Which device provides emergency 24 VDC for the emergency lighting system and the standby horizon?

• A. Separate power module (battery)
• B. External Power Unit (EPU)
• C. Generator
• D. GCU

Answer: (A)

In an aircraft, emergency lighting and the standby horizon must stay powered even if the main electrical system fails. That requires a source of power that is independent of the engines and primary generators, providing a constant 24 VDC to these critical loads for the required duration. A separate power module, which is a dedicated battery, is designed exactly for this purpose. It supplies the emergency lighting and the standby horizon directly, without relying on the main buses, and remains available when normal power is lost.

Other options are tied to the normal power system or are auxiliary rather than dedicated emergency power: an External Power Unit provides power in specific conditions but isn’t the guaranteed emergency source for these loads; a generator powers the normal electrical system and can fail to deliver the necessary emergency support; the Generator Control Unit is a control component rather than a power source.