WIRING SCHEMES AND GROUNDING Electrical Distribution SystemsFigure 18 4-Wire Delta SystemThe single-phase voltage on each side of the half-tap is one-half the voltage available in thenormal phase-to-phase relationship. This provides the same half- or full-voltage arrangement seenin the normal Edison scheme with a grounded neutral. Notice also that the legs coming from thecorners of the Delta would have a normal ungrounded appearance if it were not for the centertap of one phase. Thus, at any given location in the system, either three-phase power at fullvoltage or single-phase power with half or full voltage is equally possible. However, there areseveral strict precautions that must be observed in the operation of this system. First, all loadsmust be carefully balanced on both the single-phase and three-phase legs. Second, because thevoltage between one leg and the grounded neutral is considerably higher than the rest of thesingle-phase system, a measurement between the neutral and the phase must be taken to identifythe "high leg," or "bastard voltage." Last, the "high leg" is never used as a single-phase sourcebecause no ground or grounded neutral exists for this circuit.4-Wire,Three-PhaseWyeSystemUntil now, the voltage, the phase voltage, and the ground voltage of the three-phase systems havebeen equal, with the one exception of one phase of the corner-grounded Delta. The Wye systemhas completely different voltage characteristics from the Delta system. In the Wye system, theground voltage or voltage available from phase to ground is the phase voltage divided by 1.73.ES-15 Page 26 Rev. 0
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