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
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