Ground Service Plug Receptacle - Cessna 172 1969 Owner's Manual

GROUND SERVICE PLUG RECEPTACLE

A ground service plug receptacle may be installed to permit use
of an external power source for cold weather starting and during
lengthy maintenance work on the airplane electrical system (with
the exception of electronic equipment).
Electrical power for the airplane electrical circuits is
provided 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 semi-conductors in the electronic
equipment by transient voltages from the power
source. Therefore, 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 reversal protection.
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 airplane's electrical system,
thereby preventing any damage to electrical equipment.
The battery and external power circuits have been designed to
completely eliminate the need to "jumper" across the battery
contactor to close it for charging a completely "dead" battery. A
special fused circuit in the external power system supplies the
needed "jumper" across the contacts so that with a "dead" battery
and an external power source supplied, turning the master switch
"ON" will close the battery contactor.
STATIC PRESSURE ALTERNATE SOURCE VALVE
A static pressure alternate source valve may be installed in the
static system for use when the external static source is
malfunctioning.
NOTE
Power from the external power
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