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power supplied by the GPU.
The external power supplied to the dc electrical system can then be
monitored as described under ELECTRICAL SYSTEM INDICATORS
in this section.
The APU (if installed) can supply power to the electrical system
through the right generator bus. When the APU generator is on line, the
bus-tie will automatically close, providing power to the left generator
bus as well. Electrical power is then distributed from the generator
buses to the rest of the electrical system in the same way as if a GPU was
providing electrical power with airplane batteries selected On.
The APU generator output voltage can be monitored on the EICAS in
the same manner that GPU or airplane generator voltage is monitored.
Additionally, APU amperage draw can be monitored on an indicator
located on the APU control panel on the center pedestal or on the ELEC
schematic page of the EICAS/MFD.
Various-sized fuses are installed throughout the electrical system to
provide circuit protection. Each fuse will carry more than its rated
capacity for a short period of time. Extreme or prolonged overloading
will cause a fuse to blow, isolating a particular circuit and precluding
progressive failure of other electrical components. Fuses cannot be
reset. When a fuse has blown, it must be replaced. Fuses are located
within the aft and forward left and right Power Distribution Panels
(PDPs).
Contactors which are particularly suited for circuits with heavy
electrical loads are used throughout the electrical system. Contactors
function as remote switches to make or break power circuits. Most of
the contactors in the electrical system automatically close and open for
given conditions. Some, such as the Battery, GPU and APU, are
manually selected open and closed with the respective switches on the
electrical control panel, but can also open automatically if monitored
faults are detected.
Pilot’s Manual
4-16 PM-126A
A circuit breaker is designed to open, and interrupt current flow in the
event of a malfunction. Once opened, it may be reset by pushing it back
in. An open circuit breaker may be identified by a white base which can
only be seen when it is in the open or tripped position.
Most of the airplane’s circuit breakers are located on two circuit breaker
panels in the cockpit, one on the pilot’s left side panel and one on the
copilot’s right side panel.
The circuit breakers are thermal type mechanisms and the amperage
ratings are stamped on the top of each circuit breaker.
The circuit breakers are grouped by systems rather than by buses.
Emergency bus circuit breakers have red rings around them to easily
distinguish them from the other circuit breakers.
The individual circuit breaker labels, grouping labels and dividing
lines are illuminated with electroluminescent lighting. There are no
bulbs in the panels, but the panels glow when current is flowing
through wires imbedded in them. The silk-screened panels allow light
to shine through the lettering on the panels. The intensity of the
lighting is controlled with the CB PANEL rheostat located on the pilot’s
and copilot’s CREW LIGHTS panels.
SPLIT BUS SYSTEM
The split bus electrical system has a left and right generator bus (GEN
BUS) located in a left and right Power Distribution Panel (PDP) in the
tailcone. The generator buses supply power to the respective left and
right essential, left and right main and left and right non-essential
buses located on the left and right circuit breaker panels in the cockpit.
Under normal flight conditions, the generator buses are split (bus-tie
open), increasing safety in that any major electrical system fault will
only affect one side of the system.
GENERATOR BUSES
The generator buses are the central distribution point for the split bus
system. The left generator bus powers the left side buses and the right
generator bus powers the right side buses. Some services including
landing lights, taxi lights, navigation lights, recognition lights and baggage
heat are connected through fuses and circuit breakers directly to
the generator buses. Each generator bus is connected to a starter/
Pilot’s Manual
PM-126A 4-17
generator and a main battery. The GPU connects to the left generator
bus and the APU (if installed) connects to the right generator bus.
Normally, the two buses operate independently; however, they are
automatically “tied” through a bus-tie contactor when a GPU or APU
is connected to the electrical system, during engine starting and in
flight following a single generator failure. The generator buses can be
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