Cessna 172R, Operating Handbook

Page: 253 / 500

CESSNA
MODEL 172R
SECTION
7
AIRPLANE
&
SYSTEMS DESCRIPTION
MASTER SWITCH
The
master
switch is a split rocker
type switch labeled
MASTER,
and is ON in the up position and
OFF
in
the down position.
The|
right half of the switch, labeled
BAT, controls all electrical power
to
the airplane. The left half, labeled
ALT, controls the alternator.
A
CAUTION
PRIOR TO TURNING
THE MASTER SWITCH ON
OR OFF, STARTING THE ENGINE OR APPLYING
AN EXTERNAL
POWER SOURCE, THE AVIONICS
MASTER SWITCH SHOULD BE TURNED OFF
TO
PREVENT ANY HARMFUL TRANSIENT VOLTAGE
FROM DAMAGING THE AVIONICS
EQUIPMENT.
I
\
Normally, both sides of the master switch should be used
simultaneously; however, the
BAT side of the switch could be turned
on separately to check equipment while
on the ground. To check or
use avionics equipment
or
radios while on the ground, the avionics
master
switch
must
also be turned on. The ALT side of
the switch,
when placed in the OFF position, removes the alternator from
thel
electrical
system.
With this switch in the
OFF
position, the
entirel
electrical load is placed on the battery. Continued operation with the
alternator switch in
the
OFF position will reduce battery power
low|
enough to
open
the battery
contactor,
remove power from the
alternator field, and
prevent
alternator
restart.
Revision
8 7-33