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CESSNA
MODEL 172N
SECTION 7
AIRPLANE
&
SYSTEMS DESCRIPTIONS
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_ a d t _ e ~ - a t _ o ~ n d
a 24-volt battery (a heavy duty battery is available), located
on thereft forward side of the firewall. Power is supplied to most general
electrical and all avionics circuits through the primary bus bar and the
avionics bus bar, which are interconnected by an avionics power switch.
The primary bus i s on anytime the master switch is turned on, and is not
affected by starter or external power usage. Both bus bars are on anytime
the master and avionics power switches are turned on.
CAUTION
Prior to turning the master switch on or off, starting the
engine or applying a n external power source, the avionics
power switch, labeled AVIONICS POWER, should be
turned off to prevent any harmful transient voltage from
damaging the avionics equipment.
MASTER SWITCH
The master switch is a split-rocker type switch labeled MASTER, and
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i s 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.
Normally, both sides of the master switch should be used simultane-
ously; 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 power switch must also be
turned on. The ALT side of the switch, when placed in the off position,
removes the alternator from the electrical system. With this switch in the
off position, the entire 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.
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AVIONICS POWER SWITCH
Electrical power from the airplane primary bus to the avionics bus
(see figure 7-7) is controlled by a toggle switch/circuit breaker labeled
AVIONICS POWER. The switch is located on the left side of theswitch and
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control panel and is ON in the up position and off in the down position. With
the switch in the off position, no electrical power will be applied to the
avionics equipment, regardless of the position of the master switch or the
individual equipment switches. The avionics power switch also functions
a s a circuit breaker. If an electrical malfunction should occur and cause the
circuit breaker to open, electrical power to the avionics equipment will be
1
July 1979
7- 25
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