Emergency lighting is powered by ___ and illuminated automatically with a loss of either AC or DC essential power.

• A. Self-contained battery packs
• B. Main electrical bus
• C. External power supply
• D. Hydraulic power unit

Answer: (D)

Emergency lighting relies on an independent energy source that activates automatically when essential electrical power is lost. The best design uses self-contained battery packs that are kept charged during normal operation and immediately illuminate the lights if AC or DC essential power fails. This arrangement guarantees lighting even when the primary power sources are gone, which is crucial for safe evacuation. A hydraulic power unit serves a different function entirely and does not provide electrical illumination. Relying on the main electrical bus or an external power supply would not assure lighting once essential power is lost, since those sources depend on the very power being interrupted.