The ESIS is powered by which bus?

• A. EPS (Emergency Power System) bus
• B. ESS (Emergency Standby System) bus
• C. APU bus
• D. Main bus

Answer: (A)

The ESIS is designed to stay powered during electrical faults, so it is supplied by the Emergency Power System (EPS) bus. The EPS provides a separate power source to essential systems when the normal main power is lost, ensuring the ESIS display remains available for critical information even with a failure of the main bus. Relying on the APU bus would only work if the APU is running, and the Main bus would not be reliable during a loss of power. The ESS bus powers standby instruments, but the ESIS is specifically tied to the EPS for continuous emergency operation.