5C-16 Developed for Training Purposes Citation Bravo
September 2005
CAE SimuFlite
Battery Disable Relay
The battery disable relay (not
shown) protects the current limiters
during a ground cross generator start
by opening the battery relay. This
blocks the power from taking an
alternative route from the Main DC
bus across the current limiter to the
Battery bus and then to the Hot Bat-
tery bus and the starter. With the bat-
tery relay open, power from one
starter/generator goes directly
through the Hot Battery bus to the
second starter/generator. A squat
switch on the left main gear prevents
cross generator starts in flight.
Power Relays
Placing the generator switch in the
OFF position also opens the power
relay. The respective amber GEN
OFF annunciator illuminates once
the relay opens. The generator con-
trol unit opens the power relay (4) to
remove a failed generator from the
system and isolate the generator
from its associated Main bus.
Battery Disconnect Relay
The battery grounds to the airframe
through the battery disconnect relay
(5). This relay opens automatically
to disconnect the battery from the
electrical system when a GPU sup-
plies power during an engine start.
The relay closes automatically at the
end of the cycle.
External Power Relay
The external power relay (6) is
between the external power recepta-
cle and the Hot Battery bus. Connect-
ing a GPU to the aircraft closes the
relay; the GPU supplies power to the
Hot Battery bus. Once a generator
begins supplying power, the relay
opens to prevent the flow of current
from the electrical system to the
GPU. The external power relay also
opens when the overvoltage sensor
detects excessive GPU voltage.
Moving the battery switch to BATT
connects GPU power to the rest of
the electrical system.
DC Protection
Three major items provide protec-
tion for the electrical system:
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Generator Control Units (GCUs)
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Circuit Breakers (CBs)
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Current limiters.
Generator Control Units
A GCU for each starter/generator
provides voltage regulation at 28.5V
DC, overvoltage and ground fault
protection, reverse current sensing,
starter cut-off control, and field
weakening during engine start.
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As a voltage regulator, the GCU
monitors generator output so it
can regulate the generator to meet
the demands of the system.
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Once a generator initially excites,
the GCU begins sensing for a
ground or feeder fault. If a feeder
fault occurs, the field relay trips to
shut the generator down.
If the GCU senses an overvoltage or
feeder fault, it takes the generator
off-line by opening the field relay.
The GCU senses the resulting under-
voltage and opens the power relay to
isolate the generator from its Main
bus. Pressing the ENGINE FIRE
switch also opens the field relay. An
open field relay may be reset by
placing the generator control switch
in the momentary RESET position.
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