Cessna Skyhawk 172N Pilot Operating Handbook page 162

GROUND SERVICE PLUG
PILOT'S OPERATING HANDBOOK
RECEPTACLE
SUPPLEMENT
MODEL 172N
SECTION 2
LIMITATIONS
The following information must be presented in the form of a placard
located on the inside of the ground service plug access door:
CAUTION
24 VOLTS D.C.
This aircraft is equipped with alternator
and a negative ground system.
OBSERVE PROPER POLARITY
Reverse polarity will damage electrical
components.
SECTION 3
EMERGENCY PROCEDURES
There is no change to the airplane emergency procedures when the
ground service plug receptacle is installed.
SECTION 4
NORMAL PROCEDURES
Just before connecting an external power source (generator type or
battery cart). the avionics power switch should be turned off, and the
master switch on.
WARNING
When turning on the master switch. using an external
power source. or pulling the propeller through by hand,
treat the propeller as if the ignition switch were 0 N. Do not
stand, nor allow anyone else to stand, within the arc of the
propeller, since a loose or broken wire, or a component
malfunction, could cause the propeller to rotate.
1 July 1978 2