to L1 and L2 to L2), as above. With most equipment  with  which  the  AC  unit  may be  used,  and  with  all  ordinary  lighting, the  relationship  is  unimportant. 4-3. Equipment Conversion (AC Unit) Position  the  voltage  change  switch,  S2,  either for  120  volt  or  240  volt  operation,  as  required. Refer to figure 2-3, View A. 4-4. Procedures for Constructing Revetment a. General.  This  equipment  is  designed  to  oper- ate  in  the  open  with  unrestricted  ventilation.  In some  situations  it  may  be  necessary  to  operate the equipment from the protection of a revetment. This  paragraph  provides  information  on  the  con- struction  of  a  revetment  to  protect  the  equipment should  it  become  necessary. b.  Dimensions.  The minimum   allowable   inside dimensions  are  shown  in  figures  4-1  and  4-2.  The entrance  to  the  revetment  is  shown  in  figure  4-3. These  minimum  dimensions  are  based  solely  on consideration  of  engine  cooling  and  ventilation allowing a minimum practical amount of space for service  and  maintenance. c. Foundation   and   Drainage. If   the   generator set  is  attached  to  a  shipping  platform,  this  will provide  an  adequate  foundation.  Otherwise,  use planks,  timbers,  logs,  ammunition  boxes,  or  other available  material  to  prevent  the  skids  from  sink- ing  into  soft  earth  or  sand.  In  no  case  should  the foundation   exceed   a   height   of   six   inches;   the generator  set  should  be  situated  on  a  level  plane not  to  exceed  a  tilt  of  15°  in  any  direction.  Drain- age  must  be  provided  to  insure  run-off  of  water away  from  the  generator  set  and  out  of  the revetment  through  drain  holes  at  inside  ground level.  Where  there  is  no  natural  slope  for  the water  to  run  away  from  the  revetment,  a  sump and  drainage  trench  must  be  provided  outside  of the  revetment  for  each  required  drain  hole. d. Wall Constructwn. The  walls of the revetment may   be   constructed   with   sandbags,   ammunition boxes  filled  with  sand  or  dirt,  or  any  other  mate- rial available. The wall height should not exceed 3 feet  (0.91  meters)  and  should  be  constructed  as shown  in  figures  43  and  44. e. Roof Construction. The roof  can  be  supported by  any  means  possible,  but  should  be  1  foot  (0.31 meters)   above   the   wall   of   the   revetment   and provide  as  much  open  space  around  the  top  as possible.  Refer  to  figures  4-2,4-3,4-4,4-5,  and  4-6. TM   5–6115-323-1 The  materials  used  in  the  roof  construction  con- sist of two pieces of lumber (4 inches by inches) or logs  (4  inches  in  diameter)  about  10  feet  (3.1 meters)   long   and   the   necessary   cross   pieces   of lumber,  logs,  or  steel  planking  to  cover  the  entire roof  as  shown  in  figure  4-6.  These  cross  pieces should  be  about  8  feet  (2.44  meters)  long.  If  the above  materials  are  not  readily  available,  any available  material  of  a  like  nature  may  be  used. The  thickness  of  the  roof  (figs.  4-3  and  4-5)  will depend  upon  the  amount  and  type  of  protection desired.   Caution   should   be   taken   when   adding protection  to  the  roof  to  insure  that  the  roof  is supported  properly  to  carry  the  additional  weight. f.  Miscellaneous  Construction. A  compartment should  be  constructed  for  fuel  storage  outside  of the  revetment  as  shown  in  figure  4-1.  The  size  of the  fuel  storage  area  will  depend  upon  the  size  of the  fuel  container  being  used.  Since  air  tempera- tures  within  the  revetment  will  build  up  consider- ably  above  the  outside  ambient  when  the  genera- tor  set  is  operating,  always  locate.  the  fuel  supply outside  the  revetment  to  minimize  safety  hazards associated   with   gasoline   at   high   temperatures. For  the  same  reason,  it  is  recommended  that  the integral   fuel   tanks,   with   which   some   generator sets  are  equipped,  not  be  used.  The  exhaust  from the   generator   set   is   to   be   ducted   out   of   the revetment.  The  installation  should  be  similar  to that  shown  in  figure  4-7.  If  flexible  pipe  is  not available  to  use  for  an  exhaust  extension,  a  piece of exhaust pipe or any other similar material may be  utilized.  The  point  at  which  the  exhaust  dis- charges  goes  through  the  revetment  wall  depends entirely upon the equipment that is available. The exhaust  may  be  discharged  into  an  external  ex- haust chimney constructed as shown in figure 4-1. This  chimney  will  aid  in  ducting  the  exhaust gases  away  from  the  revetment  and  will  reduce the  noise  level.  A  revetment  doorway  shield  must be constructed as shown in figures 4-1, 4-2, and 4– 6.  This  shield  is  a  wall  3  feet  (0.91  meters)  high and  7.5  feet  (2.26  meters)  long,  constructed  in  a manner  similar  to  the  revetment  walls  to  prevent direct   entrance   of   projectiles   or   fragments through  the  doorway  into  the  revetment. g.   Alignment   Instructions. When   constructing the  revetment,  the  orientation  of  the  revetment will  be  such  that  its  entrance  is  facing  into  the direction  of  the  prevailing  wind  (fig.  4-1).  Each generator  set  will  be  located  within  the  revetmen so  that  its  long  axis  is  parallel  to  the  long  axis  of the  revetment  and  is  centered  within  the  revet- 4 – 3