Street lighting may be supplied from either series or multiple circuits.
Series circuits and lamps have been extensively used in the past
and many such systems are still in operation. Multiple supply, however,
has been used almost exclusively in later installations, especially with
the development of economical light-sensitive control devices, which has
made practical the connection of such lighting to the same secondary
mains that also supply other types of customers.
Multiple Circuits
Multiple street lighting is usually served from secondary mains
at normal utilization voltages. The secondary may be a separate circuit
serving only street lighting loads, or part of the secondary circuit supplying
other customers. The former may have controls exclusive to the
circuit, while the latter may have individual controls for individual
lamps or groups of lamps. As mentioned earlier, such circuits may be
controlled from a variety of relays, some actuated from series street
lighting circuits. Photocells, activated by the light intensity of the ambient,
and connected between the secondary supply and the streetlight,
have proven economical and desirable, as they simplify controls, eliminating
additional wires and improving the appearance of overhead
lines.
The principal advantage of multiple supply is the use of the secondary
distribution systems, with only minor effects on transformer
load and voltage conditions; additional advantages include greater
safety because of the low voltages involved as well as lower handling
and installation costs, as the equipment used is essentially that in use
on distribution circuits. In most instances, separate lightning or surge
arresters are not required.
Series Circuits
As the name denotes, lamps in the series type of circuit are connected
in series and supplied from a constant-current transformer; the
usual current output rating is 6.6 A, with the outgoing voltage varying
with the load connected. The light output for different-sized lamps at
this fixed current rating depends on the length of the filament; for larger-
size lamps operating at 15 or 20 A, the filaments are also of heavier
cross section in order to carry the greater current safely. Series lamps
often are rated in lumens, the unit of light flux, while the wattage may
vary with the size of the lamp. Lamp efficiency is expressed in lumens
per watt, with 600-lm lamps operating at 6.6 A requiring about 43 W
while 6000-lm lamps require approximately 320 W. Light efficiency is
further increased with reflectors and refractors.
Advantages of series circuits include high efficiency for a widely
distributed load, the ability to use a high-voltage and low-current supply
that permits long lengths of relatively small-size conductors to be
used with low loss and small voltage drop, and the ability to keep the
variation in light intensity at a minimum because of the constant current
value at each lamp. Disadvantages mainly center around the need
for special transformers and control devices, as well as the need for
separate lightning or surge arresters at the transformer, the switches,
and certain points along an extended circuit.
Constant-current Transformers
To obtain a constant current output from an essentially constant
voltage supply, the two coils of the transformer are so arranged that the
distance between them may be varied. The primary coil, receiving power
at a constant voltage, is fixed in position. The secondary coil is movable
along the common core; its position depends on the load. The secondary
coil is balanced by a weight that reacts proportionally with the repulsion
forces to maintain its output at the rated value.
When the two coils are close together, the magnetic field or flux
produced by the primary interlinks maximally with the secondary coil,
inducing a maximum secondary voltage. The coils act as magnets and
tend to repel each other. As the load on the secondary decreases, the
current in the secondary will tend to increase, increasing the repulsion
between the coils. The secondary coil is raised, decreasing the magnetic
field or flux linking the two coils, thereby decreasing the secondary voltage,
and the current is dropped to its rated value.
The majority of series street lighting circuits are operated without
grounds. When an accidental ground occurs, the circuit is generally not
affected. When a second ground occurs, the lamps between the two
grounds will go out or burn dimly, giving ready indication of the location
of the grounds. In some instances, however, an intentional ground
is placed at mid-circuit. This not only reduces the normal stresses on the
insulation of the several units in the circuit to one-half the voltage of the
ation. For purposes of comparison, design voltage-drop limits are determined
beforehand, and these same limits are used in all of the studies.
Series Lamps
Series lamps are usually rated at 6.6 A. Larger lamps, generally
4000 1m or larger, are supplied at 15 or 20 A through transformers that
supply an individual lamp (referred to as IL transformers) or transformers
that supply several lamps (referred to as SL transformers).
Lamp failure in a series circuit causes a momentary opening of the
circuit and a high voltage at the terminals of the lamp. A film disk cutout,
connected between the terminals of the
lamp, normally acting as an insulator, breaks down and short-circuits the
failed lamp.
Control Devices
Series circuits may be controlled from the substation manually or
through time switches that automatically turn them on and off at predetermined
times; one such switch operates in accordance with the hours
of sunset and sunrise, and is called an astronomical time switch.
