Upon powering up an aircraft for pre-flight...... found complete left A/C bus dead. APU would not start to try an alternate power source.
No related faults on Bus Power Control Unit and a swap of Generator Control Units did not help.
Problem was isolated to a drained main battery. Unit only showed 1.5 VDC.
No actual description in the AMM, but from digging through the prints, it was noted that power for the Left Tie Bus Contactor "close" coil comes from a Battery Bus breaker. This control voltage goes through the Left GCU, a Auto-Land relay, and the relaxed contacts of the Left Generator Contactor.
The Right Tie Bus Contactor does not need to see battery voltage which explains why that bus was powered.
As a guess..... I'm assuming that because the left bus plays a part in standby power, the system must see a functional battery before the left side will come up.