Blackmail,
Isn't it the Bus Tie Relay that opens during G/S intercept, thus isolating DC Bus 1 from DC Bus 2.
According to my notes, the TR3 Xfer Rly can only switch between "Left" and "Right", thus selecting either AC Xfr Bus1 or AC Xfr Bus2 as TRU3's power source.
Also, your statement indicating that it is possible to connect external power with the transfer switch in the off position seems to contradict what NSEU stated. For this to work, BTB1/2 would already have to be in the closed position...
Not trying to sound smart just trying to understand the thing
