TM 55-1510-220-10 b. Generator  Switches.    Two  switches  (fig.    212), placarded No.1 GEN and No.2 GEN are located on the overhead  control  panel  under  the  MASTER  SWITCH. 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  GEN switch in the OFF position, the affected unit cannot have its output restored to aircraft use until the GEN switch is moved to RESET, then ON. c. Master  Switch.    All  electrical  current  may  be shut  off  using  the  MASTER  SWITCH  gangbar  (fig.    2- 12)   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 forces each switch to the OFF position. d. Volt-Loadmeters.    Two  meters  (fig.    2-12),  on the overhead control panel display voltage readings and show   the   rate   of   current   usage   from   left   and   right generating  systems.    Each  meter  is  equipped  with  a spring-loaded  pushbutton  switch  which  when  manually pressed   will   cause   the   meter   to   indicate   main   bus voltage.   Each  meter  normally  shows  output  the  output amperage reading from the respective generator, unless the   pushbutton   switch   is   pressed   to   obtain   the   bus voltage reading.  Current consumption is indicated as a percentage  of  total  output  amperage  capacity  for  the generating system monitored. e Battery   Vol-Amp   Meter.      The   mission   control panel (fig.  4-1 ).  located on the right fuselage sidewall adjacent  to  the  copilot's  seat,  has  a  battery-amperage meter that displays battery voltage on the left side of the meter and (battery charge/discharge current) amperage on the right side of the meter. NOTE Battery voltage required for engine start is 24 VDC. f. Battery 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 yellow BATTERY CHARGE annunciator light and the master  fault  caution  light  will  illuminate.    Following  a battery   engine   start,   the   caution   light   will   illuminate approximately six seconds after the generator switch is placed   in   the   ON   position.      The   light   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 light may also illuminate for short intervals after landing gear and/or flap operation.  If the caution   light   should   illuminate   during   normal   steady- state  cruise,  it  indicates  that  conditions  exist  that  may cause  a  battery  thermal  runaway.    If  this  occurs,  the battery  switch  shall  be  turned  OFF  and  may  be  turned back ON only for gear and flap extension and approach to landing.  Battery may be used after a 15 to 20 minute cool down period. g. Generator Out Warning Lights. Two caution/advisory annunciator panel lights inform the pilot when  either  generator  is  not  delivering  current  to  the aircraft   DC   bus   system.      These   lights   are   placarded No.1   DC   GEN   and   No.2   DC   GEN.      Two   MASTER CAUTION   lights   and   illumination   of   either   fault   light 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. 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  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    power    plug    and applied directly to the battery bus after passing through the  external  power  relay.    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   BATT   switch   is   in   the   ON position.    The  GPU  shall  be  adjusted  to  regulate  at  28 volts    maximum    to    prevent    damage    to    the    aircraft battery. i. Security   Keylock   Switch.      The   aircraft   has   a security   keylock   switch   (fig.      2-12)   installed   on   the overhead control panel, placarded OFF ON. Change 2   2-66