If the AC ESS XFR switchlight illuminates, what does it indicate?

If the AC ESS XFR switchlight illuminates, what does it indicate?

• A. AC ESS BUS is being powered by AC BUS 2, not 1
• B. AC ESS BUS is being powered by AC BUS 1, not 2
• C. AC ESS BUS is offline
• D. Both buses are tied and powered equally

Answer: (A)

In aircraft electrical systems, the essential AC bus can be fed from two sources, and a transfer switch light shows which source is currently supplying it. When the AC ESS XFR switchlight is illuminated, it points to the active source feeding the essential bus. If the light corresponds to AC BUS 2, that means the AC essential bus is being powered by AC BUS 2, not by AC BUS 1. This indicates BUS 2 is supplying the essential bus while BUS 1 is not feeding it at that moment. If the light were indicating BUS 1, the situation would be the opposite. If the bus were offline or neither source was feeding it, you wouldn’t have a single illuminated indicator for power.

In aircraft electrical systems, the essential AC bus can be fed from two sources, and a transfer switch light shows which source is currently supplying it. When the AC ESS XFR switchlight is illuminated, it points to the active source feeding the essential bus. If the light corresponds to AC BUS 2, that means the AC essential bus is being powered by AC BUS 2, not by AC BUS 1. This indicates BUS 2 is supplying the essential bus while BUS 1 is not feeding it at that moment. If the light were indicating BUS 1, the situation would be the opposite. If the bus were offline or neither source was feeding it, you wouldn’t have a single illuminated indicator for power.