Figure 6-19.Multiple street-lighting circuit.
high-mechanical strength overcomes the difficulty.
Conductor sag should be the same as for primary
Constant-current regulators should be protected on
overhead circuits by lightning arresters on both the
primary and secondary sides.
The multiple streetlight system uses a distribution
transformer of the proper size as service equipment.
(See fig. 6-19.) Notice that the transformer is fed
directly through fuse cutouts from the base primary
distribution system. The control for the circuit is
connected into one line of the secondary side. The
selection of output voltage of the transformer depends
on the voltage required for the individual lamps that are
installed. Depending on the types of lamps selected,
this voltage may be from 120 volts to 480 volts. You
must know the type of lamp that will be used in the
circuit before you can properly select the transformer to
feed the streetlight system.
COMPONENTS AND CONTROLS
There are many components required to construct
an area lighting system. These include constant-current
transformers, relays, controls, fixtures, wiring, and
lamps. Controls can be manual, automatic, or a
Figure 6-20.Method of relieving slightly overloaded regulators with a distribution or booster transformer.
The constant-current transformer, usually called a
regulator, has a movable secondary winding that
automatically changes position to provide constant
current for any load within its full-load rating. The
balance point between coil weight and magnetic force
may be adjusted to provide the desired output current.
A moving-coil regulator is recommended because
of the close regulation required for streetlighting work.
It consists of a fixed primary coil and a movable
secondary coil on a laminated core. Voltage applied to
the primary winding causes voltage to be induced in the
secondary winding. When the secondary circuit is
closed, the magnetic field in the secondary reacts with
the primary-coil field to push the movable coil up. The
balance point between coil weight and magnetic force is
designed to provide the desired secondary current
(usually 6.6 amperes).
On most existing installations, the constant-current
regulator is of the outdoor type. Three main types of
installation are used for these regulators: two-pole
platform, timber or steel construction single-pole
platform, and pole mounted. Any regulator larger than
20 kilovolt amperes should be mounted on a platform.
Constant-current regulators should be loaded as
near 100 percent as possible since both efficiency and
power factor are best at this load. Specifications of the
American Institute of Electrical Engineers (AIEE)
require constant-current transformers to deliver the
rated secondary current at lo-percent overload. A
larger size regulator should not be installed before this
10-percent overload is reached. When larger regulators
must be installed and are not readily available, a booster
transformer may be used with its secondaries connected
into the series street circuit and its primaries connected
to the primary feeder supplying the regulator (fig. 6-20).