Hi.
First of all - failure in a distribution transformer is lethal. The local power company had such a failure in a transformer in a nearby transformation station (big concrete bulding). That explosion was so powerful that the whole first floor was literally lifted up.
If the goal is to prevent shortcut in case of three phase transformer failure , measuring just the voltages is far from enough, and probably would not help a bit. You need to measure current at primary side and current at secondary side, and then compare. If the transformer is about to have a failure (internal current flow between broken windings) there will be a difference between input current and output current (I assume you use current transformers with a ratio adapted to the specific transformer, and have taken into account the transformer power loss), and the meassuring device will be wired to a circuit breaker to prevent a fatal incident.
There is another thing you must take in account, that is how the transformer is connected. A very common scheme is Dyn11, wich means that there will be a phase shift between secondary and primary side, and if one phase at primary side is disconnected, yoy'll still have around half the rated voltage at secondary side.