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 mercury arc will be extinguished at about 20 percent undervoltage. The lamp life is related
inversely to the number of starts. If low-voltage conditions require repeated starting, the lamp
life will, therefore, be reduced. Excessively high voltage raises the arc temperature which could
damage the glass enclosure if the temperature approaches the glass softening point. Sodium and
metal halide lamps have similar characteristics to mercury lamps, although the starting and
operating voltages may be different.
7.2.2.6 Capacitors. The reactive power output of capacitors varies with the square of the
impressed voltage. A drop of 10 percent in the supply voltage, therefore, reduces the reactive
power output by 19 percent.
7.2.2.7 Solenoid-Operated Devices. The pull of alternating current solenoids varies
approximately as the square of the voltage. In general, solenoids are designed to operate
satisfactorily on 10 percent overvoltage and 15 percent undervoltage.
7.2.2.8 Solid-State Equipment. All solid-state devices are very sensitive to change in
voltage and temperatures with respect to time. All solid-state circuits, therefore, incorporate
adequate regulation circuitry which then makes the devices very stable.
7.3 STREET LIGHTING SYSTEMS. For many years lighting service was powered by special
circuits that could be energized only during periods when lighting was required. These special
circuits usually had many lights connected in series. They, therefore, operated at a fairly high
voltage and low current and were operated manually or by time-clock-controlled switches.
Modern circuits use photoelectric controls which have a high order of reliability at reasonable
cost. Photoelectric controls make it feasible and economical to connect streetlight luminaires
directly to area distribution. This eliminates the separate circuits and control systems.
7.3.1 Multiple-Circuits. Multiple lighting is used extensively today and is usually connected
directly to local low-voltage (120, 208, 240, or 277 V) distribution. The lights ordinarily are
controlled by photocells. For special applications they may be operated continuously or may be
switched by time clocks, pilot wires, carrier-current signals, or other means.
Lamp units used for multiple street lighting originally were incandescent bulbs ranging in size
from 100 to 1,000 watts (W). Today there is a much broader range, including fluorescent and
high-intensity discharge lamps (mercury vapor, high pressure sodium, and metal halide). All
lamps are available in a wide range of mountings designed to direct the light output in a variety
of patterns to match requirements, as well as in a choice of designs to suit the environment.
7.3.2 Series Street Lighting. Series street lighting is still extensively employed. Unlike
general distribution, it is powered by a variable-voltage constant-current system.
7-3
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