TM 1-1510-224-10 CABIN  AIR MODE SELECT switch is placed in the  OFF position.      In   the    OFF    position,    the    blower    will    not operate. (5) The   aft   vent   blower   is   controlled   by   the switch placarded VENT BLOWER AFT - AUTO - ON. The single speed blower operates automatically through the N₁  speed  switch  when  the  aft  vent  blower  switch  is placed   in   the   AUTO   position.      The   blower   operates continuously    when    the    switch    is    placed    in    the    ON position  with  the  air  conditioner  compressor  running.    In the OFF position, the blower will not operate. Section IX.  ELECTRICAL POWER SUPPLY AND DISTRIBUTION SYSTEM 2-72.  DESCRIPTION. The  aircraft  employs  both  direct  current  (DC)  and alternating    current    (AC)    electrical    power.        The    DC electrical power supply (fig.  2-29 and 2-30) is the basic power system energizing most aircraft circuits.  Electrical power  is  used  to  start  the  engines,  power  the  landing gear  and  flap  motors,  operate  the  standby  fuel  pumps, ventilation  blower,  lights,  and  electronic  equipment.    AC power  is  obtained  from  the  DC  power  system  through inverters.  The single phase AC power system is shown in  figure  2-32,  and  the  three  phase  AC  power  system  is shown  in  figure  2-33.    The  three  sources  of  DC  power consist  of  one  20  cell  34-ampere/hour  battery  and  two 400-ampere    startergenerators.        DC    power    may    be applied    to    the    aircraft    through    an    external    power receptacle   on   the   underside   of   the   right   wing,   just outboard   of   the   nacelle.      The   startergenerators   are controlled by generator control units.  The output of each generator   passes   through   a   cable   to   the   respective generator  bus.    Other  buses  distribute  power  to  aircraft DC loads, deriving power from the generator buses.  The generators    are    paralleled    to    balance    the    DC    loads between   the   two   units.      When   one   of   the   generating systems is not on line, and no fault exists, all aircraft DC requirements may be supplied by either the other on-line generating system or by an external power source.  The generator  system  is  designed  to  allow  cross  starting  of the  other  engine.    When  one  generator  is  on  line,  all current   limiters   are   bypassed   while   starting   the   other engine.    Most  DC  distribution  buses  are  connected  to both    generator    buses    but    have    isolation    diodes    to prevent power crossfeed between the generating systems, when connection between the generator buses is lost.  Thus, when either generator is lost because of a ground  fault,  the  operating  generator  will  supply  power for  all  aircraft  DC  loads  except  those  receiving  power from  the  inoperative  generator's  bus,  which  cannot  be crossfed.    When  a  generator  is  not  operating,  reverse current    and    over-voltage    protection    is    automatically provided.        Two    inverters    operating    from    DC    power produce   the   required   single-phase   AC   power.      Three phase   AC   electrical   power   for   the   inertial   navigation system   and   mission   avionics   is   supplied   by   two   DC powered  three  phase  mission  inverters  (fig.    2-33,  2-34, and 2-35). The   mission   AC/DC   power   cabinet   (fig.      2-36)   is located in the mission rack, aft of the copilot's seat.  AC power    may    be    applied    through    an    external    power receptacle located on the underside of the left wing, just outboard of the engine nacelle. 2-73.  DC POWER SUPPLY. One   nickel-cadmium   battery   furnishes   DC   power when  the  engines  are  not  operating.    This  24-volt  34- ampere/hour   battery,   located   in   the   right   wing   center section,  is  accessible  through  a  panel  on  the  top  of  the wing.    DC  power  is  produced  by  two  engine-driven  28 volt,    400-ampere    starter-generators.        Controls    and indicators   associated   with   the   DC   supply   system   are located  on  the  overhead  control  panel  (fig.    2-15)  and consist    of    a    single    battery    switch,    two    generator switches,  two  DC  digital  voltmeters,  and  two  DC  digital loadmeters. a. Battery Switch.  A switch, placarded  BATTERY - OFF/RESET - ON (fig.  2-15), is located on the overhead control  panel  under  the   MASTER  SWITCH  (gang  bar). The  BATTERY  switch  controls  DC  power  to  the  aircraft bus system through the battery relay, and must be  ON to allow external power to enter aircraft circuits.  When the MASTER    SWITCH    (gang     bar)     is     placed     aft,     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.    Two  switches  (fig.    2-15), placarded GENERATOR # 1 and  # 2 are located on the overhead     control     panel.          These     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   (springloaded   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 gang bar (fig. 2-15)   which   extends   above   the   battery   and   generator 2-72