If the GPU is connected and selected ON, what is the status of the battery contactors?

If the GPU is connected and selected ON, what is the status of the battery contactors?

• A. They close
• B. They open
• C. They remain unchanged
• D. They short

Answer: (B)

When a Ground Power Unit is connected and selected ON, the system prioritizes that external power source and isolates the battery from the DC bus by opening the battery contactors. This prevents current flow between the battery and the external power source, so the GPU alone powers the aircraft’s electrical system and the battery remains out of the circuit. The battery is still available as a backup if the GPU is removed or turned off, at which point the contactors would reclose to reconnect the battery. Opening is the correct state during GPU operation to avoid backfeeding and ensure proper power source priority. Closing would reconnect the battery while external power is active, which isn’t the desired condition, and remaining unchanged or shorting would not reflect the normal operating logic.

When a Ground Power Unit is connected and selected ON, the system prioritizes that external power source and isolates the battery from the DC bus by opening the battery contactors. This prevents current flow between the battery and the external power source, so the GPU alone powers the aircraft’s electrical system and the battery remains out of the circuit. The battery is still available as a backup if the GPU is removed or turned off, at which point the contactors would reclose to reconnect the battery. Opening is the correct state during GPU operation to avoid backfeeding and ensure proper power source priority. Closing would reconnect the battery while external power is active, which isn’t the desired condition, and remaining unchanged or shorting would not reflect the normal operating logic.