High-Intensity-Discharge System Troubleshooting 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:

FIXTURES There are fixture configurations to meet almost any lighting requirement or design. While the basic purpose of the fixture is to hold and prevent damage to the lamps and lamp sockets, the fixture also helps direct the light beams into the lighting patterns desired. The fixture, with its reflector and lens, determines the quality of the light being produced. Reflectors can either concentrate or diffuse light rays, and the lens can pass or refract light rays. Quite often, the lens may be used to do both from one light source; that is, part of the light rays are refracted to produce a soft, even spread of light in the outer part, while the light rays are concentrated in other areas of the lens to produce a bright, hard light at a specific area. Some streetlight fixtures are examples of this. The sides of the lenses produce a general diffused lighting to prevent blinding automobile operators and, at the same time, they produce a bright light pattern below the lamp along the curb.

Flood or security lighting fixtures may be either open or enclosed. The open fixtures provide higher maintained efficiency and more accurate beam control. The open fixture will, under some conditions, require a "hard glass" bulb to prevent bulb breakage.

Most fixtures will have provisions for mounting ballasts (transformers) within the fixture and will provide protection for the ballast. In some cases, particularly in light pole lighting, the ballasts may be mounted in the pole base and not mounted in the fixture. Several methods of fixture attachment are possible and should be considered when fixtures for a particular job are ordered The location and job determine whether the fixture is suspended, bracket-mounted, or arm-mounted. Most brackets can be attached either to wood or metal support structures. In either case, the fixture should be firmly attached to the structure so that precise aiming for light distribution can be made.