AC REF BUS DEGRAD occurs when which condition?

• A. When one transformer fails
• B. When both generators fail
• C. When the battery is low
• D. During normal operation

Answer: (A)

AC REF BUS is normally powered by two transformer-rectifier units tied to the generators, giving redundant sources to keep the bus within spec. If one transformer fails, the remaining transformer has to carry the full load, which can cause the voltage on the AC REF bus to droop or become unstable. That degradation is why a single transformer failure leads to AC REF BUS degradation. If both generators fail, you’d lose AC power altogether rather than just see a degraded reference bus. A low battery affects DC buses and does not directly cause degradation of the AC reference bus during normal operation.