What powers the emergency lighting system and when are they illuminated?

The emergency lighting system is crucial for passenger safety and operational integrity during emergencies or power failures. The correct answer indicates that the emergency lighting system is powered by self-contained battery packs, which automatically activate when there is a loss of AC or DC essential power. This feature ensures that emergency exit pathways are illuminated as soon as the aircraft experiences a power failure, thereby aiding in safe evacuations and maintaining visibility in critical situations.

Self-contained battery packs are designed specifically for this purpose, ensuring that illumination is provided independently of the aircraft's primary power sources. This design enhances reliability, as it guarantees that the emergency lighting will function regardless of the status of the aircraft's main electrical systems, a critical factor during emergencies.

In contrast, other answer options suggest mechanisms or conditions that do not align with the designed functionality of emergency lighting systems. For instance, using a generator bus with backup batteries implies reliance on systems that may not be available during a complete power loss or emergency. Similarly, relying solely on backup generators or AC power would not provide immediate illumination in an emergency scenario. Thus, the self-contained battery packs ensuring illumination specifically upon the loss of essential power exemplify the intended operational design of the emergency lighting system.