Al, ignore the voltage to ground for a moment.

In a Delta system, there are three transformer secondaries that each have 240v between the end trerminals. Connect them in a triangle, and you have three phase wires.

Until you ground a wire somewhere, the voltages are "floating" relative to ground, but there is always 240v between any two phases.

If you ground one of these hot wires, you have a "corner-grounded" Delta. There is still 240v between any two phases, but now there is also 240v between either of the two non-grounded phases and ground.

Now, let's say that one of these three 240v secondaries has a center tap that is grounded (instead of one phase), exactly like the 1-phase transformer feeding your house.

Now, you will have 120v between these two phases and ground, but the third phase will have 208v to ground. This is due to the 1-1/2 secondaries (240 and 120), "adjusted" by the 120 deg. between phases.

This "adjustment" accounts for why there is 208v between phases on a Y system, instead of 240v. Because the phase angle is greater than 90 deg., the net result of adding the 240v and 120v is a subtraction.

The point is that, to a 3-phase load, it doesn't matter whether, or where, any wire of the system is grounded.

Larry Fine
Fine Electric Co.
fineelectricco.com