Figure 6-15. - Mercury-lamp ballast circuits.
Ballasts are classified into three major categories depending on the basic circuit involved: nonregulating, lead-type regulating, and lag-type regulating. Each type has different operating characteristics.
HID lighting systems include the power supply system (wiring, circuit breakers, and switches), lighting fixture (socket, reflector, refractor or lens, and housing), ballast, lamp, and frequently a photoelectric cell to turn on the fixture at dusk. When an HID system does not operate as expected, the source of the problem can be in any part of the total system.
It is important to understand normal lamp-failure characteristics to determine whether or not operation is abnormal. All HID lamps have expected lamp-failure patterns over life; these are published by lamp manufacturers. Rated life represents the expected failure point for one third to one half of the lamps, depending on the lamp type and the lamp manufacturer's rating.
The end-of-life characteristics vary for the different HID lamps of the following types:
1. Mercury. Normal end of life is a nonstart condition or low-light output, resulting from blackening of the arc tube that is due to electrode deterioration during the life of the lamp.
2. Metal halide. Normal end of life is a nonstart condition, resulting from a change in the electrical characteristic when the ballast can no longer sustain the lamp. Lamp color at the end of life will usually be warmer (pinker) than that of a new lamp due to arc-tube blackening because of changes in thermal balance within the tube. The lamp manufacturers' recommendations regarding metal-halide lamp enclosures should be reviewed.
3. High-pressure sodium. Normal end of life is on- off cycling. This results when an aging lamp requires more voltage to stabilize and operate than the ballast is able to provide. When the normally rising voltage of the lamp exceeds the ballast output voltage, the lamp is extinguished. Thea, after a cool-down period of about 1 minute, the arc will restrike and the cycle is repeated. This cycle starts slowly at first and then increases in frequency if the lamp is not replaced. Ultimately, the lamp fails because of overheating of the arc-tube seal.
There are four basic visual variations in the lamp of a HID lighting system that indicates when a problem may exist: (1) the lamp does not start, (2) the lamp cycle is on and off or is unstable, (3) the lamp is extra bright, or (4) the lamp is dim. The following table indicates the most likely possible causes for each of these system conditions.Continue Reading