Which busses are powered with no AC power available and the Battery Master OFF?

• A. Main Battery Direct; APU Battery Direct; EMER DC Bus
• B. AC ESS Bus; Main Battery Direct; External Power
• C. ACMP 2A Pump; DC Bus; Avionics Bus
• D. Nose Gear Lights; Cabin Hum; Electrical Bay

Answer: (A)

When there’s no AC power available and the Battery Master is OFF, only the buses that have their own independent battery source stay energized. The Main Battery Direct bus stays powered because it’s connected directly to the main battery, providing essential DC power without needing AC or the Battery Master to be on. The APU Battery Direct bus remains energized through the APU’s dedicated battery connection, allowing APU-related systems to operate even with no main power. The EMER DC Bus is fed by the emergency power system (an emergency battery), designed to keep critical systems alive for a limited time during an electrical emergency. These three paths are specifically intended to preserve essential operations when normal power sources are unavailable. Other buses rely on AC power or the Battery Master being on, so they wouldn’t have power in this scenario.