DISTRIBUTION
A multiple bus, multiple conductor system is
used (Figures 2-11 and 2-12). Buses and major
circuits are protected by strategically placed
relays, current limiters, overload sensors, and
circuit breakers to preclude total failure and
to simplify the isolation of any partial system
malfunction. All circuit breakers are accessi-
ble to the crew during flight.
Battery Bus
Airplanes with a Single Battery
Switch
The battery bus is connected directly to both
batteries at all times. The battery switch ener-
gizes a relay that connects the battery bus to
the battery charging bus when turned on. If
voltage on the small pin of the GPU recepta-
cle does not exceed 33 ±2 VDC, external power
is applied to both batteries and the battery bus.
This permits power from a GPU or the batter-
ies to supply the airplane electrical system. It
also permits the generators to charge the bat-
teries. During engine start, a relay connects the
battery bus to the starter. Hot-wired items are
connected directly to the battery bus.
Airplanes with Dual Battery
Switches
The battery bus is connected to the two bat-
teries via the battery relays when the corre-
sponding battery switch(es) is turned on. The
battery bus may be powered by external power
if the voltage from the ground power unit is
less than 33 ±2 VDC on the small pin of the
GPU receptacle, and at least one battery switch
is on. As long as both battery switches are on,
either engine DC generator charges both bat-
teries. The engine generators are able to sup-
ply DC power to all DC-powered equipment
on the airplane with both battery switches in
OFF except for the hot-wired items. All hot-
wired items are connected directly to the bat-
teries. The battery bus is connected to the left
or right starter through the associated start
relay when the respective generator switch is
placed in the START position.
Battery Charging Bus
The battery charging bus is located in the cur-
rent-limiter panel in the tailcone. It distributes
power from the batteries or GPU to the airplane
electrical system and generator power back to
the batteries for charging. The battery charg-
ing bus distributes power to the generator
buses, essential DC buses, and the battery bus.
Generator Buses
The left and right generator buses are a part of
the current-limiter panel in the tailcone. The
generator bus may be powered by the battery
charging bus or by the output of the generator.
Main DC Power Buses
The two main DC power buses receive power
from the respective generator bus through a 10-
ampere current limiter. The power buses sup-
ply their respective starter switches, inverter
switches, and main bus relays.
Main DC Buses
The two main DC buses receive power from
their respective generator buses. Circuit pro-
tection for each bus is provided by a 70-am-
pere overload sensor. The left and right main
buses are interconnected by a 50-ampere DC
bus-tie circuit breaker.
Essential DC Buses
The two essential DC buses receive power
from the battery charging bus. Each essential
DC bus is protected by a 50-ampere current
limiter and a 40-ampere left or right bus cir-
cuit breaker. The buses are interconnected by
a 20-ampere DC bus-tie circuit breaker. On
SNs 25-368 through -373 and airplanes mod-
ified by AMK 85-1, the essential buses are
powered by their respective generator buses.
2-11
FOR TRAINING PURPOSES ONLY
LEARJET 20 SERIES PILOT TRAINING MANUAL
FlightSafety
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