"Icebound" wrote in message
...
I am reading from the generic POH for 1976 Cessna 172, section 7, Airplane
and Systems Descriptions, Electrical System:

quote:
Both sides of the bus are on at all times except when either an external
power source is connected, or the *starter switch is turned on*; then a
power contactor is automatically activated to open the circuit to the
electronic bus. Isolating the electronic circuits in this manner prevents
harmful transient voltages from damaging the transistors in the electronic
equipment.
:unquote.

By 1981, though, the 172P POH says: "The primary bus is 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."

Perhaps Cessna decided that the electric-cutout switches created an
unnecessary risk of accidental loss of electricity.

--Gary