The Fluorescent Lighting System

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Section 5: Rapid Start Fluorescent Fixtures

Most fluorescent fixtures with two or more lamps are known as "rapid start". In this system, there is no starter. Instead, the ballast keeps a low flow of current running through the filaments at all times or during the start-up period, and uses a capacitor or other techniques to start the lamp by ionizing the gas, which is another way to reduce the initial resistance of gas to a flow of electrical current.

Most of these rapid-start systems establish a brief but high voltage potential between one or both cathodes of the lamp and a metal surface in the fixture that must be within a certain distance (usually less than 0.5 inches) of the lamp along its full length. Some lamps that are formed into a "U" shape actually have a conductive metal stripe painted on the glass tubing to assist in the starting process.

By ionizing the gas, the ballast is able to get a low current flow of electricity moving through the lamp, which causes the lamp to glow dimly. However, this feeble flow of current does start heating the gas throughout the length of the lamp, and the light being produced creates even more ionization, both of which work to lower the resistance of the gas further. Meanwhile, the ballast is providing current to the filaments at each end of the lamp to further assist the starting process. Combined, all of these things work to rapidly lower the electrical resistance of the gas and encourage the lamp to draw more current and become brighter. This is why rapid start fixtures light almost instantly (but dimly), and then take a few more seconds to achieve most of their normal operating brightness.

Typical Rapid-Start Fixture Wiring Diagram

The most common types of Rapid-Start Fluorescent fixtures contain two, three or four T-12 (1.5" diameter) lamps with one ballast controlling a maximum of two lamps each.

When three or four lamps are present, two ballasts are typically used.

The wiring for the typical T-12, 40 watt two-lamp rapid-start ballast is shown below.

+---------------------------------------+ +-----=|-- --|=-----------+ | | ) FLUORESCENT LAMP ( | | | +--=|-- --|=--------+ | | | +---------------------------------------+ | | | | | | +--|--|--========METAL=FIXTURE=REFLECTOR============ | | | | | | | | | | +---------------------------------------+ | | | +-----=|-- --|=-----+ | | | | | | ) FLUORESCENT LAMP ( | | | | | | +--=|-- --|=--+ | | | | | | +---------------------------------------+ | | | | | | | | | | | | | | | | | | | | | | | | | | | | +---------------+ | | | | | | +-----YELLOW1----| |----RED1------+ | | | | | | | | | | | +--------YELLOW2----| RAPID- |----RED2---------+ | | | | | | | --GROUND-+-----------GREEN-----(| START | | | LINE | | | | aka --NEUTRAL------------WHITE------| BALLAST |----BLUE1-----------+ | MAINS | | | --HOT----------------BLACK------| |----BLUE2--------------+ | | +---------------+

Notes:

Wiring color and how the wires are to be connected may vary by ballast manufacturer, so always refer to the diagram on the ballast used and follow its instructions.

Since the entire rapid-start fixture is grounded, there may not be an actual wire connecting the ballast and reflector together. Instead, they usually make electrical contact via the metal parts of the fixture. To ensure proper starting, it is essential that there be electrical contact between the ballast and a metal surface close to the entire length of the lamps. There are also safety reasons for having the fixture and ballast properly grounded.

Typically, the pairs of red, yellow and blue wires are polarity independent and can be connected to either contact of the same end of the lamp as shown. The same color wires must be used together, i.e. both red wires must be connected to the same end of the lamp and so on. The yellow wires must be connected to ends of the lamps opposite the red or blue pair.

Rapid-Start and pre-heat fixtures frequently use the same type of lamp.

Both rapid-start and pre-heat lamps require filaments that are used in the starting process, so in almost all cases, a given rapid-start lamp can be used in a pre-heat fixture and in a rapid-start fixture. The reverse isn't always true, because the smaller straight-line lamps of 20 watts or less are used only in pre-heat fixtures.

Some Electronic Rapid-Start ballasts behave like Pre-Heat ballasts with "smart" starters.

It has been found that some "Electronic" or "Solid State" Rapid-Start ballasts behave more like Pre-heat ballasts than the traditional Rapid-Start ballast. In these models of electronic Ballasts, the filaments are initially supplied with a high current level, causing them and the ends of the lamp to glow brightly when power is first applied. This lasts for about 0.5 to 1.0 seconds and then the current flows through the length of the lamp at or near its full brightness. The initial "dim" period that standard Rapid-Start ballasts exhibit doesn't occur, so visually these can be mistaken for Pre-Heat configurations, but they are really Rapid-Start systems. In these models, the electronics appear to heat the filaments for exactly the amount of time needed to start the lamp, rather than heating the filaments for however long it takes the semi-mechanical starter to reset itself, as is done in Pre-Heat fixtures, or leaving minimal filament current on all the time, as is done in traditional Rapid-Start fixtures. The behavior difference is noticible, and may be objectionable if it is expected that some amount of light (however minimal) is provided the instant that the power is applied. (If the delayed-illumination behavior is a problem, conversion to a T-8 lighting system with an Instant Start Ballast and T-8 lamps may be desired. See Section 6 for information on the Instart Start systems.)

Rapid-Start fixtures consume power even when the lamps have burned-out or are missing.

Because of the design of a Rapid Start fixture, the fixture will consume power if the lamps are unable to start because one or more of the lamps have been removed or rotated so that it does not make electrical contact with the socket. This is because the ballast will consume power trying to start dead or electrically missing lamps. The worst case situation is to have at least one lamp still in place, as the cathode filaments will continue to receive current for heating the filaments in an effort to get all the lamps going.

This may seem like an insignificant point, but many commercial organizations have removed lamps from thousands of rapid-start fixtures under the false assumption that they are saving electricity. In fact, they may be using just as much power (or more) than if working lamps were installed, and they may be violating OSHA safety rules, exposing the organization to legal liabilities.

This is not a new misunderstanding of how these fixtures work. During the 1974 Energy Crisis, the Fort Worth Independent School District (like many other organizations nationwide) removed every other pair of lamps in the fluorescent light fixtures in their schools, leaving students and instructors with gloomy hallways and under-lit classrooms, while the still-powered fixtures continued to consume and waste electricity with absolutely no benefit. The removed lamps in the Fort Worth ISD were not re-installed until 1975, probably after they noticed it wasn't helping on the power bill as much as expected, plus the teachers and students were complaining.

Subsequently, in situations where the light is not needed or you are trying to conserve energy, you must disconnect the electricity supply to rapid start fixtures rather than just remove or "unscrew" the lamps.