CIRCUIT COMPONENTS
Current Limiters
A current limiter panel is installed on the gen-
erator control panel in the tailcone. A current
limiter is comparable to a slow blow fuse and,
once blown, must be replaced. There is a spare
current limiter box in the tailcone with one
spare current limiter for each size used in the
airplane. This spare current limiter box has the
inventory printed on the top cover. Current
limiters are used to provide power to some
systems, to back up circuit breakers, and to in-
terconnect buses.
Circuit Breakers
All circuit breakers are located in the cockpit
on two circuit-breaker panels (Figures 2-9
and 2-10), one on each side of the cockpit. All
circuit breakers are the push-pull type. The DC
circuit breakers operate thermally, and the AC
circuit breakers operate magnetically. The
amperage ratings are stamped on the top of
each circuit breaker.
Relays
Relays are used at numerous places through-
out the electrical distribution system, partic-
ularly in circuits with heavy electrical loads.
The relays function as remote switches to
“make” or “break” power circuits. This arrange-
ment allows the control circuit and wiring to
the control switches to be a much smaller gage
wire. Battery relays, external power relays,
starter relays, generator relays, and inverter re-
lays are used to connect/control the power cir-
cuits. Instrument panel switches complete the
control circuits and operate the relays. Relays
are also used to connect the power circuits to
the left and right main DC buses.
Overload Sensors
The DC overload sensors are located in the tail-
cone. An overload control sensor consists of
a 70-ampere thermal circuit breaker mechan-
ically connected to a set of switch contacts.
During normal operation, the main bus power
relay is energized through the closed contacts
of the overload control sensor and the main bus
circuit breaker. If an overload condition exists,
the thermal breaker contained within the sen-
sor positions the contacts to ground the circuit
from the applicable bus circuit breaker. The
opened circuit breaker removes power from the
main bus relay. The overload sensor automat-
ically resets when the thermal breaker has
cooled. However, power is not restored to the
power relay and bus until the DC bus circuit
breaker is reset.
Reverse Current Diodes
Two reverse current diodes are installed on the
aft side of the engine beam in the tailcone
(one for each generator system). The reverse
current diodes isolate the generator systems
from each other and prevent the starter from
motoring when not selected.
2-8
FOR TRAINING PURPOSES ONLY
LEARJET 20 SERIES PILOT TRAINING MANUAL
FlightSafety
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