two-, T for three-, and F for four-conductor cables. The number following this letter denotes the wire’s circular mil area in thousands. For example, the cable supplying distribution  box  symbol  615  (fig.  6-2)  is  marked (2-38-1)-L-Al-T-g.  This  marking  identifies  a  three-con- ductor, 9000-circular mil, 120-volt, ship’s service sub- main lighting cable supplied from panel 2-38-1. Note that you would need the isometric wiring diagram for the main deck and above to follow the complete run of this  cable.  This  print  would  show  lighting  main 2(38-l)-lL-A-T-30 supplying a distribution box some- where on the main deck (or above), and submain cable (2-38-l)-IL-Al-T-9  coming  from  this  distribution  box to supply distribution box symbol 615 on the second platform, frame 19 starboard. Remember, the isometric wiring diagram shows only the general location of the various cables and fixtures. Their exact location is shown on the wiring plan discussed briefly in the next paragraphs. Wiring Deck Plan The wiring deck plan is the actual installation dia- gram for the deck or decks shown and is used chiefly in ship construction. It helps the shipyard electrician lay out his or her work for a number of cables without referring to individual isometric wiring diagrams. The plan includes a bill of material that lists all materials and equipment necessary to complete installation for the deck  or  decks  concerned.  Equipment  and  materials except cables are identified by a symbol number both on the drawing and in the bill of material. Wiring deck plans are drawn to scale (usually 1/4 inch to the foot), and they show the exact location of all fixtures. One blueprint usually shows from 150 to 200 feet of space on one deck only. Electrical wiring equipment symbols from MIL-STD-15-2 are used to represent fixtures just as they do in the isometric wiring diagram. Elementary Wiring Diagram These diagrams show in detail each conductor, terminal, and connection in a circuit. They are used to check for proper connections in circuit or to make the initial  hookup. In   interior   communication   (IC)   circuits,   for example, the lugs on the wires in each connection are stamped  with  conductor  markings.  The  elementary wiring  diagrams  show  these  conductor  markings alongside each conductor and how they connect in the circuit.  Elementary  wiring  diagrams  usually  do  not show the location of connection boxes, panels, and so on; therefore, they are not drawn to any scale. Electrical System Diagrams Navy  ships  have  electrical  systems  that  include many types of electrical devices and components. These devices and components may be located in the same  section  or  at  various  locations  throughout  the ship. The electrical diagrams and drawings necessary to operate and maintain these systems are found in the ship’s  electrical  blueprints  and  in  drawings  and diagrams   in   NAVSHIPS’   and   manufacturers’ technical  manuals. BLOCK  DIAGRAM.—These     diagrams     of electrical systems show major units of the system in block form. They are used with text material to present a general description of the system and its functions. Figure 6-3 shows a block diagram of the electrical steering system for a large ship. Look at the diagram along   with   the   information   in   the   following paragraphs to understand the function of the overall system. The steering gear system (fig. 6-3) consists of two similar  synchro-controlled  electrohydraulic  systems; one for each rudder (port and starboard). They are separate systems, but they are normally controlled by the same steering wheel (helm) and they move both port and starboard rudders in unison. Each port and star- board system has two 100 hp main motors driving a variable-stroke pump through reduction gears. Each also has two 5-hp servo pump motors interconnected electrically  with  the  main  pump  motors  so  both  operate simultaneously.  During  normal  operation,  one  main pump motor and one servo pump motor are used with the other units on standby. If the normal power supply fails, both port and starboard transfer switchboards may be transferred to an emergency 450-volt supply. The steering system may be operated from any one of three steering stations located in the pilothouse, at a   secondary   conn,   and   on   the   open   bridge.   A transmitter selector switch in the IC room is used to assign steering control to any of the three. To transfer steering control from the pilothouse to the open bridge station, the selector switch in the IC room must be in the  pilothouse  position.  Duplicate  power  and  control cables (port and starboard) run from a cable selector in the IC room to port and starboard cable selector switches  in  the  steering  gear  room.  From  these switches, power and control cables connect to receiver selector  switches.  These  selector  switches  allow selection of the appropriate synchro receiver for the system in operation. 6-5