TM 55-1510-222-10 to  enter  aircraft  circuits.    When  the  MASTER  SWITCH (gangbar)   is   placed   down,   the   BATTERY   switch   is forced OFF. NOTE With battery or external power removed   from   the   aircraft   electrical system, due to fault, power cannot be restored     to     the     system     until     the BATTERY switch is moved to OFF/RESET, then ON. b. Generator  Switches.    The  two  switches (fig 2-13), placarded #1 GENERATOR and #2 GENERATOR    are    located    in    the    overhead    control panel.  The toggle switches control electrical power from the  designated  generator  to  paralleling  circuits  and  the bus distribution system.  Switch positions are placarded RESET,   ON   and   OFF.      RESET   is   forward   (spring- loaded   back   to   ON),   ON   is   center,   and   OFF   is   aft. When a generator is removed from the aircraft electrical system,    due    either    to    fault    or    from    placing    the GENERATOR  switch  in  the  OFF  position,  the  affected unit cannot have its output restored to aircraft use until the GENERATOR switch is moved to RESET, then ON. c. Master   Switch.      All   electrical   current may  be  shut  off  using  the  MASTER  SWITCH  gangbar (fig    2-13)    which    extends    above    the    battery    and generator  switches.    The  MASTER  SWITCH  (gangbar) 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  in  the  overhead  panel,  display  voltage readings  and  show  the  rate  of  current  usage  from  the left and right generating systems.  The two load meters 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/Amp  Meter.    The  mission control panel (fig. 4-1), located in the fuselage sidewall adjacent to the copilot's seat, incorporates a digital volt/ amp  meter  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 fault 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 until the beginning of the approach. 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     volts     maximum     to     prevent damage to the aircraft systems. h. DC External Power Source.  See figure 2-32.  External DC power can be applied to the aircraft through  an  external  power  receptacle  on  the  underside of   the   right   wing   stub,   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 power plug, through the external power 2-70