TM 1-1510-224-10 the  DC  power  system.    Both  inverters  are  rated  at  750 volt-amperes   and   provide   single   phase   output   only. Each  inverter  provides  115  volts,  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   on   the   overhead control   panel   and   on   the   caution/advisory   annunciator panel. (1) AC power WARNING/CAUTION annunciators. Illumination of the two MASTER CAUTION  annunciators,  and  the  illumination  of  the   #   1 INVERTER and/or   #  2  INVERTER  caution  annunciator indicates inverter failure. (2) Instrument  AC  annunciator.    A  red   INST AC  warning  annunciator,  will  illuminate  if  all  instrument AC buses fail. (3) Inverter  control  switches.    Two  switches, placarded  1f  INVERTER  #  1 and   #  2  on  the  overhead control  panel  (fig.    2-15),  control  the  single-phase  AC inverters. b. Volt-Frequency Meters. Two digital volt- frequency    meters    (fig.        2-15)    are    mounted    on    the overhead control panel. These meters provide continuous monitoring of voltage and frequency on each 115 VA. bus.  Normal bus conditions will be indicated by a reading of 115 VA. and 400 Hz on each meter. c. Three  Phase  AC  Power  Supply.    Three  phase AC   electrical   power   (fig.      2-33)   for   operation   of   the inertial    navigation    system    and    mission    avionics    is supplied   by   two   DC-powered   3000   volt-ampere   solid state three phase inverters. (1) Three    phase    inverter    control    switches. Two three-position switches, placarded  #  1  IN  RESET  - ON -OFF and  #  2  IN  RESET  -ON  -OFF,  located  on  the mission   control   panel   (fig.      4-1),   control   three   phase inverter operation. (2) Three  phase  volt/frequency  meters.    Two digital   three-phase   volt/frequency   meters,   mounted   on the  mission  control  panel  (fig.    4-1),  monitor  and  display the   voltage   and   frequency   outputs   of   the   three   phase inverters. (3) Three    phase    loadmeters.        Two    digital three-phase loadmeters, mounted on the mission control panel (fig.  4-1), monitor inverter output level. (4) Three    phase    AC    off    annunciator.        An annunciator    placarded    3f    AC   OFF,    located    on    the mission annunciator panel (fig.  4-1), indicates a problem with one of the three phase AC power busses. (5) Three phase AC external power.  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 on the underside of the left wing just outboard of the engine nacelle (fig.  2-1 and 2-2).  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. (a) External AC power annunciator. The  annunciator  placarded   EXT  AC  PWR  ON,  located on  the  mission  annunciator  panel  (fig.    4-1),  indicates that   external   AC   power   is   connected   to   the   3   phase busses.    The    EXTERNAL  POWER  annunciator  in  the advisory  annunciator  panel  indicates  that  an  AC  GPU plug is mated to the AC external power receptacle. (b) External   AC   power   control   switch. The   switch   placarded   EXT   PWR   -RESET   -ON   -OFF, located  on  the  mission  control  panel  (fig.    4-1),  controls application of three phase AC power to the aircraft. 2-83