Almost 20 years ago when we built our boat, I chose fluorescent lights as our principal belowdecks light source. We have four 26-watt, dual-tube, 12-volt fluorescent fixtures on board Nada, our 39-foot sloop. These are mounted in the fore cabin, the head, the main saloon, and the engine room. Between them, they provide more than enough light to illuminate the entire boat. As supplementary lighting, we have individual incandescent reading lamps over each berth.
The choice of fluorescents as the primary lighting on a voyaging boat is an obvious one. According to General Electric, manufacturer of both fluorescent and incandescent lamps, fluorescents typically produce around 72 lumens (a measure of light output) per watt of energy consumed, whereas incandescent lights typically produce around 17.5 lumens per watt. In other words, fluorescent lights are approximately four times more efficient than incandescent lights. Or, to put another way, we can light an entire cabin with a 26-watt fluorescent unit that puts out around 1,700 lumens, while for the same power consumption we only get around 400 lumens of light output from our incandescent reading lights.
Since they produce considerably less heat than incandescents, fluorescent lights are a benefit to voyagers such as ourselves who spend most of their time in tropical climates. They also have a far greater life expectancy than incandescent lamps. “A standard Pl tube will last about 10,000 hours,” said Reese Bischoff, sales manager at Hella, Inc., a manufacturer of marine lighting. “An incandescent lasts between 400 and 500 hours.”
None of our fluorescent lights, however, have come close to this life expectancy. I must have changed the tubes in all our lights at least once, and some of them two or three times. I have also changed all the ballasts (the device that powers the fluorescent tubes) at least once. I recently discovered from Patricia Cramer, the inside sales manager at Thin-Lite Corporation (all of the fixtures on Nada are Thin-Lite units), not only that ballasts “almost never fail,” but also that our tube failure rate was way too high. I decided it was time to do some research.
How they work
The first thing to do is get a mental picture of how a fluorescent light works. There are two components: the ballast unit and the fluorescent lamp.
The ballast unit is built around a transformer that takes the incoming line voltage on an AC circuit (DC is covered below) and converts it to a higher voltage. This high voltage is fed to the fluorescent lamp, which consists of a sealed glass tube, with cathodes at each end. The cathodes are formed from coated tungsten coils. Inside the tube is a vacuum, more or less, except for the addition of a small amount of a special starting gas (argon, argon-neon, or krypton) that ionizes (becomes electrically conductive) in the presence of a high voltage. The tube is internally coated with fluorescent powder, a substance that converts ultraviolet energy into light. A few drops of mercury are also added to the mix.
When the light is turned on the ballast produces a high voltage that causes a stream of electrons to be emitted from the cathodes, generating an arc down the length of the tube that ionizes the starting gas. The ionized gas reduces the resistance in the electrical circuit, which both lowers the voltage required to maintain the arc and increases the current flow. This current vaporizes the mercury droplets and causes them to radiate ultraviolet energy. The fluorescent coating on the tube converts the UV into light.
One of the trickier parts of the process lies in striking the first arc down the tube. It can be done by increasing the voltage on the circuit to a level at which it will make the necessary long arc without additional assistance (known as an instant start unit), but unless the lamps are carefully designed, the high voltage causes the coating to burn off the cathodes, drastically shortening lamp life.
The essential initial arc can be struck with a lower voltage if the cathodes are first pre-heated. Pre-heated lamps are quite common. They can be recognized by the fact that, after the light is turned on, it takes a second or two to light up. In one type, a separate pre-heater supplies current to the cathodes until they get hot, after which the pre-heat circuit is turned off and the main (lighting) circuit is automatically turned on; in another type (many desk lamps and small kitchen-counter lamps) the switch has to be held on for a second or two to energize the pre-heat circuit, after which the switch is released, activating the light circuit. In both cases, once the lamp is in operation, the cathode temperature is maintained by the arc.
