Lineman's Test C Practice

Understanding how three-phase primaries are fed helps here. A delta primary system needs three separate high-voltage connections—one for each phase—so the transformer’s primary winding must be accessible at three distinct points. A transformer described as having a single primary bushing provides only one external high-voltage connection. With only one HV terminal, you can’t energize all three corners of a delta. That’s why a single primary bushing transformer isn’t suitable for a delta primary system. It can be used for single-phase service or as part of a bank where each unit handles one phase, but not as one unit connected to a delta.