Old school, pole mounted transformers.
There were four cutouts on the cross-arm.
When a bank was taken out of service locally and was to be re-energized, a fuse would be installed in the neutral cutout.
With the neutral connected, the three phase line fuses would be installed.
Then the fuse would be removed from the neutral cutout and taken away in the line truck.
Steve, are you floating the primary wye points?
We had a customer In Canada with a 480 Volt service.
They were a branch of an Ontario company where 600 Volts was the norm.
We got a lot of 600 Volt equipment to install.
We had small open delta auto-transformers installed beside the 575(600) Volt motors.
Then they moved to a new plant.
600 volt service was now the utility standard.
The power was incoming to a power distribution center at 600 Volts. On the end of the PDC was a section with a pair of open delta auto-transformers stepping down to 480 Volts.
Have you considered suggesting to your customers a 120/208 Volt wye system with a pair of auto-transformers feeding a 240 Volt three phase equipment panel?
Then you don't have to worry about the issues with a lost phase on a wye/delta bank.
Bill
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"Why not the best?"
Jimmy Carter