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.
As we stated earlier, a number of light systems are in use today, such as streetlights, floodlights, and security lights. These systems are either series or multiple (parallel), depending on how they are used and the equipment available.
The series circuit is supplied by a regulating transformer that gives a constant current, usually of 6.6 amperes, to the lighting circuit. If a higher amperage is required, autotransformers are available for stepping up the current to 15 or 20 amperes. This higher amperage permits the use of more rugged lamp filaments that give longer life for lamps of equal candlepower and higher lamp efficiency.
The multiple (parallel) circuit consists of a number of streetlights supplied by a distribution transformer, delivering a constant low voltage to a circuit or secondary main that also supplies other loads; however, running secondary conductors any great distance to supply a parallel connected lamp or a group of lamps is impractical because of the excessive voltage drop.
The cost of the multiple luminaire is low compared to the series type because the low voltage allows for the elimination of other luminaire accessories. This saving is largely offset, however, by the increased requirement for control devices and the copper wire cost. Lamp life and efficiency are comparatively low, and the illumination is not as uniform as in a series circuit.
In choosing a system, here are a few suggestions that may aid in your selection.
If the total wattage of the circuit exceeds 2 kilowatts or more than 15 lights, consider a series lighting system.
When extending an existing system, use the existing circuit.
If low-voltage capacity exists at the proposed location, use a multiple system, even though the load exceeds 2 kilowatts.
When several small lights are to be spaced rather far apart and no low-voltage secondary exists along the route, use the series system regardless of the load size or the number of lights.
When estimates show that one type of system will save money and time, use the more economical system.
Let us consider a series streetlight system. The power for the circuit will be supplied from the base primary distribution lines, through fuse cutouts, to an oil switch, and from the oil switch to a constant-current regulator (fig. 6-16). The constant-current regulator will supply power to the series loops and, thus to the individual lamps. While the current (normally 6.6 amperes) remains constant, the voltage of the circuit is equal to the sum of the voltages of all the lamps plus the voltage drop in the wire. With enough lamps connected in series, the circuit can become a high-voltage circuit.
The series circuit is easily controlled, but any break, such as a burned-out filament in a lamp, interrupts the
Figure 6-16. - Series street-lighting circuit.Continue Reading