TM 5-6115-590-12 c.  Power Unit electrical System (fig.  1-7 and FO-1). The  power  unit  electrical  system  consists  of  a  24V  DC system   powered   by   batteries,   a   120/240V   AC   single phase,  60  Hz  system  powered  by  the  engine  driven  60 Hz generator, and a 120/208V AC three phase, 400 Hz system powered by the engine driven 400 Hz generator. The 24V DC system provides power for engine starting and   control,   control   power   for   the   other   power   unit systems  (electrical,  conditioned  air  and  water)  control components,   and   emergency   lighting   power   for   other elements    of    the    MUST    hospital.        The    24V    DC emergency  light  power  is  distributed  through  the  60  Hz system convenience receptacles and cable assemblies. The 60 Hz system provides 60 Hz power to elements of the  MUST  hospital  as  required.    The  400  Hz  system provides 400 Hz power for operation of the conditioned air  system,  water  pumps,  water  hose  heating,  battery changing, and for 400 Hz power requirements for other elements  of  the  MUST  hospital.    All  of  the  electrical systems  incorporate  safety  devices  to  disconnect  the electrical output and prevent damage to components in the event of an overload, undervoltage, overvoltage, or underfrequency  condition  (except  60  Hz  system).    The power unit includes electrical circuits for connection as a standby   unit   and   will   automatically   start   and   deliver power should the operating power unit shut down.  The 24V    DC    system    also    includes    circuits    and    cable assemblies for connection of an external source of 24V DC  power  for  starting.    The  power  unit  may  also  be connected to another power unit to provide 24V DC and 400    Hz    power    to    the    nonoperating    power    unit    for operation of the ventilating fans in the nonoperating unit, provided  the  cooled  air  system  in  the  operating  unit  is not operating. (1)   24V  DC  Electrical  System  (fig.    1-7  and  FO-1). The 24V DC system provides power for engine starting and for the switches, relays, contactors, solenoids, etc., which control operation of the engine electrical system, 60  Hz  and  400  Hz  electrical  systems,  conditioned  air system, and water system.  The batteries which provide the   DC   power   will   also   provide   up   to   one   hour   of emergency light service to other elements of the MUST hospital   in   the   event   of   engine   or   electrical   system failure.          The     batteries     are     recharged     through     a transformer-rectifier  type  battery  charger  powered  from the   400   Hz   electrical   system.      Provisions   are   also incorporated   in   the   24V   DC   system   to   connect   an external source of 24V DC power for engine starting in the event the batteries are discharged. (2)   60  Hz  Electrical  System  (fig.    1-7  and  FO-1). The 60 Hz electrical system provides 120/240V AC, 60 Hz, single phase output power for external requirements.  The system consists of a brushless, self- excited,   air-cooled   generator,   a   voltage   regulator,   a generator  protective  panel,  output  power  convenience receptacles, and interconnecting wiring and connectors. The  60  Hz  system  neutral  conductor  is  not  grounded. Control  components  in  the  60  Hz  system  are  powered from the 24V DC system. Figure FO-1 Power plant electrical system schematic (Sheet 1 of 2) (Located in back of manual) Figure FO-1 Power plant electrical system schematic (Sheet 2 of 2) (Located in back of manual) (3)   400  Hz  Electrical  System  (fig.    1-7  and  FO-1). The  400  Hz  electrical  system  provides  120/208V  AC, 400    Hz,    three    phase    power    for    operation    of    the conditioned air system compressors, condenser cooling fans, air circulating fans, hot and cold water pumps, and water  hose  heaters,  24V  DC  battery  charger,  and  for 400  Hz  power  requirements  in  other  elements  of  the MUST   hospital.      The   400   Hz   system   consists   of   a brushless,   self-excited,   air-cooled   generator,   voltage regulator,    generator    protective    panel,    output    power receptacles, and interconnecting wiring and connectors. The  400  Hz  generator  incorporates  an  integral  cooling fan  and  receives  additional  cooling  air  from  the  engine cooling  fan.    The  400  Hz  system  neutral  conductor  is grounded.    Control  components  in  the  400  Hz  system are powered from the 24V DC system. d.    Conditioned  Air  System  (fig.    1-8,  1-9,  and  1-10). The  conditioned  air  system  supplies  cooled  air,  heated air,   or   ventilating   air   under   slight   pressure   to   other elements   of   the   MUST   hospital.      The   system   is   a recirculating system with recirculating fans in the return air    plenum.        Adjustable    vents    in    the    return    air compartments permit adding make-up air to the system. The   conditioned   air   system   consists   of   a   cooled   air system,   a   heated   air   system,   and   a   ventilating   air system.    The  three  systems  function  independently  but use some components interchangeable between systems.  The four conditioned air outlets are located at the   rear   of   the   power   unit   and   provide   connection flanges  for  attaching  the  distribution  ducts.    The  four return air inlets are located on the right side of the power unit  and  provide  connection  flanges  for  attaching  the return air ducts.  Covers are provided for the outlets and inlets  when  not  in  use.    The  four  return  air  inlets  are provided   with   permanent   high-velocity-type   cleanable air filters.  Two temperature sensors are located at each of   the   four   return   air   inlets   to   provide   temperature control   of   the   output   air   in   response   to   temperature changes  in  the  return  air.    Also,  a  remote  temperature sensor   may   be   connected   to   an   electrical   connector provided on the return air inlet panel to provide 1-13