TM  55-1510-221-10 Section  IX.  ELECTRICAL  POWER  SUPPLY  AND  DISTRIBUTION  SYSTEM 2-73. DESCRIPTION. The  aircraft  employs  both  direct  current  (DC) and   alternating   current   (AC)   electrical   power.   The DC  electrical  power  supply  (fig.  2-22)  is  the  basic power  system  energizing  most  aircraft  circuits.  Elec- trical  power  is  used  to  start  the  engines,  to  power  the landing  gear  and  flap  motors,  and  to  operate  the standby   fuel   pumps,   ventilation   blower,   lights   and electronic  equipment.  AC  power  is  obtained  from DC  power  through  inverters.  The  single  phase  AC power  system  is  shown  in  figure  2-23,  and  the  three phase AC power system is shown in figure 2-24. 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  under- side  of  the  right  wing  leading  edge  just  outboard  of the  engine  nacelle  (refer  to  Section  XII  for  GPU requirements).   The   starter-generators   are   controlled by  generator  control  units.  The  output  of  each  gener- ator  passes  through  a  cable  to  the  respective  genera- tor  bus  (fig.  2-22).  Other  buses  distribute  power  to aircraft  DC  loads,  and  derive  power  from  the  gener- ator  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 either  by  the  other  on-line  generating  system,  or  by an  external  power  source,  but  not  by  both.  Most  DC distribution   buses   are   connected   to   both   generator buses  but  have  isolation  diodes  to  prevent  power crossfeed  between  the  generating  systems,  when  con- nection   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  auto- matically  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   mission   inverters.   AC   power may  be  applied  through  an  external  power  receptacle located  on  the  left  nacelle.  The  mission  power  sys- tem  is  shown  in  figure  2-25. 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.  2-12)  and  consist  of  a  single  bat- tery   switch   (BATT),   two   generator   switches   (No.1 GEN  and  No.2  GEN),  and  two  volt-loadmeters. a. Battery  Switch.  A  switch,  placarded  BATT (fig.  2-12)  is  located  on  the  overhead  control  panel under  the  MASTER  SWITCH.  The  BATT  switch controls  DC  battery  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  is  placed  down,  the  BATT 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  sys- tem  until  the  BATT  switch  is  moved  to OFF/RESET,  then  ON. b.  Generator  Switches.  Two   switches   (fig. 2-12),  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  out- put  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. 2-74. 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 d.   Volt-Loadmeters.   Two  meters  (fig.  2-12), on  the  overhead  control  panel  display  voltage  read- ings 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   the output  amperage  reading  from  the  respective  genera- tor,  unless  the  pushbutton  switch  is  pressed  to obtain  the  bus  voltage  reading.  Current  consumption 2-58