So we talk about a" 240 volt delta 3 phase" converted to "one with high leg " delta true?
Whether there's a high leg depends on which conductor is grounded. If the center of one secondary is grounded, the opposing line is the high leg. That's what I described above.
Otherwise, either any line is grounded, creating a corner-grounded Delta; or no lines are grounded, and you have a floating Delta, which requires ground-detection.
so you dont use the other conductor of the 3 phase delta ?
Not for line-to-neutral loads, nor for line-to-line loads that require the neutral. Line-to-line loads that do not use the neutral can use any two lines, even the high leg.
So why delta breaker is manufactured and why was banned ?
Imagine an existing 120/240v 1-ph service like your home has, and you want to add a 3-ph motor. You have two lines, so the POCO must add one more, either one more transformer and supply an open Delta, or two and supply a full Delta.
The meter can would be swapped out for a 3-ph one, and the additional line added to the service entrance, and run directly to the "in" terminal on a Delta breaker, which has four bodies, like a 3-ph shunt-trip breaker.
The original two poles of the breaker receive power from the stabs as usual, and the third pole is fed via a terminal on the fourth body. The back of the breaker has two stab clips and two empty holes.
So, even though the existing service has only two lines, a 3-ph load could be supplied from the original panel. It probably became illegal because throwing the main did not deenergize the third phase.