Mike — On the service-head open neutral, the transformer would still remain at “120+120V” on Aø and Bø. {For simplicity’s sake leave out Cø at the moment.} If the service-neutral connection opens up, and [most likely] the 120V phase-to-neutral loads will not be equal on Aø-N and Bø-N, causing “neutral” at the building to shift {or float/drift} making voltages-to-building ground something more like “140+100 volts” while the transformer is happy serving “120+120V.”
—Bjarn’
