I don't often disagree with you, David but this time.....
Assuming three PTs, when one phase is grounded, the voltage to ground on the ungrounded phases rises to 1.73 of the line to neutral voltage.
The two ungrounded PTs will be developing 58 Volts.
These voltages are displaced by 120 electrical degrees so the resultant voltage across the open or break will be 58 Volts.
The voltage across the grounded phase PT will drop to zero.
Each PT may be monitored separately to determine which phase is grounded.
80, 90, 100 years ago, it was common to connect lamps across the PT secondaries.
In the event of a grounded phase, the lamp for that phase goes out and the other two lamps become brighter.
Often transformer type pilot lamps were used.
The units would be selected for line to line voltage.
Bill
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"Why not the best?"
Jimmy Carter