TM 1-1510-218-10 2-108 or  voltage  is  insufficient  to  keep  it  connected  to  the power distribution system. CAUTION The   Ground   Power   Unit   (GPU)   shall   be adjusted  to  regulate  at  28-volts maximum to  prevent  damage  to  the  aircraft.    Do not turn  generators  on  when  GPU  is  supplying power. j.    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  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 keylock and   BATT / BATTERY switches are in the ON position.   The GPU shall  be  adjusted  to  regulate  at  28-volts maximum to prevent damage to the aircraft battery and electronics. k.   Security Keylock Switch.   The aircraft has a security   keylock   switch   installed   on   the   overhead control  panel,  placarded  OFF  /  ON.     The  switch  is connected  into  the  battery  relay  and  external  power circuits, in series with the battery master power switch.   The key cannot be removed from the lock when in the ON position.   The key will not fit the keylocks on other Army aircraft. l.    Circuit    Breakers.         The    overhead    circuit breaker    panel    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.  Refer to Figure 2-16, Sheet 1 for  C configuration,  Sheet  2  for   D1,  Sheet  3  for   D2,  and sheet 4 for  T . 2-84.  AC POWER SUPPLY. a.   Description C D1 .  AC power for the aircraft is  supplied  by  inverter  units,  numbered  #1  and  #2, which   obtain   operating   current   from   the   dc   power system.  Refer to Figure 2-29. Both inverters are rated at 750 VA 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.    Aircraft  equipment  operating  from single-phase     ac     include     the     following:     autopilot navigation   receivers,   the   tuning   portion   of   the   ADF receiver, gyro magnetic compass, and engine instruments  for  fuel  flow  and  torquemeters.    Controls and indicators of the ac power system are located on the    overhead    control    panel    and    on    the    caution/ advisory   annunciator   panel.      The   pilot   selects   ac sources using the two inverter select switches located on the overhead control panel. NOTE  T2 AC  Power  for  ANVIS  cathode  tube  lighting is provided by cathode power packs totally independent    of    the    #1    and    #2    aircraft inverters.  b.  Description  D2  T .  AC power for the aircraft is supplied by inverter units, numbered #1 and #2, that obtain   operating  current  from  the  dc  power  system.   Refer to Figure 2-29, Sheets 2 and 3.  Both inverters are  rated  at  750  VA  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.    Aircraft  equipment operating  from  single-phase  ac  include  the  following: autopilot/flight  director,  the  tuning  portion  of  the  ADF receiver, gyro magnetic compass, RMI, pilot's altimeter,    transponder    IFF    computer,    and    engine instruments  for  fuel  flow  and  torquemeters.    Controls and indicators of the ac power system are located on the    overhead    control    panel    and    on    the    caution/ advisory   annunciator   panel.      The   pilot   selects   AC sources using the two inverter select switches located on the overhead control panel. c.   AC  Power  Warning  Lights.      Two  flashing MASTER  CAUTION  lights  and  the  illumination  of  an annunciator     caution     light     #1     INVERTER     or #2 INVERTER, indicate an inverter failure. d.  Inverter  Control  Switches.     Two  switches, placarded   INVERTER    #1   or    #2   on   the   overhead control  panel  give  the  pilot  a  choice  of  inverters  to provide   single-phase   ac   power.      Two  inverters  are involved in this selection. e.   Volt-Frequency    Meters    C    D    T1.        Two volt-frequency  meters  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.