What condition will cause the generators to de-energize the power contractor and disconnect the system from the main AC bus requiring a reset?

• A. The presence of under-speed, over-voltage, under-voltage, over-frequency, under-frequency, feeder-fault, excessive waveform distortion or excessive dc voltage content.
• B. Loss of external electrical power
• C. Manual switch fault
• D. Overheating of the PMG

Answer: (A)

Protection for the generators is designed to automatically open the generator’s contactor and isolate it from the main AC bus whenever electrical parameters go out of acceptable limits. If any monitored condition is out of spec—under-speed or over-speed, over- or under-voltage, over- or under-frequency, a feeder-fault, or poor power quality such as excessive waveform distortion or excessive DC voltage content—the protective logic trips and the system disconnects. A reset is then required after the fault is cleared to re-energize the generator onto the bus. Other scenarios like loss of external power or a manual switch fault don’t trigger this automatic protective trip in the same way, and overheating of the PMG is a different fault mode, not the collective set described here.