TM 1-1510-224-10 switches.  The  MASTER  SWITCH  (gang  bar)  is  moved forward when a battery or generator switch is turned on. When  moved  aft,  the  bar  positions  each  switch  to  the OFF position. d. DC  Load  and  Voltmeters.    Four  digital  meters, located   on   the   overhead   control   panel   (fig.      2-15), display  voltage  readings  and  show  the  rate  of  current usage  from  the  left  and  right  generating  systems.    The two  loadmeters  indicate  output  amperage  as  a  percent of rated capacity from the respective generator.  Current consumption is indicated as a percentage of total output amperage   capacity   for   the   generating   system   being monitored.    The  two  voltmeters  indicate  bus  voltage  for the respective generating system. e. Battery    Volt/Ammeter.        The    mission    control panel (fig.  4-1), located in the fuselage sidewall adjacent to  the  copilot's  seat,  incorporates  a  digital  volt/  ammeter that   displays   available   battery   voltage   and   amperage. Minimum battery voltage for engine starting is 22 VDC. f. Battery Charge Monitor. Nickel-cadmium battery overheating will cause the battery charge current to increase if thermal runaway is imminent.  The aircraft has  a  charge-current  sensor  which  will  detect  a  charge current.      The   charge   current   system   senses   battery current   through   a   shunt   in   the   negative   lead   of   the battery.    Any  time  the  battery  charging  current  exceeds approximately  7  amperes  for  6  seconds  or  longer,  the amber     BATTERY     CHARGE annunciator and the MASTER CAUTION annunciator will illuminate. Following a battery engine start, the caution annunciator will    illuminate    approximately    six    seconds    after    the generator   switch   is   placed   in   the   ON   position.      The annunciator   will   normally   extinguish   within   two   to   five minutes, indicating that.  the battery is approaching a full charge.  The time interval will increase if the battery has a  low  state  of  charge,  the  battery  temperature  is  very low, or if the battery has previously been discharged at a very low rate (i.e., battery operation of radios or lights for prolonged  periods).    The  caution  annunciator  may  also illuminate   for   short   intervals   after   landing   gear   and/or flap    operation.        If    the    caution    annunciator    should illuminate during normal steady-state cruise, this indicates  that  conditions  exist  that  may  cause  a  battery thermal   runaway.      If   this   occurs,   the   battery   current should   be   monitored   using   the   battery   ammeter.      If battery  current  continues  to  increase,  the  battery  is  in thermal runaway and should be selected   OFF  and  may be turned back  ON  only  for  gear  and  flap  extension  and approach to landing. g. Generator    Out    Warning    Annunciators.        Two caution/advisory   annunciator   panel   fault   annunciators inform  the  pilot  when  either  generator  is  not  delivering current to the aircraft DC bus system.  These annunciators are placarded   #  1  DC  GEN and   #  2  DC GEN.        Illumination    of    the    two    MASTER    CAUTION annunciators  and  either  fault  annunciator  indicates  that either the identified generator has failed or voltage is not sufficient  to  keep  it  connected  to  the  power  distribution system. CAUTION The GPU shall be adjusted to regulate   at   28   ±.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  and  2-2).    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.      2-15)   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.  2-9) 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.  2-32), which obtain operational current from 2-82