(5) Aft Vent Blower. The aft vent blower is
controlled by the switch placarded
continuously when the switch is placed in the ON
position with the air conditioner compressor running.
Section IX. ELECTRICAL POWER SUPPLY AND DISTRIBUTION SYSTEM
The aircraft employs both direct current (dc) and
alternating current (ac) electrical power. The dc
electrical power supply is the basic power system
energizing most aircraft circuits. Refer to Figure 2-27
and Table 2-7 R ; Figure 2-28 and Table 2-8 T3 ;
and Figure 2-29 and Table 2-9 F3 . Electrical power
is used to start the engines, power the landing gear
and flap motors, operate the standby fuel pumps,
ventilation blower, lights, and electronic equipment.
AC power is obtained from the dc power system
through inverters. The single phase ac power system
is shown in Figure 2-30, Sheets 1 through 3. The
three sources of dc power consist of one 20 cell
34-ampere/hour battery and two 250-ampere starter-
generators. DC power may be applied to the aircraft
through an external power receptacle on the underside
of the right wing, just outboard of the nacelle.
Generator control units control the starter generators.
The output of each generator passes through a cable
to the respective generator bus. Other buses
distribute power to aircraft dc loads, deriving power
from the generator buses. The generators are
paralleled to balance the dc loads between the two
units. When one of the generating systems is not on
line, and no fault exists, all aircraft dc requirements
may be supplied by either the other on-line generating
system or by an external power source. The generator
system is designed to allow cross starting of the other
engine. When one generator is on line, all current
limiters are bypassed while starting the other engine.
Most dc distribution buses are connected to both
generator buses but have isolation diodes to prevent
power crossfeed between the generating systems,
when connection between the generator buses is lost.
Thus, when either generator is lost because of a
ground fault, the operating generator will supply power
for all aircraft dc loads except those receiving power
from the inoperative generator's bus, which cannot be
crossfed. When a generator is not operating, reverse
current and over-voltage protection is automatically
provided. Two inverters operating from dc power
produce the required single-phase ac power.
2-71. DC POWER SUPPLY.
One nickel-cadmium battery furnishes dc power
when the engines are not operating. This 24-volt
34-ampere/hour battery, located in the right wing
center section, is accessible through a panel on the
top of the wing. DC power is produced by two engine-
Controls and indicators associated with the dc supply
system are a battery switch, two generator switches,
and two dc volt-loadmeters.
a. Battery Switch. A switch, placarded BATT,
OFF / ON, is located on the pilot's subpanel under the
MASTER SWITCH (gang bar). The BATT switch
controls dc power to the aircraft bus system through
the battery relay, and must be set to ON to allow
external power to enter aircraft circuits. When the
MASTER SWITCH (gang bar) is placed down, the
BATT switch is forced OFF.
With battery or external power removed
from the aircraft electrical system due to
fault, power cannot be restored to the
system until the BATT switch is moved to
OFF, then ON.
placarded GEN 1 and GEN 2 OFF / ON / GEN
RESET, are located on the pilot's subpanel. These
switches control electrical power from the designated
generator to paralleling circuits and the bus distribution
system. When a generator is removed from the
aircraft electrical system, due either to fault or from
placing the GEN switch in the OFF position, the
affected unit cannot have its output restored to aircraft
use until the GEN switch is moved to RESET, then
c. Master Switch. All electrical current may be
shut off using the MASTER SWITCH gang bar that
extends above the battery and generator switches.
When moved downward, the bar positions the
switches to the OFF position.
d. DC Volt-Loadmeters. Two-volt loadmeters,
located on the overhead control panel, Figure 2-31,
display bus voltage and current load as a percentage
of maximum from the left and right generating
systems. The volt-loadmeters normally display load.
Voltage may be read by pressing a push-button switch
on the respective volt-loadmeter.