TM 55-1510-219-10 CAUTION The   GPU   shall   be   adjusted   to regulate  at  28  volts  maximum  to prevent damage to the aircraft. g. 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  leading edge    just    outboard    of    the    engine    nacelle.        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   plug   and   applied directly   to   the   battery   bus   after   passing   through   the external  power  relay.    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 BATT switch is in the ON position.  The GPU shall be adjusted to regulate at 28 volts maximum to prevent damage to the aircraft battery. h. Security   Keylock   Switch.      The   aircraft   has   a security   keylock   switch   (fig.      2-18)   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  power  switch.    The key  cannot  be  removed  from  the  lock  when  in  the  ON position. i. Circuit  Breakers.    The  overhead  circuit  breaker panel   (fig.      2-26)   contains   circuit   breakers   for   most aircraft systems.  The circuit breakers on the panel are grouped   into   areas   which   are   placarded   as   to   the general  function  they  protect.    A  DC  power  distribution panel is mounted beneath the floor forward of the main spar.    This  panel  contains  higher  current  rated  circuit breakers  and  is  not  accessible  to  the  flight  crew  under normal conditions. 2-73.  AC POWER SUPPLY. a. Single Phase AC Power Supply.    AC  power  for the  aircraft  is  supplied  by  inverter  units,  numbered  #1 and  #2  (fig.    2-23)  which  obtain  operating  current  from the  DC  power  system.    Both  inverters  are  rated  at  750 volt-amperes   and   provide   single   phase   output   only. Each inverter provides 115 volt and 26 volt 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. b. AC  Power  WARNING/CAUTION  Lights.   Two MASTER/CAUTION   lights   and   the   illumination   of   an annunciator caution light #1 INVERTER or #2 INVERTER indicates an inverter failure. c. Instrument  AC  Light.    A  red  warning  light, located   on   the   warning   annunciator   panel,   placarded INST   AC,   will   illuminate   if   all   instrument   AC   busses should fail. d. Inverter  Control  Switches.    Two    switches  (fig. 2-18), placarded INVERTER #1 and #2 on the overhead control  panel  give  the  pilot  control  of  inverters  single- phase AC power. e. Volt-Frequency    Meters.        Two    volt-frequency meters (fig.  2-18) are mounted in the overhead control panel to provide monitoring capability for both 115 VAC buses.      Normal   display   on   the   meter   is   shown   in frequency   (Hz).      To   read   voltage,   press   the   button located  in  the  lower  left  corner  of  the  meter.    Normal output of the inverters will be indicated by 115 VAC and 400 Hz on the meters. f Three  Phase  AC  Power  Supply.    Three  phase AC    electrical    power    for    operation    of    the    inertial navigation  system  and  mission  avionics  is  supplied  by either  of  two  DC  powered  3000  volt-ampere  solid  state three phase inverters (fig.  2-24). (1) Three  phase  inverter  control  switch.      A three position switch placarded #1 MISSION INV, OFF, #2  MISSION  INV,  located  on  the  mission  control  panel controls three phase inverter operation. (2) Three   phase   volt-frequency   meter.      A three    phase    volt/frequency    meter,    mounted    on    the mission  control  panel,  monitors  output  of  the  selected three    phase    inverter.        Frequency    (Hz)    is    normally displayed  on  the  meter.    To  read  voltage  press  button located in the lower left corner of the meter. (3) Three  phase  loadmeter.    A  three  phase loadmeter,    mounted    on    the    mission    control    panel, monitors inverter output level. (4) Three phase AC off annunciator light.  An indicator   light   placarded   3(   AC   OFF,   located   on   the mission annunciator panel indicates that three phase AC power is not being supplied. (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  leading  edge just  outboard  of  the  engine  nacelle.    The  receptacle  is installed   inside   the   wing   structure   and   is   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. 2-66