Final answer:
The ALTN light on the AC ESS FEED pushbutton indicates that the aircraft's electrical system has switched to an alternate power source for the essential feed bus, usually due to a primary power source issue.
Step-by-step explanation:
The ALTN light on the AC ESS FEED pushbutton is an indicator used in electrical panels of aircraft systems. When illuminated, this light indicates that the electrical system has automatically switched to an alternate (ALTN) power source for the essential feed bus.
The transition to an alternate power source typically happens in response to a failure or abnormal condition in the primary power source. The design of electrical panels ensures that any such change in power source is made apparent to the operators, often through both an audible 'click' and a visual indication on a digital or analog meter.
An ALTN light indicates an alternate power source on the AC ESS FEED pushbutton of an electric panel.
For example, in aviation, the AC ESS FEED pushbutton is used to transfer electrical power from the aircraft's main electrical system to the emergency electrical system. When the ALTN light is illuminated, it means that the alternate power source is engaged and providing electrical power to the system.
Other industries may also use similar systems with ALTN lights to indicate the activation of an alternate power source.