Figure 6-19.Multiple street-lighting circuit.
high-mechanical strength overcomes the difficulty. Conductor sag should be the same as for primary distribution.
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.
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 combination.
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).Continue Reading