A short in the Main DC Bus would cause which two lights to illuminate?

• A. Battery Relay OPEN light and Generator FAIL light
• B. Hydraulic light and Engine Fail light
• C. Battery Relay OPEN light and PUMP light
• D. Generator FAIL light and PUMP light

Answer: (A)

When the Main DC Bus has a fault, the aircraft’s electrical protection responds by isolating sources and signaling the loss of proper bus power. A short on the Main DC Bus typically causes the battery relay to open to protect the system, so the Battery Relay OPEN light comes on. At the same time, the generator’s ability to supply the bus is compromised or detected as not meeting specs, which triggers the Generator FAIL light. These two indicators directly reflect the power-distribution fault: one shows the battery relay has opened, the other shows the generator isn’t maintaining proper bus power.

Hydraulic and engine indicators relate to those specific systems and aren’t directly caused by a fault on the Main DC Bus, so they wouldn’t illuminate from this fault. The PUMP light is tied to hydraulic pump status, not the electrical bus condition, so it wouldn’t be expected to come on due to a bus short.