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Pilot’s Manual
PM-132A 4-21
ESSENTIAL BUSES
All essential power loads are connected to the left and right essential
buses, including cockpit warning systems and the engine, flap,
hydraulic, pressurization, and spoiler controls. Normally, the essential
buses are fed by the left and right generator buses respectively through
150-amp fuses, essential bus contactors and 80-amp circuit breakers.
The left and right essential bus contactors automatically close when the
respective left and right BATT switches are turned on and they
automatically open during starter operation. The essential buses are
tied together by a left and right isolation contactor so that they are both
powered by the emergency battery during engine start and they will be
powered by either generator bus if an essential bus contactor should
fail in flight.
ISOLATION CONTACTORS
There are two isolation contactors located within the electrical system.
The right isolation contactor is located between the right essential
contactor and the emergency battery bus, and the left isolation
contactor is located between the left essential contactor and the
emergency battery bus. The isolation contactors are automatically
controlled and there are no provisions for the crew to manually
override their operation.
Except during starter-assisted engine start, the left isolation contactor is
normally open and the right isolation contactor is normally closed.
With the right isolation contactor closed, the right generator bus
powers the emergency battery bus and charges the emergency battery.
During ground engine starting and for starter-assisted airstarts, the
isolation contactors close and the left and right essential contactors
automatically open. This allows the emergency battery to power the
emergency battery bus, essential buses, and essential avionics buses
and isolates these buses from voltage fluctuations caused by starter
operation. After starter drop-out, the left isolation contactor opens and
the essential contactors close.
Pilot’s Manual
4-22 PM-132A
ISOLATION CONTACTORS (CONT)
There are two abnormal conditions that will cause the left isolation
contactor to automatically close. If either essential contactor fails
(opens), the left isolation contactor will close. Since the right isolation
contactor is normally closed, either generator bus can then provide
generator power through the essential contactor that is closed to the left
and right essential buses and to essential avionics buses, the emergency
battery bus and to charge the emergency battery. This failure will be
displayed as a white L/R ESS BUS FAULT message on the CAS.
The left isolation contactor will also close in the event that the right
isolation contactor fails (open). This will allow the emergency battery
bus and emergency battery to receive power from the left generator
bus. There is no apparent indication of this condition to alert the crew.
ESSENTIAL AVIONICS BUSES
The left and right essential avionics buses are powered from the left and
right generator buses respectively through the left and right essential
contactors and left and right essential avionics contactors. In addition
to the 150-amp fuses protecting the essential bus feeders, the essential
avionics buses are also protected with 35-amp circuit breakers. Under
normal conditions, the essential contactors will automatically be
closed, providing power to the essential avionics contactors. The
essential and main avionics contactors are closed and opened by
manual selection of the left and right AV MSTR switches. These
essential avionics buses, like the essential buses, are powered by the
emergency battery during starter-assisted airstarts while the main
avionics buses are depowered.
EMERGENCY BATTERY BUS
The emergency battery bus is normally powered from the right
generator bus through the right essential contactor and right isolation
contactor, but for engine starting and in the event of dual generator
failure, it is powered directly from the emergency battery. Emergency
battery bus services include landing gear control and indication,
inboard brakes/anti-skid, standby instruments, and the #1 RMU for a
backup EICAS display.
Pilot’s Manual
PM-132A 4-23
In the event of a dual generator failure, the right isolation contactor
opens (left one already open), leaving the main aircraft batteries to
power the respective essential buses and the emergency battery to
power the emergency battery bus. Hence, three independent battery
power sources are operating following a dual generator failure. The
standard 10-ampere-hour emergency battery will power the emergency
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