Electrical Distribution Systems WIRING SCHEMES AND GROUNDINGThe first scheme (Figure 15A) provides for the connection of the load from a phase leg to anyFigure 15 Three-Phase To Single-Phase Connectionsground point and is referred to as a phase-to-ground scheme. The remaining scheme (Figure15B) connects the single-phase load between any two legs of the three-phase source and isreferred to as a phase-to-phase connection. The choice of schemes, phase-to phase or phase-to-ground, allows several voltage options depending on whether the source three-phase system isa delta or wye configuration. This will be discussed in the three-phase segment of this chapter.The only approved method of wiring single-phase power is the scheme commonly referred to asthe 3-wire, single-phase Edison system. The illustration in Figure 16 depicts the use of a center-tapped transformer, with the center tap grounded, providing half voltage (120 V) connections oneither side or full voltage (240 V) across both sides.Rev. 0 Page 23 ES-15
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