Auto change over with 2 incomer and Bus Coupler scheme.

In auto change over scheme with 2 incomer (Normally closed) and Bus coupler (Normally open), why we used Line PT under voltage signal to initiate auto change over. Can't we use Bus-PT under voltage to initiate Auto change over?
Let's call Incoming Breakers, 51-1 & 51-2, and the Tie-Breaker 24 !
1) Are 52-1 and 52-2, tripped by operation of any protective device upstream of them ?
2) Are 52-1 & 52-2 tripped by Voltage-loss devices ? If so, there should be 2-types, one Instantaneous, and the other delayed !
3) Are 52-1 & 52-2 tripped by Instantaneous and Delayed Overcurrent devices respectively ?
4) Is the system Grounded/Earthed ? Solid or Impedance connected ?
5) Are the sources synchronized ?
6) A schematic Diagram would be helpful !

Regards, Phil Corso
The answer to your specific question is:
Using a BUS-PT rather than an INCOMING-LINE-PT prevents knowing the condition of SOURCE & LINE VOLTAGE before attempting reclosure or transfer !