With the cockpit switch in ARM, emergency lighting activates if there is a loss of power to which buses?

• A. AC ESS BUS or DC ESS BUS.
• B. AC ESS BUS only.
• C. DC ESS BUS only.
• D. Neither; power loss is required from the essential bus.

Answer: (A)

The main idea is that emergency lighting is designed to come on when the aircraft’s essential power sources fail, so you have lighting to find exits. With the cockpit switch in ARM, the system will automatically illuminate if either essential power feed loses power—meaning the AC essential bus failing or the DC essential bus failing. Those two buses power critical systems, and the ARM setting ensures the emergency lights are connected to that protection. If either one loses power, the emergency lights activate to provide safe illumination during an evacuation. The concept isn’t about a single “essential bus” but about the loss of power to either of the essential feeds, which is why the correct understanding is that the lights come on if power is lost to the AC essential bus or the DC essential bus.