Overhead transmission line voltage
The delta transformer is galvanically isolated from ground.
If both connected phases are neither connected to ground (e.g. via incorrect installation or defective device etc.), there is no potential difference between the disconnected phase and ground - even with the phase connected, there is no potential difference - in other words, the ground is not involved at all in this circuit, unless there are no other unshown elements with connections to ground. 

In the other case with the grounded star, any unshown device that is connected to one of the 2 or both uninterrupted phases and the interrupted phase could produce dangerous voltages at the "disconnected" phase.

That is why e.g. in Europe the power transmission lines are switched off alltogether (all 3 phases), even if there is a problem with 1 phase only , i.e short, undervoltage, load limit etc.