What does an illuminated fault light on the Auto Xfer switchlight indicate?

• A. AC bus 1 or 2 has a complete power loss
• B. The generator is producing excessive power
• C. The APU is inoperative
• D. Auto Xfer was inhibited due to a fault on AC Bus 1 or 2

Answer: (D)

An illuminated fault light on the Auto Xfer switch means automatic transfer between AC buses has been inhibited because a fault has been detected on AC Bus 1 or AC Bus 2. The Auto Xfer system monitors the health of both buses, and when a fault is found, it prevents automatic crossfeed to avoid energizing a faulty bus or spreading a problem. This light signaling the inhibition is the crew’s warning that auto transfer cannot be relied upon until the fault is cleared. Other indicators might appear for generator or APU status, but they are not what this fault light denotes.