When do the EMER lights activate due to loss of power?

The emergency lights are designed to activate in specific scenarios to ensure safety during a loss of power. In this case, the correct answer indicates that the emergency lights activate when there is a loss of power on either the AC or DC ESS BUS. This is crucial because the emergency lighting circuits are typically powered through these essential buses.

If either the AC or DC ESS BUS fails, it signifies a critical power loss situation, and the emergency lights automatically switch on to provide illumination, enabling occupants to navigate safely in the event of a blackout or reduced visibility conditions. The emphasis on the AC or DC ESS BUS reflects the reliance on these systems as the primary source of emergency power.

Options referring to the loss of power of any main bus or the failure of the backup generator do not directly correspond to the specific triggering of emergency lights. The condition of cabin pressure is unrelated to the activation of emergency lights, making that option irrelevant in this context.