Module 4 - Introduction to Electrical Conductors, Wiring Techniques,
and Schematic Reading
Navy Electricity and Electronics Training Series (NEETS)
Chapter 3:  Pages 3-1 through 3-10

CHAPTER 3
 
SCHEMATIC READING

LEARNING OBJECTIVES

 
Upon completing this chapter, you should be able to:
 
1.    Recognize the marking system for cables to include shipboard and test equipment systems.
 
2.    Recognize the marking system for wire to include aircraft and shipboard electronic equipment
systems.
 
3.    Recall the seven types of electrical diagrams and the functional design of each.
 
4.    Recall basic safety practices and precautions for working around electrical and electronic
systems.
 

SCHEMATIC READING

 
This chapter is divided into three subtopics - (1) cable and wire-marling systems, (2) electrical and electronic diagrams, and (3) safety precautions. First, we will discuss the systems used for marking cables and wires. We will then explain each of the types of diagrams you will encounter when troubleshooting, testing, repairing, or learning about circuit or system operation. Finally, we will briefly discuss safety practices relating to working around electrical and electronic systems.
 

CABLE- AND WIRE-MARKING SYSTEMS

 
Cables and wires are marked to give the technician a means of tracing them when troubleshooting and repairing electrical and electronic systems.
 
Numerous cable- and wire-marking systems are used in ships, aircraft, and equipment throughout the Navy. A few of these systems are briefly discussed here to acquaint you with how marking systems are used. For a specific system or equipment, you should refer to tile applicable technical manual.
 
CABLE-MARKING SYSTEMS
 
Two typical cable-marking systems you are likely to see are the (1) shipboard and (2) test equipment
cable-marking systems.
 
Shipboard Cable-Marking Systems
 
Metal tags embossed with the cable markings are used to identify all permanently installed shipboard electrical cables. These cable tags (figure 3-1) are placed on cables close to each point of connection, and on both sides of decks, bulkheads, and other barriers to identify the cables. The markings on the cable tags identify cables for maintenance and circuit repairs. The tags show (1) the SERVICE LETTER, which identifies a particular electrical system, (2) the CIRCUIT LETTER or LETTERS, which identify a specific circuit within a particular system, and (3) the CABLE NUMBER, which identifies an individual cable in a specific circuit.
 
 

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Figure 3-1. - Cable tag.

 
In figure 3-1, note that the cable is marked "C-MB144." The letter C denotes the service; in this case, the IC (interior communication) system. The letters MB denote the circuit; in this case, the engine-order circuit. The number 144 denotes cable number 144 of the MB circuit.
 
Q1.    Why must cables and wires be identified?
 
Q2.    Where would you find the wire identification system for a specific piece of equipment?
 
Q3.    What does the cable number identify?
 
Test Equipment Cable-Marking Systems
 
View A of figure 3-2 shows apiece of test equipment that is used to check out electrical or electronic equipment or a system. It also shows the cables that are used to hook the tester to the equipment. The cables have metal or plastic tags at each end showing the cable number and the connector number.
 
 

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Figure 3-2. - Test equipment cable marking.

 
View B of figure 3-2 shows the method of connecting the tester to the piece of equipment to be tested. (For a specific tester, the technical manual supplied with the tester shows the method of connection.) The tester shown has four cables. These are numbered W1, W2, W3, and W4. Each cable has two connectors (plugs), one on each end, that are numbered P1 and P2. The cable tag on one end of the cable reads W1-P1, and the other end reads W1 -P2. As shown in the figure, W1-P1 is connected to the receptacle J1 on the tester. W1-P2 is then connected to receptacle J1 on the equipment to be tested. The same procedure is followed for connecting the remaining three cables. The hookup is then complete.
 
The shipboard and the test equipment cable markings just discussed are only two of many cable- marking systems you may encounter. There are too many systems to attempt to discuss them all. As stated earlier, you should study an equipment or installation technical manual before attempting repairs or connections.
 
 

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WIRE-MARKING SYSTEMS
 
Wire-marking systems are used to identify wires in aircraft, shipboard electronic equipment, and
power tool and appliance cables.
 
