulb. Replace the bulb and this common problem is fixed.
Less likely, the problem is a reflector that returns light to the arc tube in the bulb and overheats it.
High Pressure Sodium Lamp is Knocked Out for 1-3 minutes When A Heavy Electrical Load Is Started:
Although poor load distribution and/or wiring undersized for the length of a wiring run is likely to exist, this is usually a variation of "end of life cycling". Replacing the bulb usually fixes this, although often some wiring upgrades are needed. Slight chance it is the problem of a reflector returning light to the arc tube.
Sodium, Mercury, or Metal Halide Lamp Cycles On-Off and Goes Out Before Warmup Is Complete:
a) Bulb is at end-of-life. Replace the bulb.
b) Bulb is wrong type for the ballast - bulb/ballast mismatch. Wattages must match, except for slightly different wattage in some cases. (Sodium bulbs of mercury retrofit type have nominal wattage slightly below that of the compatible mercury, for example.)
Matching wattage alone is not a guarantee. 150 watt sodium comes in three different types - S55 or 55 volt nominal arc voltage, S56 or 100 volt nominal arc voltage, and mercury retrofit. 1000 watt mercury comes in H34 and H36 which are not compatible with each other. Some specialty metal halides and high pressure sodiums such as "white SON" are not interchangeable with the more common types of same wattage but require different ballasts.
In a few rare cases, a metal halide bulb in a mercury ballast may start but go out during warmup due to some "fussy" characteristics of a warming-up metal halide lamp. Metal halides often don't start and generally do not reliably work properly with mercury ballasts.
Note that mercury bulbs can be used with metal halide ballasts of the same wattage in the event the wattage is 175 to 400 watts. This normally also works for 50-100 watts, but those are pulse start metal halide ballasts and applying power with a bad bulb or one that was just shut off may damage the bulb and may even be a fire hazard and is not recommended. There are two 1,000 watt mercury bulbs; H34 won't work properly in a metal halide ballast but H36 will.
Please note that mercury and metal halide bulbs will usually be overpowered by pulse start sodium ballasts for 100 volt bulbs. Mercury and metal halide bulbs will usually go out before full warmup if powered by pulse start sodium ballasts for 55 volt bulbs (most sodiums 35 to 100 watts).
c) Less likely - bulb has burning position restrictions being violated.
d) Less likely - Photocell (if any) is catching light from the fixture and turning the fixture off.
Mercury Retrofit Sodium Bulb is Unreliable in Brightness (usually gets brighter with age):
Mercury retrofit sodium bulbs are sometimes not suitable for some types of mercury fixtures nor fixtures that are suitable for metal halide bulbs. Some are only suitable if the ballast is a "high leakage reactance" autotransformer with the proper open circuit output voltage or a "reactor" (plain choke or inductor) ballast (whether power factor corrected or not) with the proper line voltage, and this voltage is usually 230-280 volts or so. Check instructions that come with the bulb and the bulb packaging!
Leading, lead-peaked, "CWA", and metal halide ballasts typically mishandle variations in retrofit bulb characteristics that change as the bulb ages unless the bulb is rated for all mercury and metal halide ballasts of the proper wattage (slightly higher than the sodium bulb's nominal wattage). Brand-new bulbs may be underpowered and older bulbs may be overpowered.
Metal Halide Bulbs Change Color and do so in Unlike Ways as They Age:
This is normal. But truly radical changes, especially with a noticeable reduction in light output, usually indicate that the bulb is at or near the end of its life. Remove any HID bulb that is rapidly changing or has suddenly made a major change in characteristic. Metal halide bulbs that turn pink may work another month or two but will fail soon. Urgency to remove a bulb that has a major change is less if the fixture is fully enclosed and rated to contain exploding bulbs. Metal halide bulbs not of "protected" type (rated for being safe in open fixtures) have the greatest need to be operated only in suitable enclosed fixtures and to be removed if malfunctioning in fixtures not known to be "proper" for them. Sodium bulbs tend to be non-catastrophic with severe failure.
Ignitors in pulse-start ballasts suffer extra wear if operated without a bulb in place that stays on during operation.
Bulb is Flickering and it Did Not Flicker Before:
Usually this means the bulb is approaching end of life. If the bulb has been used a lot, this really probably means it is approaching end of life. Replace the bulb.
High Pressure Sodium Bulb has Short Life or otherwsie is Not Quite Right:
You can have the right wattage but still have the wrong bulb for the ballast. The 150 watt comes in different types, "white SON" is not interchangeable with others including a standard-compatibility improved-color type, and some retrofit bulbs have restrictions against metal halide ballasts and some types of mercury ballasts.
Replacement Bulb is Dim or Fails to Fully Warm Up:
You probably have an incompatible bulb type despite the wattage being the same, such as H34 1000 watt mercury on an H36 ballast or S55 150 watt sodium on an S56 ballast.
Extremely rare and usually only from overpowering the bulb via bulb/ballast mismatch, incorrectly wired ballast, or very excessive line voltage. Much less likely but possible is a bad ballast. More rarely, bulbs do this when not overpowered and usually only when operated past their life expectancy - mercury and metal halide bulbs should be replaced after exceeding their life expectancy of running time. It is sometimes recommended to use fixtures of a design to contain exploding bulbs. Metal halide bulbs other than of "protected" type should only be operated in proper enclosed fixtures since their chance of exploding is higher.
Replace or discontinue using any HID bulb if it starts operating erratically or has a major color change or loss of light output that develops over a couple weeks or less. Replace any bulbs with an unusual bulge or "bubble" on the arc tube.
High pressure sodiums have less of a risk - sodium arc tube ruptures normally do not cause the outer bulb to break.
It is sometimes recommended to turn off metal halide and mercury lamps at least once every couple weeks as opposed to running them 24 hours a day 365 days a year - bulbs that are approaching catastrophic failure sometimes do not restart when turned off or give warning signs of obviously erratic operation when restarted.
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