A fault on which buses would cause the Auto Xfer to be inhibited, as indicated by the fault light?

• A. AC bus 1 or 2
• B. DC bus 1 or 2
• C. AC power fault on bus 1
• D. External power source

Answer: (A)

Auto transfer relies on healthy AC buses to switch sources without spreading a fault. When an AC bus becomes faulty on either bus 1 or bus 2, that bus can no longer carry power reliably, and trying to auto-transfer loads through it would risk energizing a faulted path or losing power to essential systems. To protect the system, the auto-transfer logic is inhibited, and the fault light indicates this inhibited state.

Faults on DC buses don’t directly affect the AC transfer logic, since they’re on a separate part of the distribution system. Likewise, an external power source fault removes one supply but doesn’t automatically disable auto transfer if the other sources remain healthy. Therefore, a fault on the AC buses themselves is what triggers the Auto Xfer inhibition.