TM 1-1500-204-23-4 (b)   If   adequate   isolation   cannot   be accomplished, the entire fuel system will be drained and purged. 3-3.        Installation    of    Electrical    Connectors    and Common  Plugs.    Plugs  and  receptacles  are  provided throughout the aircraft to provide a means of connecting individual  components  to  the  electrical  system  and  to facilitate    their    removal    and    installation.        Electrical connectors     also     are     provided     at     some     firewalls, bulkheads,  and  decks  to  provide  wire  routing  through the   aircraft   structure   and   to   facilitate   removal   and replacement    of    wiring    harness    segments.        Those connectors installed in areas of high vibration or in areas not  accessible  for  periodic  inspection  are  secured  with wire    or    otherwise    mechanically    locked    to    prevent opening of the connector due to vibration. a.        Types    of    Connectors.        Connectors    are identified  by  AN  or  MS  numbers  and  are  divided  into classes with the manufacturer's variations in each class. The manufacturer's variations are differences in appearance and in the method of meeting a specification.      Some   commonly   used   connectors   are shown in figure 3-4.  There are five basic classes of AN or  MS  connectors  used  in  most  aircraft.    Each  class  of connector has slightly different construction characteristics.    Classes  A,  B,  C,  and  D  are  made  of aluminum, and class K is made of steel. · Class A-Solid, one-piece back shell, general-purpose connector. · Class B-Connector back shell separates into two parts lengthwise.  Used primarily where it  is  important  that  the  soldered  connectors be   readily   accessible.      The   back   shell   is held   together   by   a   threaded   ring   or   by screws. · Class C-A pressurized connector with inserts that are not removable.  Similar to a class  A  connector  in  appearance,  but  the inside   sealing   arrangement   is   sometimes different.  It is used on walls of bulkheads of pressurized equipment. · Class D-Moisture-and vibration-resistant connector  which  has  a  sealing  grommet  in the back shell.  Wires are threaded through tight-fitting    holes    in    the    grommet,    thus sealing against moisture. · Class K-A fireproof connector used in areas where  it  is  vital  that  the  electric  current  is not  interrupted,  even  though  the  connector may be exposed to continuous open flame. Wires   are   crimped   to   the   pin   or   socket contacts  and  the  shells  are  made  of  steel. This  class  of  connector  is  normally  longer than other classes of connectors. (1)   MS   connectors.      MS   connectors   as shown in figure 3-4 are electric connectors with solder or removable crimp contacts (both front and rear release). These   connectors   are   for   use   in   electronic,   electric power, and control circuits. (2) Special-purpose connectors.  In addition to    the    MS    connectors,    special    purpose    types    are commonly    found    in    military    aircraft.        These    are described in the following paragraphs. (a) Moisture-proof connectors (Cannon  KE  series).    These  connectors  are  similar  to the   MS-K   series,   with   the   additions   of   a   moisture- resisting   seal.      They   will   mate   with   MS-K   plugs   or receptacles, but retain the moisture sealing feature only when   mated   with   corresponding   KE   series   plugs   or receptacles. (b)  Subminiature  connectors  (Cannon US Series).  These connectors, as shown in figure 3-5, are  wire  connecting  types  only;  they  have  no  mounting flanges,  but  can  be  mounted  with  nut  and  lockwasher. They     are     used     on     instruments,     switches,     relays, transformers, amplifiers, etc. (c) Rectangular shell connectors (Bendix   SR;   Cannon   DPD).      These   connectors,   as shown in figure 3-6, are flanged for panel or equipment mounting.  They consist of an aluminum alloy shell, rigid or  resilient  insert,  and  pin  or  socket  contact.    They  are usually potted to protect connections against moisture at the  back  of  the  connector.    The  mating  faces  are  not moisture sealed. Change 3  3-10