TM 1-1510-225-10 2-108 e.   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   that   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    yellow    BATTERY   CHG annunciator and the  MASTER  CAUTION annunciator will  illuminate.    Following  a  battery  engine  start,  the caution     annunciator     will     illuminate     approximately 6-seconds  after  the  generator  switch  is  placed  in  the ON position.  The annunciator will normally extinguish within   2   to  5 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 volt-loadmeters.  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. f.    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  L  DC  GEN   and R 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 28.0    to    28.4    volts.        The    GPU    shall    be capable of producing 1000 amps capacity, 500 amperes for 2 minutes, and 300 amps maximum continuous.  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,   just   outboard   of    the    engine    nacelle.        The receptacle  is  installed  inside  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 28.2  volts maximum to prevent damage    to    the    aircraft    battery.        The    EXT   PWR annunciator  indicates  that  the  dc  external  power  plug is connected. h.  Circuit Breakers.   The right and left sidewall circuit  breaker  panels  contain  the  circuit  breakers  for most  aircraft  systems.    The  circuit  breakers  on  the panels are grouped into areas that are placarded as to their  general  function.    A  DC  power  distribution  panel is   mounted   beneath   the   aisle,   forward   of   the   main spar.    This  panel  contains  higher  current  rated  circuit breakers and is not accessible to the flight crew under normal conditions. 2-72.  AC POWER SUPPLY. a.   AC   Power.      AC   power   for   the   aircraft   is supplied  by  two  single-phase  inverters,  which  obtain operational  current  from  the  dc  power  system.  Each inverter provides 115 and 26 volts, 400 Hz ac output.   The    inverters    are    protected    by    circuit    breakers mounted  on  the  dc  power  distribution  panel  mounted beneath the floor. (1)   AC  Power  Annunciators.    Illumination  of the two   MASTER  WARNING   annunciators,  and  the illumination   of   the   INVERTER   warning   annunciator indicates inverter failure. (2)   Inverter  Control  Switches.    The  inverters are controlled by a switch placarded INVERTER NO. 1 / OFF / NO. 2, located on the pilot's subpanel. b.  Volt-Frequency    Meter.        A    volt-frequency meter  is  located  on  the  overhead  control  panel.    A reading  of  115  Vac  and  400  hertz  will  indicate  normal bus   conditions.      The   volt-frequency  meter  normally displays frequency.  Voltage may be read by pressing a push-button switch on the meter.