TM 55-1510-222-10 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. i. 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 position. j. 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 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. 2-13) 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 annunciator panel. (1) AC Power WARNING/CAUTION Annunciators.    Illumination  of  the two MASTER CAUTION annunciators, and the illumination of caution annunciator #1 INVERTER and/or #2 INVERTER indicates inverter failure. (2) Instrument  AC  Annunciator.    A red  INST  AC  warning  annunciator,  will  illuminate  if  all instrument AC buses should fail. (3) 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. b. 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. c. 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 phase inverters. (1) 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. (2) Three phase volt/frequency 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. (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.  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 busses. (5) Three phase AC external 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. 2-71