TM 1-1500-204-23-4 (6) Suitable    mechanical    guards,    such    as enclosures or barricades, shall be permanently installed on   all   machinery   not   already   equipped   with   such   to eliminate danger of injury from moving parts. (7) Machinery shall not be adjusted, repaired, oiled, or cleaned while machine is in operation or power is on. (8) Personnel operating machinery shall wear protective   clothing   as   prescribed.      A   protective   face shield or goggles shall be worn when operating a grinder regardless of whether grinder is equipped with attached shields. (9) Jewelry shall not be worn while performing any maintenance. d. Fire   Safety.      A   constant   vigilance   must   be maintained  to  seek  out  fire  hazards.    Fire  hazards  are constantly present in the shop where sparks, friction, or careless   handling   can   cause   an   explosion   that   may destroy    equipment    or    buildings,    and    injure    or    kill personnel.      Refer   to   AR   385-10,   The   Army   Safety Program and The Occupational Safety and Health Act of 1971. (1) NO  SMOKING  signs  shall  be  placed  in areas where smoking could create a fire hazard. (2) Personnel   shall   be   trained   in   the   use, knowledge, and location of shop fire fighting equipment. (3) Each   shop   shall   be   equipped   with   fire extinguishers suited for type fire most likely to occur. (4) Use  correct  fire  extinguisher  for  class  of fire as follows: · Class A fire (wood, paper, trash, etc).  Use water or bromotrifluoromethane fire extinguisher. · Class   B   fire   (oil,   paint,   fuel,   grease,   etc).      Use bromotrifluoromethane or carbon dioxide fire extinguisher, or dry chemical extinguisher with class B rating. · Class C fire (electrical equipment). Use bromotrifluoromethane or carbon dioxide fire extinguisher, or dry chemical extinguisher with class C rating. · Class    D    fire    (combustible    metals)    magnesium, titanium, zirconium, sodium, lithium, and potassium).  Use dry powder type fire extinguisher. (5) Oily  waste,  rags,  and  similar  combustible materials    shall    be    discarded    in    self-closing    metal containers which shall be emptied daily. (6) Use only approved cleaning solvents. (7) Flammable  materials  shall  not  be  stored in the shop. e. Maintenance of Shop Equipment.  Maintenance of   shop   equipment   consists   of   cleaning,   preventive maintenance,    and    replacement    of    defective    parts. Preventive maintenance includes before-operation, during-operation, after-operation, services performed by operator,  and  scheduled  services  to  be  performed  at designated intervals.  Consult the operation and service instructions manual for specific maintenance instructions on particular types of equipment. 2-2.  Basic Principles and Theory for Direct Current. Direct Current (dc) flows constantly in only one direction with   a   constant   polarity.      It   changes   magnitude   only when the circuit is opened or dosed, as shown in the dc wave form in figure 2-1. 2-3.      Basic   Principles   and   Theory   for   Alternating Current.  Alternating current (ac) flows periodically first in  one  direction  and  then  in  the  opposite  direction,  as shown in figure 2-2.  The time necessary for the current to go from 0 to a maximum positive, to 0, to maximum negative, and return again to 0 is the time for one cycle. That portion of the cycle during which the current flows in one direction only is called an alternation.  Frequency is the number of I cycles occurring per second, and an alternating current may be any frequency. 2-4.          Wiring     Diagram     Symbols,     Charts,     and Schematics.      See    figures    2-3    and    2-4    for    wiring diagram symbols and schematics. 2-5.    Basic  Circuits.    The  basic  electrical  circuits  of series,   parallel,   and   compound   are   explained   in   the following paragraphs. a. Series.  The series circuit, as shown in figure 2- 5, is the most basic type of electrical circuit.  It provides a complete path for current to flow from the negative to Change 3 2-2