A modified pre-heat system (known as rapid start) is also popular. The ballasts have a separate winding that heats the cathodes continuously, reducing the time to start when compared to a conventional pre-heat unit, but also reducing the voltage required to start when compared to an instant-start type. Pre-heat and rapid-start units use the same bulbs; the difference between the two types lies in the method used to pre-heat the lamp. The bulbs can be identified by the fact that they have two pins at each endone for the heating circuit and one for the lighting circuit.
The electric arc that occurs inside a fluorescent tube when it is operating has what is sometimes called a “negative resistance,”as the current flow increases, the internal resistance falls. Unless there is some current-limiting device in the circuit, the ballast and lamp will rapidly burn up, so, aside from providing the high voltage necessary to make the light work, a ballast also contains a mechanism, often a simple resistor, for restricting the current flow.
DC fluorescent lights
So far I have presupposed that fluorescent lights are running on alternating current (AC). The alternating current is necessary to transform low voltages to high voltages (you can’t use a transformer on DC), and also to avoid a situation in which the “one way” current on a DC circuit would cause the mercury to accumulate at one end of the tube, with a consequent loss of light output and efficiency.
To get AC out of DC, an additional component has to be added to the ballast units on DC-powered fluorescent lights. This component is a small inverter, a device that converts DC to AC by switching the polarity of the DC input from positive to negative and back again at high speed. The inverter produces alternating current that is fed to the ballast transformer, after which the process is the same as with any other fluorescent light. (This means the lamps in the DC units are the same as in a “regular” AC unit; for years I had replacement lamps shipped from the manufacturer, not realizing that I could buy them at less expense, and with less hassle, from the local hardware store!)
On a normal household AC circuit, the polarity reverses 60 times a second (60 Hz); in the units manufactured by Thin-Lite, it reverses around 20,000 times a second. This has several significant advantages: it reduces considerably the size and weight of the transformers needed to produce the high-voltage AC output required to start and power the lamps; it eliminates the annoying flicker that is sometimes visible on 60-Hz-frequency fluorescent lights (caused by the light dimming as the polarity reverses); and, above all, it increases the efficiency of the lights by up to 18%, which more than compensates for the losses in the inverter (typically less than 10%). On the down side, the high-frequency switching causes radio frequency interference (RFI) in the AM band, with a particularly significant impact on loran performance. Fluorescent lights in or near navigation stations need to be specially screened. This must be specified when ordering the light; ask for an RFI-suppressed fixture.
The Thin-Lite ballasts, which are typical of contemporary DC fluorescent fixtures, end up with four principal components: 1) an inverter, which converts 12- or 24-volt battery power to high frequency (approximately 20,000 Hz) AC at 12- or 24-volts; 2) a transformer, which steps up this low-voltage, high-frequency AC input to the transformer into a higher-voltage, high-frequency AC output; 3) a capacitor, which is the key component in the lamp-lighting process, generating starting voltages as high as 1,500 volts (as opposed to typical running voltages between 55 and 90 volts); and 4) a resistor, which is a current-limiting device that keeps the ballast and lamp from burning up .
Temperature and voltage variations
Although this brief description of fluorescent lamp operation begs all kinds of questions, it’s adequate to describe the primary failure modes on boats and what we can do to minimize the likelihood of failure.
The Thin-Lite units are designed on the assumption that the voltage on the DC system, and at the light fixture, is 12.6 volts. However, this is rarely the case. On the one hand, a number of factors, such as discharged batteries, undersized wiring (causing voltage drop), or other heavy loads on at the same time as the lights, may cause the voltage at the fixture to fall. It is not uncommon to see operating voltages around 11 volts, a loss of approximately 13% when compared to the nominal voltage, while even lower voltages are likely to occur from time to time.
On the other hand, charging voltages from alternators, battery chargers, and so on are commonly set to output around 14.0 volts, with many of the newer, “high-capacity” devices operating at voltages of 14.4 and higher. A voltage of 14.4 at a lighting fixture is 114% of rated voltage.
The nature of the inverters and transformers in the ballasts is such that any decrease or increase in system voltage will be reflected proportionately in the final AC voltage outputted by the device, so lamp voltage will vary by the same amount. This has a proportional effect on light output (a 1% decrease or increase in voltage will produce a 1% decrease in lumen output; this is in contrast to incandescent lamps, which show a disproportionate change in lumen output). Of more significance is the fact that the lower the voltage, the lower the current that will pass through the lamp, and the higher the voltage, the higher the current.
