AC Power System

Question 1

AC (Alternating current) composition

Answer 1

The AC power system:

• Transfer busses
• Main busses
• Galley busses
• AC standby bus
• Ground service busses

Question 2

Transfer busses (Overview)

Answer 2

-The transfer busses are connected by a tie bus and bus tie breaker system

Question 3

Transfer busses (insight)

Answer 3

• If 2 power sources are connected, the bus tie breakers open to prevent parallel operation of the generators
• If only 1 power source energizes the tie bus. The tie breakers close to supply power to the 2 transfer busses

Question 4

Sources of AC power

Answer 4

• Ground power
• APU generator
• Engine-driven IDG (Integrated Drive Generators

Question 5

AC system annunciator lights

Answer 5

• ‘Transfer buss off’ = Transfer bus not energized
• ‘Source off’ = No power source is selected to energize the transfer bus (If gen does not connect to bus or if connected source is replaced by other source)
• ‘Gen off bus’ = Transfer bus not supplied with power from related IDG (Integrated drive generator)
• ‘APU gen off bus’ = Shows when APU is available but does not supply power to a bus. Light extinguishes once light is connected to a bus
• ‘GRD power available’ Ground power of good quality is connected to the airplane. The light does not extinguish after ground power is connected to a bus

Question 6

APU and Ground Power

Answer 6

• The APU and GPU can energize 1 or 2 transfer busses
• They cannot be connected to any part of the electrical system at the same time
• If the APU is on after take-off and fail the IDGs are automatically connected to the busses

Question 7

Normal AC indications

Answer 7

• AC electrical meters show voltage (normal voltage 115 +)
• Normal frequency 400 +/- 10 C
• AMPs change with different electrical sources

Question 8

CABIN/UTILITY and INFLIGHT ENTERTAINMENT/PASS SEAT

Answer 8

-CAB/UTIL and IFE/PASS power switches control sub-busses off the AC transfer busses.

• The busses control:
• Galley power
• Passenger convenience items
• Inflight entertainment systems

-The switches are normally put on during pre-flight

Question 9

During flight non-normal

Answer 9

-If 1 power source is available. The electrical system automatically removes power to some busses to crease the total electrical load (Load shedding)

• Load shedding:
• Galley busses first
• Main buses 1 at a time
• Until the load is less than the system limit
• If more power is available, some, or all the busses are automatically energized again

Question 10

APU normal conditions

Answer 10

-You can connect the APU generator after the APU GEN OFF BUS light illuminates