B737-800 DC electrical System
 

Hi, one quick question about the TR3 even though i am aware that is not pertinent to the topic of this specific video: on the FCOM of the 737-800 it states that the TR3 serves as a back up source should either TR1 or TR2 fail, while in the DC section it states that all TR 1 2 and 3 power the DC BUS 1, 2 and the DC STBY bus. Now under normal conditions with the cross bus tie relay closed it is my understanding the TR1 and 2 power the DC BUS 1 and 2 and the DC STBY bus via the DC stby bus normal relay, sharing the load, while the TR3 powers the DC BATTERY BUS via the battery bus normal relay, while in the event of either TR 1 or 2 failure the TR3 will assist the remaining TR to power the DC BUS 1 and 2 and the DC STBY bus while still powering the DC BATTERY bus. Is that correct or is the TR3 supplying the DC BUS 1 and 2 and the DC STBY BUS together with the TR1 and 2 even during normal operations? Thanks

TR1 and TR2 share the load and the amperage shown should be same, while TR3's amperage should be lower. The cross bus tie relay opens at GS intercept (in normal ops). If you had TR1 failure, the instruments on the captains side would show several failure flags, CMD A and F/D are unavailable. That's why the TR UNIT says not to use approach mode.