At lower voltages the lights may have trouble striking the first critical arc. Since temperature is an important factor here, if low voltage occurs in cold conditions (which it commonly does, because batteries operate far less efficiently when cold) the voltage and current may be insufficient for proper heating of the cathodes, causing unreliable or delayed starting. The capacitor circuit in the ballast unit repeatedly produces a high voltage in an attempt to get the lamp started. The high voltage breaks down the protective coating on the tungsten filament cathodes in the lamps, considerably shortening the life expectancy of the lamp. At the same time, the repeated starting attempts overheat the ballast, reducing its life expectancy as well. These kinds of starting problems are exacerbated by cold drafts, which will also affect performance even if the lamp does get going. In extreme cases the mercury condenses on the inside of the tube (sometimes forming dark streaks along the length of the tube), and light output drops sharply. Below an ambient temperature of 65° F, light loss may be 2% or more per 1° F fall in temperature.
At higher voltages and currents, both lamp life and ballast life are once again shortened. The higher operating voltage and current breaks down the protective coating on the cathodes, while at the same time generating excess heat that burns out components in the ballast unit (for every 1% increase in voltage above the light’s rated voltage, the temperature of the ballast case rises approximately 2° F). If the high voltage occurs in a high ambient temperature, this will accelerate both lamp and ballast failure. Ironically, in spite of the higher power consumption that occurs, light output also declines. This is because higher mercury vapor pressure inside the lamp negatively impacts the release of ultraviolet radiation. Above an ambient temperature of around 77° F, light loss is approximately 1% per 2° F rise in temperature.
On a 12-volt system the lamp and ballast should not see anything below 11.4 volts or above 14.2 volts. It should also be noted that most lamps and ballasts are designed to operate at peak efficiency in an ambient temperature of around 70°F, with the assumption that the light is installed in an area free of drafts. In such conditions, the bulb temperature will be around an optimum 100° F. At lower or higher ambient temperatures, lamp efficiency, and maybe lamp and ballast life expectancy, will decline.
As soon as I found out these voltage and temperature facts, I knew we had a problem. For on our boat, as is typical of many modern voyaging boats, we converted some years ago to a high-output alternator controlled by a multi-step regulator, with the absorption phase on the regulator set to 14.4 volts (i.e., 114% of rated voltage for the fluorescents). When I wired our boat I used oversized cables to keep voltage drop to an absolute minimum, so there are almost no line losses between the alternator and the lights; when the alternator is running, the lights are seeing the full 14.4 volts. On top of this, we do much of our cruising in the tropics, where the ambient temperature inside the boat, even in the evenings when the lights are operating, may be above 80° F. It is clear that we, and many other voyagers, are regularly stressing our ballasts and lamps beyond their designed limits. Our galley light, the one that experiences the highest temperatures, has had two ballast failures and several lamp failures.
At the other end of the spectrum, many voyagers have chronic battery and charging system problems, combined with wiring harnesses that result in significant voltage losses. As a result, fluorescent lights are frequently operated at well below design voltage, drastically shortening lamp and ballast life. In fact, according to Alan Griffin, the technical wizard at Thin-Lite, this is probably the primary reason for premature failures on voyaging boats.
Elevated charging voltages and high ambient temperatures, however, do not fully explain the failure rate on our second most troublesome fluorescent light unitthe one installed in the head compartment. This light sees very little service and is installed at one of the cooler locations on the boat, in the direct path of the airflow from the forward wind scoop back into the saloon.
What I have discovered is that operation with frequent starts, followed by short burning times, appreciably shortens the life expectancy of both ballasts and lamps. This is, of course, precisely the operating conditions of a head light. Once again, the culprit is the loss of the coating on the cathodes caused by the higher voltages required to get the lamps going from a cold start. Most times, the light is not on for much more than a minute, so it never properly warms up (a process that takes up to five minutes). This kind of a regimen is a death sentence for a fluorescent lamp.
