Sunday, July 29, 2012

Lighting Loads

Included under lighting are incandescent and fluorescent lamps,
neon lights, and mercury vapor, sodium vapor, and metal halide lights. Nominal voltages specified for lighting are usually 120, 240, and 277 Volts (variations may exist from the base 120-V value, e.g., 115 and 125 V). All operate with dc or single-phase ac; the discussion will be in terms of ac, with comments concerning dc operation where applicable.

Incandescent Lighting

Incandescent lamps operate at essentially unity power factor.Their light output drops considerably at reduced voltage, being some
16 percent less with a 5 percent lowered voltage, and decreasing at a geometrically faster rate from then on. They are also sensitive to sudden rapid voltage variations, producing a noticeable (and annoying) flicker at variations of as little as 3 Volts (on a 120-V base). Street lighting of the incandescent type can be operated in a multiple or a series fashion. The former operates as other lighting in a multiple or parallel circuit, while the light output for the series type depends on the amount of deviation from the standard value of current flowing through it (usually 6.6, 15, or 20 A); it is sensitive to variations of as little as 1 percent in the value of the current. The life of incandescent lamps is considerably reduced at voltages appreciably above normal.

Fluorescent and Neon Lighting

Fluorescent lamps and neon lights operate at power factors of
about 50 percent, but usually have corrective capacitors included so
that, for planning purposes, they may also be considered to operate at 100 percent or unity power factor. Their light output, per unit input of electrical energy, is considerably greater (25 percent or more) than that of a similarly rated incandescent lamp. The life of fluorescent lamps and neon lights is affected by the number of switching operations they undergo. If fluorescent lamps are used on dc circuits, special auxiliaries and series resistance must be employed; operation is inferior to that on ac, with much less light produced per unit of energy and rated life reduced 20 percent. Neon lights are not usually employed on dc circuits.
Fluorescent lamps, neon lights, mercury and sodium vapor, and metal halide lights may, if improperly installed or when deteriorating, cause radio and TV interference.

High-intensity Vapor Lighting

Mercury vapor (high pressure) and sodium vapor (high and low
pressure) and metal halide lights operate at power factors of 70 to 80 percent, but also are associated with capacitors to raise the effective value to 100 percent. They are not as susceptible to voltage variations as are incandescent lamps. Their light output and life expectancy are greater than those for fluorescent lamps. They may be employed on dc circuits, but require additional starting auxiliaries. They are generally restricted to applications where large amounts of lighting are desirable, such as on expressways, in large manufacturing areas, or in photographic work;they are somewhat more expensive than other types and have the disadvantage of taking some time after being energized before maximum light output occurs.

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