relay, directly to the battery bus. Turn off all external
power while connecting the power cable to, or removing
it from, the external power supply receptacle. The
holding coil circuit of the relay is energized by the
external power source when the BATTERY switch is in
the ON position. The GPU shall be adjusted to regulate
at 28 volts maximum to prevent damage to the aircraft
battery. The EXTERNAL POWER annunciator (fig. 2-5)
indicates the DC external power plug is connected. The
EXT DC ON annunciator indicates external power is
connected to the aircraft DC bus.
Security Keylock Switch. The aircraft
has a security keylock switch (fig. 2-13) installed on the
overhead control panel, placarded ON OFF. The switch
is connected to the battery relay circuit and must be ON
when energizing the battery master power switch. The
key cannot be removed from the lock when in the ON
Circuit Breakers. The overhead circuit
breaker panel (fig. 2-6) contains circuit breakers for
most aircraft systems. The circuit breakers in the panel
are grouped into areas which are placarded as to their
general function. A DC power distribution panel is
mounted beneath the aisleway, forward of the main
spar. This panel contains higher current rated circuit
breakers and is not accessible to the flight crew under
AC POWER SUPPLY.
Single Phase AC Power Supply. AC
power for the aircraft is supplied by inverter units,
numbered #1 and #2 (fig. 2-13) which obtain operational
current from the DC power system. Both inverters are
rated at 750 volt-amperes and provide single-phase
output only. Each inverter provides 115 volts and 26
volts and 400 Hz AC output. The inverters are
protected by circuit breakers mounted on the DC power
distribution panel mounted beneath the floor. Controls
and indicators of the AC power system are located in the
overhead control panel and in the caution/advisory
WARNING/CAUTION Annunciators. Illumination of the
illumination of caution annunciator #1 INVERTER and/or
#2 INVERTER indicates inverter failure.
Instrument AC Annunciator. A
red INST AC warning annunciator, will illuminate if all
instrument AC buses should fail.
Inverter Control Switches. Two
switches, placarded 1 INVERTER #1 and #2 on the
overhead control panel (fig. 2-13), control the single-
phase AC inverters.
Volt-Frequency Meters. The two digital
volt-frequency meters (fig. 2-13) are mounted in the
overhead control panel to provide continuous monitoring
of voltage and frequency on each 115 VAC bus.
Normal bus conditions will be indicated by a reading of
115 VAC and 400HZ on each meter.
Three Phase AC Power Supply. Three
phase AC electrical power for operation of the inertial
navigation system and mission avionics is supplied by
two, DC powered, 3000 volt-ampere solid state three
Three phase inverter control
switches. The two, three-position switches, placarded
#1 INV - RESET - ON - OFF and #2 INV RESET ON
OFF, located in the mission control panel (fig. 4-1),
control three phase inverter operation.
meters. Two, digital, three phase volt/frequency meters
mounted in the mission control panel (fig. 4-1), monitor
and display the voltage and frequency outputs of the
three phase inverters.
Three phase loadmeters. Two,
digital, three phase loadmeters mounted on the mission
control panel (fig. 4-1), monitor inverter output level.
annunciator. The annunciator placarded 3 AC OFF,
located in the mission annunciator panel (fig. 4-1),
indicates a problem with one of the three phase AC
power. See figure 2-32. External three phase AC
power for operation of the inertial navigation system or
mission equipment, can be applied to the aircraft
through an external power receptacle located in the
underside of the left wing stub just outboard of the
engine nacelle. The receptacle is installed inside the
wing structure, accessible through a hinged access
panel. The AC electrical system is automatically
isolated from the external power source if: the external
power is over or under voltage, over or under frequency,
or has an improper phase sequence.