Over the years I have changed the head ballast at least twice and the tubes several times. To reduce this failure rate, we have learned to change our operating practices. If the trip to the head is just a short one, the incandescent light gets used. For longer visits, the fluorescent is turned on.
Old age and troubleshooting
Regardless of the operating environment and practices, eventually tubes and ballasts wear out. The primary failure mechanism is the slow loss of the coating on the lamp cathodes, leading to a failure of one of the cathodes. This may or may not be evidenced by a blackening of the insides of the ends of the tubes. Once the coating is lost for whatever reason (repeated starts, cold starts, over voltage/temperature, old age, etc.), the lamp becomes harder and harder to start, and as a result the ballast attempts to continuously output the high voltage necessary for starting (up to 1,500 volts) as opposed to the lower voltage needed for running (between 55 and 90 volts on most small fluorescents). The ballast then overheats and fails prematurely.
Aside from maintaining the rated voltage and ambient temperature at a light fixture, and avoiding repeated starts with little operating time, the key to protecting ballasts is to replace the lamps as soon as they begin to show signs of failure rather than waiting for them to fail altogether. In addition to the tube blackening already mentioned, signs of impending failure include longer starting times, the light operating more dimly than normal, a purplish glow at the ends of the tube when starting (the glow may only occur for a fraction of a second), and flickering of the lamp. Similar symptoms may occur in cold weather or with low voltages, so before tossing the tube, check the operating environment.
If one lamp in a dual fixture unit is acting up, the other will either go out or operate dimly. Both should be replaced; failure of the second will not be far behind the first. Since lamp performance is critical to ballast life, when replacing lamps only quality, name-brand bulbs should be used (GE, Phillips, or Osram/Sylvania; other brands may not be made to the same tight specifications and may cause ballast failure).
If a light fails altogether, the first thing to do is to switch its bulbs with those from a normally operating unit. When doing this, check the sockets for damage, replacing them if necessary (they can be bought at local hardware stores). If the light that had failed now works, but the one that was formerly working does not, it is the bulbs that are at fault. If, however, the failed light still does not work, the problem lies in the power supply or the ballast.
Check the voltage at the unit. Maybe a fuse has blown, or a breaker is tripped, or the switch is not working. Assuming the power supply is at rated voltage, and the problem persists, change the ballast (one or two spares should always be carried; note that ballasts are lamp specific and not interchangeable for lamps of different wattage). When ordering replacement ballasts, if the boat is to be operated in a hot and humid environment, such as the tropics, it is worth specifying high-humidity (HH) ballasts, in which the windings are specially sealed against moisture penetration.
Knowing what I do now, I see that we have been giving our fluorescent lights a tough time. Some of the abuse we can remedy, but the more significant abuse (overvoltage in high ambient temperatures) is harder to deal with. One way is to use the incandescent lights when the engine is running and the alternator is putting out high voltages, but it should be noted that these high voltages also significantly reduce the life expectancy of incandescent light bulbs. At 14.4 volts, a bulb rated for 12-volt use will have its life-expectancy cut to one fifth of normal. Of course, these bulbs are easier and cheaper to replace than fluorescent lamps and ballasts.
Another way to limit the damage from high voltages and temperatures would be to convert our existing 26-watt lights, which use two 13-watt lamps, to 30-watt units, using two 15-watt lamps. The difference in power consumption is minimal, while there would be a significant increase in light output. But the real reason for the change is that the 15-watt lamps are somewhat larger in diameter, which results in a different internal structure. The end result is that the lamps are more efficient while at the same time tolerant of more abuse. If installing lighting in a new boat, fixtures using 15-watt lamps are the ones to go for.
These measures would ameliorate existing problems, but the long-term answer to premature burn-out of lamps and ballasts is for the fluorescent light manufacturers to re-design their units to take account of the higher voltages that are commonly being used in boat charging systems today. n
Contributing editor Nigel Calder is the author of several books, including Boatowner’s Mechanical and Electrical Manual, published by International Marine.