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during:
• airplane power-up
• APU start (assists APU battery)
• refueling operations (using refueling panel BATT switch)
• towing operations
• electric braking (backup power in the event of TRU failure)
• loss of all AC power in flight (powers instrument buses until RAT
deployment)
The main battery charger is powered by the captains flight instrument bus.
APU Battery and Battery Charger
The APU battery powers the APU hot battery bus, providing power to the airplane
during:
• APU start
• airplane power-up (assists main battery)
• refueling operations (assists main battery)
• towing operations (assists main battery)
The APU battery is designed to support two consecutive 45 second unsuccessful
start attempts, followed by a 45 second successful start after 5 minutes.
The APU battery charger is powered by the first officers flight instrument bus.
Start Power Unit (SPU)
The SPU receives at its input:
• 28 Vdc APU hot battery bus power, and then outputs 115 Vac power
The SPU will provide all necessary power for the APU start, but only when no
other AC power sources are available. The SPU powers the R2 ATRU, which
provides the necessary power to the controller for an APU Start attempt (using R
ASG), whether in the air or on the ground. With AC power available, the SPU only
supports the APU start by powering the APU generator field.
Electrical Brake Power Supply Unit (E-BPSU)
The E-BPSU receives:
• 28 Vdc TRU power, and then outputs ±135 Vdc power
The E-BPSU are a regulated power supply that provides power for the exclusive
use of the airplanes electric brake system. Each of the four E-BPSUs receives two
separate 28 Vdc inputs. One from their associated TRU output, and one from the
hot battery bus. The associated TRU input is the primary input, while the hot
battery bus provides the backup power source. The E-BPSU then output ±135 Vdc
for use by the electric brake motors.
787 Flight Crew Operations Manual
Electrical -
System Description
Boeing Proprietary 6.20.11
Brake controllers signal dedicated motors within the brake system when
commanded from the brake pedals in all braking conditions, including parking.
Electrical Brake Power Supply Units (E-BPSU) Schematic
Standby Electrical System
If AC power fails in flight, the standby electrical power system automatically
provides 28 Vdc power to selected instrument, communication, and navigation
equipment. Both flight instrument buses connect together and receive power from
the main battery until the RAT is deployed. With the RAT deployed and powering
the backup bus, both flight instrument buses are powered by their respective
TRUs. Power for center pitot heat is provided by the backup bus.
The standby electrical system consists of the:
• RAT generator
• C1 and C2 TRUs
• captain and first officer flight instrument buses
• backup bus
• hot battery bus
• main battery.
HOT BATTERY BUS
L
TRU
C1
TRU
C2
TRU
R
TRU
EBPSU
EBPSU
EBPSU
EBPSU
±135 Vdc
±135 Vdc
±135 Vdc
±135 Vdc
Primary Power Secondary/Backup Power
PRELIMINARY
787 Flight Crew Operations Manual
Electrical -
System Description
6.20.12 Boeing Proprietary
Standby Electrical System Schematic
Ram Air Turbine (RAT) Generator
A Ram Air Turbine (RAT) is installed in the right wing-to-body fairing as an
emergency source of electrical and hydraulic power. The turbine is dedicated to
drive an electrical generator and a hydraulic pump.
The is RAT deployed automatically in the air with loss of:
• C1 and C2 TRU electric power or,
• hydraulic power or,
• both engines.
The RAT can also be deployed manually by pushing the RAM AIR TURBINE
switch on the hydraulic panel.
It has no operating time limits, airspeeds, or altitudes. The RAT drops off-line
momentarily when the landing gear extends, due to the effects of reverse flow
within the hydraulic system on the RAT hydraulic pump.
Condition: RAT deployed. HBB - Hot Battery Bus
PRELIMINARY
RAT
BKUP
C1
TRU
C2
TRU
HBB CAPT F/O
MAIN
BATT
787 Flight Crew Operations Manual
Electrical -
System Description
Boeing Proprietary 6.20.13
The 235 Vac RAT output is connected to the 235 Vac backup bus. With the
exception of one load (center pitot probe heat) all of the RAT output is converted
to 28 Vdc power to power the captains and first officers flight instrument buses
　
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