4-Wire, Three-Phase Delta System

Electrical Distribution Systems WIRING SCHEMES AND GROUNDING The upper diagram depicts the Figure 17 3-Wire, Three-Phase Delta Scheme ungrounded Delta, normally confined to protected environments such as fully enclosed ducts or overhead transmission lines that cannot b e r e a c h e d w i t h o u t extraordinary means. Each conductor’s ground voltage is equal to the full phase voltage of the system. The lower diagram shows a ground point affixed to one corner of the Delta, which effectively lowers one phase’s voltage reference to ground to zero, but retains a phase-to- phase voltage potential. The corner-grounded phase acts in much the same way as the grounded neutral of the single- phase Edison system, carrying current and maintaining ground potential. The corner-grounded Delta system has an obvious economy in wiring costs, and the grounded phase can be used to physically protect the other two phases from accidental grounding or lightning strikes in outdoor settings. This system is rarely used for low voltage (under 600 V), however, because of the absence of a safety ground required by many facilities for circuits involving potential worker contact. 4-Wire, Three-Phase Delta System The 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 in the example illustration in Figure 18, one side of the Delta has a grounded-neutral conductor connected to a center tap winding on one phase. Rev. 0 Page 25 ES-15