DISTRIBUTION TRANSFORMERS Distribution transformers are installed in the vicin-ity of each customer to reduce the voltage of the distri-bution circuit to a usable voltage, usually 120/ 240 volts. 

DISTRIBUTION CIRCUITS Distribution circuits (primary main circuits) are circuits that originate from the distribution substation.

Primary mains are circuits carrying over 600 volts, but generally they operate between 2,400 and 34,500 volts. Primaries can be found in single-phase or three-phase configurations and generally operate as three-phase, three-wire or three-phase, four-wire circuits. Two types of primary circuits are in use today. One, the delta type (A) system, (fig. 4-29) is used when most of the load in

Figure 4-29.- Three-phase delta (A) primary and secondary system.

an area is commercial, consisting of motors and other three-phase equipment. The other type of circuit is called the wye (Y) system (fig. 4-30) and is used primarily for residential use.

SECONDARY CIRCUITS Secondary circuits (secondary mains) are circuits that originate from the secondary windings of a distribution transformer and are 600 Vac or less. The secondary circuits are also configured either delta (A) (fig. 4-29) or wye (Y) (fig. 4-30) and are also used for the same type of loading as the primary circuits. Secondary circuits are either three phase- meaning those with three live conductors- or single phase, which can be one live conductor and a neutral or two live conductors and a neutral.

SERVICE DROPS A service drop is the combined conductors used to provide an electrical connection between a secondary distribution circuit and a user's facility. There are different ways of installing the service drop. Some typical secondary racks used to install service drops are shown in figure 4-31. Whether using the racks with individual conductors or self-supporting service cable, known as triplex or quadraplex, to provide a service drop, you must maintain a minimum aboveground distance. Aclearance of 12 feet over lawns or walkways accessible to pedestrians and 18 feet over roads or alleyways subject to truck traffic as recommended by the NEC(c), as shown in figure 4-32. When the nearest distribution pole is over 125 feet from the facility to be connected, an intermediate support pole must be provided.

CONTROL AND PROTECTIVE DEVICES A power distribution circuit, like any other electrical circuit, requires the use of special devices to provide control and to protect the system from internal or external influences that may damage the circuit.

DISTRIBUTION CUTOUTS A distribution cutout provides a high-voltage mounting for the fuse element used to protect the distribution system or the equipment connected to it.

Figure 4-30.- Three-phase wye (Y) primary and secondary system.

Figure 4-31.- Secondary racks and dead-end spools.

Figure 4-32.- Minimum ground clearances of service drops

Figure 4-33 shows an open types of distribution cutout being closed. Distribution cutouts are used with installations of transformers, capacitors, cable circuits, and sectionalizing points on overhead circuits.