Power distribution, load, and motor centers

Abstract

Electric power distribution is the portion of the power delivery infrastructure, taking the electricity from the highly meshed, high-voltage transmission systems and delivers it to customers. Primary distribution lines are medium-voltage circuits, usually in the range of 600 V to 35 kV. Close to the end-users, transformers stepdown the primary distribution voltages to the low-voltage secondary distribution levels, commonly 120/240 V or other utilization voltages used mainly in industrial facilities. From the power distribution transformer, the secondary distribution circuits connect to the end-users, where the connection is made at the service entrance. Distribution infrastructure is extensive and complex, the electricity has to be delivered to customers concentrated in cities, suburbs or rural areas, industrial and commercial facilities, schools, hospitals, military bases, or communication infrastructure. In industrial and large commercial facilities, electricity is provided to the loads from specialized power distribution units and the load centers containing equipment necessary to protect, operate, and control the loads. The terms switchgear and load centers are used to describe combinations of enclosures, busbars, circuit breakers, power contactors, fuses, protective relays, controls, and indicating devices. There are several load center types, with their selection based primarily on the electrical requirements and installation environment. The essential parts of electrical distribution systems are discussed in this chapter, as well as in other sections of the book. After completing this chapter, the readers will have a good understanding and knowledge of several aspects of power distribution networks, load, and motor functionalities and requirements, such as: understanding the role, configurations and topologies of power distribution, ratings and characteristics, the purpose, basic construction, and configurations of load centers, switchgear and motor control centers, purpose, specifications of switchgear and motor control centers, their ratings, structure, and major applications. Readers will also understand the basics of circuit breaker and motor starter operation and applications, as well as protection requirements for switchgear and motor control centers, and learn about the most important provisions of standards and codes used in power distribution networks and load centers.