4-Wire, Three-Phase Delta System:

The 4-wire, three-phase Delta system merges the ungrounded Delta discussed above for threephase loads along with the convenience of the Edison system for single-phase loads. Since depicted in the instance describe in Figure, one side of the Delta has a grounded-neutral conductor linked to a center tap winding on one phase.

Figure 4-Wire Delta System

The single-phase voltage on each side of the half-tap is one-half the voltage available in the normal phase-to-phase relationship. This gives the similar half- or full-voltage arrangement seen in the general Edison scheme along with a grounded neutral. Remember also in which the legs coming from the corners of the Delta would have a general ungrounded appearance if it were not for the center tap of one phase. Therefore, at any given location in a system, either three-phase power at full voltage or single-phase power with half or full voltage is equally possible. Therefore, there are various strict precautions that must be observed in the operation of this system. First, all loads have to be carefully balanced on both the single-phase and three-phase legs. Second, since the voltage among one leg and the grounded neutral is considerably higher than the rest of the single-phase system, a measurement among the neutral and the phase must be taken to identify the "high leg," or "bastard voltage." Last, the "high leg" is never used as a single-phase source since no ground or grounded neutral exists for this circuit.