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4 Three-Phase Circuits and Residential Wiring

4.1 Three-Phase Source Voltages and Phase Sequence

4.2 Balanced Three-Phase Loads

4.3 Measurement of Power

4.4 Residential Wiring and Safety Considerations

4.5 Learning Objectives

4.6 Practical Application: A Case Study—Physiological Effects of Current and Electrical Safety

Problems

We shall conclude Part 1 of this book on electric circuits with a treatment of three-phase circuits,

an aspect of circuit theory important to the discussion of electric machines and to the bulk

transfer of electric energy. After explaining the phase sequence of three-phase source voltages,

balanced three-phase loads and power in three-phase circuits are considered. Then the elements

of residential wiring, including grounding and safety considerations, are presented. The chapter

ends with a case study of practical application.

The three-phase system is by far the most common polyphase system used for generation,

transmission, and heavy power utilization of ac electric energy because of its economic and

operating advantages. An ideal three-phase source generates three sinusoidal voltages of equal

amplitudes displaced from each other by an angle of 120° in time. The voltages generated by

the giant synchronous generators in power stations are practically sinusoidal with a frequency

of 60 Hz in the United States, or 50 Hz in the United Kingdom and many other countries. Even

though voltages and currents are sinusoidal, the power delivered to a balanced load is constant

for a three-phase system. The three-phase scheme of power transmission offers the advantages of

using the ac mode, constant power flow, and high power transfer capability.

4.1 THREE-PHASE SOURCE VOLTAGES AND PHASE SEQUENCE

The elementary three-phase, two-pole generator shown in Figure 4.1.1 has three identical stator

coils (aa′,bb′, andcc′) of one or more turns, displaced by 120° in space from each other. The

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