Pilot’s Manual
PM-126A 4-21
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
4-22 PM-126A
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
channels are operating following a dual generator failure. The standard
10-ampere-hour emergency battery will power the emergency battery
bus for at least one hour after being isolated from the rest of the
electrical system. The optional 18 ampere hour emergency battery will
last for approximately two (2) hours.
NON-ESSENTIAL BUSES
The left and right non-essential buses, normally connected to the
respective generator buses, are used to supply noncritical loads,
including certain cabin lighting and domestic loads. When the airplane
electrical system is powered with airplane batteries only, the nonessential
buses are not powered and the OFF captions will be
illuminated on both NON-ESS switches. If a GPU or APU is selected
ON, but the main batteries are OFF, only the non-essential buses are
powered. When a GPU, APU, or airplane generator is powering the
electrical system, and the main batteries are On, the non-essential bus
contactors will automatically close and the OFF captions will
extinguish if the airplane is on the ground. However, if either generator
fails in flight, the non-essential buses are both automatically
disconnected to reduce the electrical load. One or both non-essential
buses may be subsequently reconnected by depressing the NON-ESS
switches on the electrical control panel.
DISTRIBUTION SUMMARY
When only main and emergency battery power is On (and the AV
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