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battery bus for at least one hour after being isolated from the rest of
the electrical system.
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 the GPU is selected ON, but
the main batteries are OFF, only the non-essential buses are powered.
When the GPU 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 nonessential
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
MSTR switches are On), the non-essential buses are not powered and
cannot be selected On.
When external power (GPU) is connected and the emergency battery is
selected On, the bus-tie connecting the two generator buses is
automatically closed. When the AV MSTR switches are selected to On,
the contactors to the left and right essential avionics buses and the left
and right main avionics buses will close. Note that the EMER BATT
switch caption EMER will no longer illuminate since the emergency
battery will no longer be discharging.
During an engine start (using airplane batteries) the essential buses and
emergency battery bus are isolated and are powered from the
emergency battery. The non-essential buses and main avionics buses
are also automatically depowered for engine start.
Pilot’s Manual
4-24 PM-132A
When one engine-driven generator comes on line, the essential bus
contactors will close and the left isolation contactor will open. Also, the
contactors to the non-essential buses will close. The bus-tie contactor
remains closed at this point so that both generator buses are powered
by the single generator.
During normal flight conditions (two engine-driven generators on
line), the bus-tie opens and will remain open as long as both generators
remain on line and the bus-tie is not manually selected closed.
ABNORMAL MODES OF OPERATION
SINGLE GENERATOR FAILURE
In the event of a single generator failure in flight, the operating
generator must supply the load for both sides of the dc electrical power
system. To prevent shock loading of the remaining generator, load
shedding of the non-essential buses automatically occurs. After a 5 second
voltage check delay, to see that there is no fault on the generator
bus associated with the failed generator, the bus-tie automatically
closes. The CAS will display an amber L or R GEN FAIL message and
a white BUS TIE CLSD message after the bus-tie automatically closes.
Indications on the electrical control panel would be illuminated OFF
captions on the left and right NON-ESS bus switches, an illuminated
OFF caption on the GEN switch of the failed generator and an illuminated
bar on the BUS-TIE switch. All other systems will continue to operate
normally. If necessary, the crew should reduce the load further
and if conditions warrant, attempt to reset the failed generator. Nonessential
buses may be regained by selecting the appropriate NON-ESS
switch. No aircraft electrical system power is required to reset a generator,
however, the corresponding L GEN or R GEN circuit breaker must
be in to sustain generator operation.
Pilot’s Manual
PM-132A 4-25
DUAL GENERATOR FAILURE
Dual generator failure would become most apparent with illumination
of the red GEN FAIL annunciator on the CWP and the accompanying
flashing Master Warning lights. Also the CAS would display a red L R
GEN FAIL message. Generator failure can be verified by noting zero
left and right amps on the EICAS/MFD SUMRY or ELEC system
schematic display and OFF illuminated in each GEN switch.
With a dual generator failure, main airplane batteries and the
emergency battery powers the dc electrical systems. Both main buses,
both main avionics buses and both non-essential buses will be
automatically depowered to immediately shed the electrical load. Also,
the right isolation contactor automatically opens leaving the
emergency battery to power the emergency battery bus. The bus-tie
　
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