Final answer:
The Essential Power Button illuminates with a striped bar to indicate activation during electrical emergencies in aircraft, ensuring essential systems are powered and non-essential ones are not.
Step-by-step explanation:
The Essential Power Button is typically found in aircraft cockpits and is used in electrical emergencies to isolate the essential power bus from non-essential systems.
When this button is pressed, a striped bar, usually illuminated, indicates that the button has been activated and that the essential electrical systems are powered, while non-essential systems are not. The illumination serves a critical role in allowing pilots to quickly confirm that the button has successfully engaged under emergency conditions, where cockpit lighting or visibility might be limited.