WIRING SCHEMES AND GROUNDING
Electrical Distribution Systems
Figure 18 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 provides the same half- or full-voltage arrangement seen
in the normal Edison scheme with a grounded neutral. Notice also that the legs coming from the
corners of the Delta would have a normal ungrounded appearance if it were not for the center
tap of one phase. Thus, at any given location in the system, either three-phase power at full
voltage or single-phase power with half or full voltage is equally possible. However, there are
several strict precautions that must be observed in the operation of this system. First, all loads
must be carefully balanced on both the single-phase and three-phase legs. Second, because the
voltage between one leg and the grounded neutral is considerably higher than the rest of the
single-phase system, a measurement between 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
because no ground or grounded neutral exists for this circuit.
4-Wire, Three-Phase Wye System
Until now, the voltage, the phase voltage, and the ground voltage of the three-phase systems have
been equal, with the one exception of one phase of the corner-grounded Delta. The Wye system
has completely different voltage characteristics from the Delta system. In the Wye system, the
ground voltage or voltage available from phase to ground is the phase voltage divided by 1.73.
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