Open Delta is a 3 phase transformer bank using 2 transformers. In essence one side of the secondary Delta is physically missing, however an electrical measurement is still possible across the missing side and so it is 3 phase. It will phase in with a closed delta bank. It is not as efficient as a 3 transformer bank, but saves one transformer. It also can save one conductor on the primary, whether using Wye or Delta primary, which can be a great savings as well. A lighting connection can be used in an open Delta bank. An open Delta connection can be used in an emergency when one transformer of three in a Delta bank is inoperable or burnt out.
No such hook up in a Y secondary is possible, always use three transformers.
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