What voltage and maximum current must the external DC power unit supply?

What voltage and maximum current must the external DC power unit supply?

• A. 110V AC 60Hz
• B. 24V and 1000A
• C. 26-30V and a maximum of 1500A
• D. 28V and 2000A

Answer: (C)

The key idea is how ground power interacts with the aircraft’s electrical system. On the Longitude 700, the DC power system runs at about 28V DC, but to keep the voltage within safe limits when connected to external power and to account for drop under load, the external DC power unit must supply a range of 26 to 30 volts. In addition, the system has substantial starting and operating loads, so the power unit needs to be capable of delivering up to about 1500 amperes to handle peak demand (engine start, systems on, air conditioning, avionics, etc.) without voltage sag. That combination—26–30V DC and up to 1500A—matches the aircraft’s documented ground-power requirements. Other options either reference AC instead of DC, use a voltage outside this acceptable range, or specify a current rating that doesn’t cover the peak loads, so they aren’t suitable for proper ground power.

The key idea is how ground power interacts with the aircraft’s electrical system. On the Longitude 700, the DC power system runs at about 28V DC, but to keep the voltage within safe limits when connected to external power and to account for drop under load, the external DC power unit must supply a range of 26 to 30 volts. In addition, the system has substantial starting and operating loads, so the power unit needs to be capable of delivering up to about 1500 amperes to handle peak demand (engine start, systems on, air conditioning, avionics, etc.) without voltage sag. That combination—26–30V DC and up to 1500A—matches the aircraft’s documented ground-power requirements. Other options either reference AC instead of DC, use a voltage outside this acceptable range, or specify a current rating that doesn’t cover the peak loads, so they aren’t suitable for proper ground power.