What occurs when the APU generator voltage is below the rated level?

When the APU generator voltage falls below the rated level, the APU MASTER switch FAULT light illuminates. This occurs as a protective measure to alert the crew that there is an issue with the APU generating adequate voltage. The illumination of the fault light indicates a discrepancy in the system's performance, allowing the crew to take appropriate action or investigate the condition further.

The operational functionality of the APU continues unless other thresholds are also violated, so while the APU will keep running, the illuminated fault light serves as a critical warning. This ensures that the flight crew is informed and can respond accordingly, maintaining safety and operational integrity.