TM 1-1510-223-10 of  the  two  MASTER  CAUTION  annunciators  and  either fault annunciator indicates that either the identified gener- ator   has   failed   or   voltage   is   not   sufficient   to   keep   it connected to the power distribution system. The GPU shall be adjusted to regulate at 28 t+.2   volts.      The   GPU   shall   be   capable   of producing 1000 amperes for 5 seconds, 500 amperes   for   2   minutes,   and   300   amperes continuously. h. DC  External  Power  Source.   External  DC  power can  be  applied  to  the  aircraft  through  an  external  power receptacle    on    the    underside    of    the    right    wing,    just outboard of the engine nacelle (fig.  2-1).  The receptacle is  installed  inside  of  the  wing  structure  and  is  accessible through  a  hinged  access  panel.    DC  power  is  supplied through the DC external power plug, through the external power   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  indicates  that  the DC external power plug is connected.  The EXT DC PWR ON annunciator indicates that external power is connected to the aircraft DC bus. i. Security   Keylock   Switch.      The   aircraft   has   a security    keylock    switch    (fig.        2-13)    installed    on    the overhead control panel, placarded OFF - ON.  The switch is connected to the battery relay circuit and must be ON when energizing the BATTERY MASTER SWITCH.  The key  cannot  be  removed  from  the  lock  when  in  the  ON position. j. Circuit  Breakers.     The  overhead  circuit  breaker panel (fig.  2-7) contains circuit breakers for most aircraft systems.    The  circuit  breakers  on  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 normal conditions. 2-74. AC POWER SUPPLY. a. Single  Phase  AC  Power  Supply.    AC  power  for the aircraft is supplied by inverter units, numbered #1 and #2  (fig.    2-28),  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,   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  INVEIER 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  10  INVERTER  #  1  and  #  2  on  the  overhead control   panel   (fig.      2-13),   control   the   single-phase   AC inverters. b. Volt-Frequency   Meters.       The   two   digital   volt- frequency    meters    (fig.        2-13)    are    mounted    on    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 400 Hz on each meter. c. Three Phase AC Power Supply.  Three phase AC electrical  power  (fig.    2-29)  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 INV - RESET - ON - OFF  and  #2  INV  -  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 30 AC OFF, located on the mission annunciator panel (fig.  4-1), indicates a problem with one 2-72