of  similar  units  proceeds  from  starboard  to  port, forward to aft, and from a lower to a higher level. Within a given compartment, then, the numbering of similar units follows the same rule; that is, LOWER takes  precedence  over  UPPER,  FORWARD  over AFT,  and  STARBOARD  over  PORT. Electrical  distribution  panels,  control  panels,  and so forth, are given identification numbers made up of three numbers separated by hyphens. The first number identifies  the  vertical  level  by  deck  or  platform number  at  which  the  unit  is  normally  accessible. Decks of Navy ships are numbered by using the main deck  as  the  starting  point  as  described  in  Basic Military  Requirements,  NAVEDTRA 12043. The numeral  1  is  used  for  the  main  deck,  and  each successive deck above is numbered 01, 02, 03, and so on, and each successive deck below the main deck is numbered 2, 3, 4, and so on. The second number identifies the longitudinal location  of  the  unit  by  frame  number.  The  third number  identifies  the  transverse  location  by  the assignment of consecutive odd numbers for centerline and starboard locations and consecutive even numbers for port locations. The numeral 1 identifies the lowest centerline  (or  centermost,  starboard)  component. Consecutive odd numbers are assigned components as they would be observed first as being above, and then outboard,  of  the  preceding  component.  Consecutive even  numbers  similarly  identify  components  on  the portside.  For  example,  a  distribution  panel  with  the identification number, 1-142-2, will be located on the main  deck  at  frame  142,  and  will  be  the  first distribution panel on the port side of the centerline at this frame on the main deck. Main switchboards or switchgear groups supplied directly from ship’s service generators are designated 1S, 2S, and so on. Switchboards supplied directly by emergency generators are designated 1E, 2E, and so on. Switchboards for special frequencies (other than the frequency of the ship’s service system) have ac generators designated 1SF, 2SF, and so on. Sections of a switchgear group other than the generator section are designated by an additional suffix letter starting with the letter A and proceeding in alphabetical order from left to right (viewing the front of the switchgear group). Some large ships are equipped with a system of distribution called zone control. In a zone control system, the ship is divided into areas generally coinciding with the fire zones prescribed   by   the   ship’s   damage   control   plan. Electrical power is distributed within each zone from load center switchboards located within the zone. Load   center   switchboards   and   miscellaneous switchboards on ships with zone control distribution are given identification numbers, the first digit of which indicates the zone and the second digit the number  of  the  switchboard  within  the  zone  as determined  by  the  general  rules  for  numbering electrical  units  discussed  previously. Cable Marking Metal  tags  embossed  with  the  cable  designations are   used   to   identify   all   permanently   installed shipboard electrical cables. These tags (fig. 6-1) are placed on cables as close as practical to each point of connection on both sides of decks, bulkheads, and other   barriers.   They   identify   the   cables   for maintenance and replacement. Navy ships use two systems of cable marking; the old system on pre-1949 ships, and the new system on those built since 1949. We  will  explain  both  systems  in  the  following paragraphs. OLD CABLE TAG SYSTEM.—In   the   old system,   the   color   of   the   tag   shows   the   cable classification:  red—vital,  yellow—semivital,  and gray or no color—nonvital. The tags will contain the following  basic  letters  that  designate  power  and lighting cables for the different services: C D F FB G MS P R RL S Interior communications Degaussing Ship’s service lighting and general power Battle power Fire control Minesweeping Electric   propulsion Radio and radar Running,  anchor,  and  signal  lights Sonar FE Emergency lighting and power Figure 6-1.—Cable tag. 6-2