TM 55-1510-222-10 by LEFT and RIGHT bleed air valve switches placarded ENVIRO & PNEU BLEED AIR PNEU ONLY ON.  When the switch is in the ON position, the environmental flow control  unit  and  the  pneumatic  valve  are  open.    When the    switch    is    in    the    PNEU    ONLY    position,    the environmental    flow    control    unit    is    closed    and    the pneumatic  bleed  air  valve  is  open;  in  the  ENVIRO  & PNEU   BLEED   AIR   position,   both   are   closed.      For maximum   cooling   on   the   ground,   turn   the   bleed   air valve switches to the PNEU ONLY position. (4) The    forward    vent    blower    is controlled   by   the   switch   placarded   VENT   BLOWER FWD   AUTO   LOW   HI.      The   HI   and   LOW   positions regulate  the  blower  to  two  speeds  of  operation.    In  the AUTO  position,  the  fan  will  run  at  low  speed  except when   the   CABIN   TEMP   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.  2-25)  is  the  basic  power system    energizing    most    aircraft    circuits.        Electrical power  is  used  to  start  the  engines,  power  the  landing gear   and   flap   motors,   and   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 2-26, and the three phase AC power   system   is   shown   in   figure   2-27.      The   three sources    of    DC    power    consist    of    one    20    cell    34- ampere/hour    battery    and    two    400-ampere    starter- generators.    DC  power  may  be  applied  to  the  aircraft through  an  external  power  receptacle  on  the  underside of the right wing stub, just outboard of the nacelle.  The starter-generators   are   controlled   by   generator   control units.    The  output  of  each  generator  passes  through  a cable  to  the  respective  generator  bus  (fig.  2-25):  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    inertial    navigation    system    and    mission avionics  is  supplied  by  two  DC  powered  three  phase mission inverters (fig. 2-28). The mission AC/DC power cabinet (fig. 2-29) located in the mission rack, aft of the copilot's seat. AC power may be applied through an external power receptacle located in  the  underside  of  the  left  wing  stub,  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  in  the  overhead  control  panel  (fig.  2-13)  and consist    of    a    single    battery    switch,    two    generator switches,  two  DC  digital  voltmeters,  and  two  DC  digital loadmeters. a.   Battery Switch. The switch, placarded BATTERY OFF RESET ON (fig. 2-13), is located on the overhead  control  panel  under  the  MASTER  SWITCH (gangbar).  The BATTERY switch controls DC power to the  aircraft  bus  system  through  the  battery  relay,  and must be ON to allow external power 2-62