Cessna 172N - Page 132

SECTION
7
AIRPLANE
&
SYSTEMS DESCRIPTIONS
CESSNA
MODEL
172N
alternator
charging has resumed;
however,
if
the light
does illuminate
again,
a
maHunction has
occurred, and the
flight should
be
terminated as
soon
as
practical.
The
warning light may
be tested
by momentarily turning off the
ALT
portion
of the
master
switch and leaving the BAT
portion
furned on.
CIRCUII
BREAKERS AND FUSES
Most
of the electrical
circuits in
the airplane are
protected
by
"push-
to-reset"
circuit breakers
mounted on the
left side
of the instnrment
panel.
Exceptions
to
this are
the
battery
contactor
closing
(external power)
cir-
cuit,
clock, and
flight hour recorder
circuits
which have
fuses
mounted
near the battery. The
control
wheel
map
light is
protected
by the NAV LT
circuit
breaker on the instnrment
panel,
and a
fuse
behind the
panel.
The
cigar
lighter
is
protected
by
a manually reset
circuit
breaker on
the back
of
the
lighter, and
by the
LAND LT
circuit
breaker.
GROUND
SERVICE
PtUG
RECEPTACTE
A
ground
service
plug
receptacle
may
be installed
to
permit
the
use
of
an
external
power
source for cold
weather
starting
and
during
tengthy
maintenance
work on the airplane electrical
system
(with
the exception of
electronic equipment).
The
receptacle is
located
behind
a
door on the
left
side of the fuselage
near the aft edge
of
the cowling.
NOTE
Electrical
power
for
the
airplane
electrical
circuits
is
pro-
vided
through
a
split
bus bar
having
all
electronic circuits
on
one
side
of
the
bus
and other
electrical
circuits
on the
other
side
of
the bus. When
an
external
power
source is
connected,
a
contactor
automatically
opens the
circuit to
the electronic
portion
of
the
split
bus
bar
as a
protection
against
damage
to
the
transistors
in
the electronic equip-
ment by
transient
voltages
from
the
power
source.
There-
fore,
the
external
power
source
can not be
used
as a
source
of
power
when checking electronic components.
Just before
connecting an
external
power
source
(generator
type
or
battery
cart), the
master
switch should
be turned
on.
The
ground
service
plug
receptacle
circuit incorporates
a
polarity
re-
versal
protection.
Power
from
the
external
power
source will
flow only
if
the
ground
service
plug
is correctly
connected
to the airplane.
If the
plug
is accidentally
connected
backwards,
no
power
will
flow
to
the
elec-
trical
system,
thereby
preventing
any damage to electrical equipment.
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