CESSNA AIRCRAFT COMPANY FOR TRAINING USE ONLY
MODEL 152 7-20
ELECTRICAL SYSTEM
Electrical energy (see figure 7-7) is supplied by a 28-volt, direct-current system powered by an engine-
driven, 60-amp alternator and a 24-volt, 14-amp hour battery (or 17-amp hour battery, if installed)
located on the right forward side of the firewall. Power is supplied through a single bus bar; a master
switch controls this power to all circuits, except the engine ignition system, clock, or flight hour recorder
(if installed). The flight hour recorder receives power through activation of an oil pressure switch
whenever the engine is operating, and the clock is supplied with current at all times. All avionics
equipment should be turned off prior to starting the engine or using an external power source to
prevent harmful transient voltages from damaging the transistors in this equipment.
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.
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. 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.
AMMETER
The ammeter indicates the flow of current, in amperes, from the alternator to the battery or from the
battery to the airplane electrical system. When the engine is operating and the master switch is turned
on, the ammeter indicates the charging rate applied to the battery. In the event the alternator is not
functioning or the electrical load exceeds the output of the alternator, the ammeter indicates the
battery discharge rate.
OVER-VOLTAGE SENSOR AND WARNING LIGHT
The airplane is equipped with an automatic over-voltage protection system consisting of an over-voltage
sensor behind the instrument panel and a red warning light, labeled HIGH VOLTAGE, under the
ammeter.
In the event an over-voltage condition occurs, the over-voltage sensor automatically removes alternator
field current and shuts down the alternator. The red warning light will then turn on, indicating to the
pilot that the alternator is not operating and the battery is supplying all electrical power.
The over-voltage sensor may be reset by turning the master switch off and back on again. If the warning
light does not illuminate, normal alternator charging has resumed; however, if the light does illuminate
again, a malfunction has occurred, and the flight should be terminated as soon as practical.
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