Electrical Distribution Systems WIRING SCHEMES AND GROUNDINGThe upper diagram depicts theFigure 17 3-Wire, Three-Phase Delta Schemeungrounded Delta, normallyconfined to protectedenvironments such as fullyenclosed ducts or overheadtransmission lines that cannotb e r e a c h e d w i t h o u textraordinary means. Eachconductor’s ground voltage isequal to the full phase voltageof the system.The lower diagram shows aground point affixed to onecorner of the Delta, whicheffectively lowers one phase’svoltage reference to ground tozero, but retains a phase-to-phase voltage potential. Thecorner-grounded phase acts inmuch the same way as thegrounded neutral of the single-phase Edison system, carryingcurrent and maintaining groundpotential.The corner-grounded Deltasystem has an obviouseconomy in wiring costs, andthe grounded phase can beused to physically protect theother two phases fromaccidental grounding or lightning strikes in outdoor settings. This system is rarely used for lowvoltage (under 600 V), however, because of the absence of a safety ground required by manyfacilities for circuits involving potential worker contact.4-Wire,Three-PhaseDeltaSystemThe 4-wire, three-phase Delta system combines the ungrounded Delta discussed above for three-phase loads with the convenience of the Edison system for single-phase loads. As depicted inthe example illustration in Figure 18, one side of the Delta has a grounded-neutral conductorconnected to a center tap winding on one phase.Rev. 0 Page 25 ES-15
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