Aircraft Wire-Marking Systems
 
All aircraft wiring is identified on wiring diagrams exactly as the wire is marked in the aircraft. Each wire is coded by a combination of letters and numbers (figure 3-3) imprinted on the wire at prescribed intervals along the wire run.
 

Figure 3-3. - Aircraft wire marking.

 
Look at figure 3-3. The circuit function letter (P in this example) identifies the basic function of the circuit concerned. The letter P indicates that the wire is in the dc power distribution system of the aircraft. The wire number, 215, indicates that it is the 215th wire in the dc distribution system. The wire segment letter (A) identifies the position of each wire segment of the circuit. The wire segments are lettered in alphabetical sequence and change each time the wire passes through a terminal or connector. For example, after the wire passes through the first terminal or connector, the segment letter A, as in this instance, would change to B.
 
The wire size number (4) is the AN wire size. AN wire sizes have more strands for flexibility and are slightly different in circular mil area than AWG (American Wire Gauge) wire sizes. The current-carrying capacity of each is almost the same. The last letter (N) is the ground or phase letter. The letter N identifies any wire that completes the circuit to the ground network of the aircraft.
 
In a 3-phase ac power distribution system, a phase letter (A, B, or C) is used as the last letter of the wire marking. If aluminum wire is used as the conductor, ALUMINUM or ALUM will be added as a suffix to the wire identification code.
 
Q4.    If a wire passes through a connector what portion of the aircraft wire identification number changes?
 
Shipboard Electronic Equipment Wire-Marking Systems
 
The following explanation is an example of the type of conductor marking used in shipboard electronic equipment. These conductors may be contained in cables within the equipment. Cables within equipment are usually numbered by the manufacturer. These numbers will be found in the technical
 
 

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manual for the equipment. If the cables connect equipment between compartments on a ship, they will be marked by the shipboard cable-numbering system previously described.
 
On the conductor lead, at the end near the point of connection to a terminal post, spaghetti sleeving is used as a marking material and an insulator. The sleeving is marked with identifying numbers and letters and then slid over the conductor. The marking on the sleeving identifies the conductor connections both "to" and "from" by giving the following information (figure 3-4):
 

Figure 3-4. - Designating conductor marking between unlike terminals.

 
The terminal "from"
 
The terminal board "to"
 
The terminal "to."

These designations on the sleeving are separated by a dash. The order of the markings is such that the first set of numbers and letters reading from left to right is the designation corresponding to the terminal "from" which the conductor runs. Following this is the number of the terminal board "to" which the conductor runs. ("TB" is omitted when the sleeve is marked.) The third designation is the terminal  "to" which the conductor runs.
 
For example, as shown in figure 3-4, the conductor is attached to terminal 2A of terminal board 101 (terminal "from" 2A on the spaghetti sleeving). The next designation on the sleeving is 401, indicating it is going "to" terminal board 401. The last designation is 7B, indicating it is attached "to" terminal 7B of TB 401. The spaghetti marking on the other end of the conductor is read the same way. The conductor is going "from" terminal 7B on terminal board 401 "to" terminal 2A on terminal board 101.
 
On occasion, it may be necessary to run conductors to units that have no terminal board numbers; for example, a junction box. In this case, an easily recognizable abbreviation may be used in place of the terminal board number on the spaghetti sleeving. The designation "JB2" indicates that the conductor is connected to junction box No. 2. A conductor to junction box No. 2 of a piece of equipment would be identified as shown in figure 3-5. In the same manner, a plug would be identified as "P." This P number would be substituted for the terminal board number marking on the sleeving.
 
 

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Figure 3-5. - Marking of conductors running to a junction box.

POWER TOOL AND APPLIANCE MARKING SYSTEMS

 
As with the wire- and cable-numbering systems discussed so far, there are many color-coding systems used in electrical and electronic applications. The color-coding system discussed here is the one used to code conductors for power tools and appliances.
 
An electrical power tool or appliance is required to have a three-wire cable. The conductors in the cable are color-coded black, white, and green. At shore bases or civilian facilities, one side of the electrical input is grounded. The grounded side is called the "common," and is color-coded white. The other side of the input is called the "line," or hot side, and is color coded "black". The green conductor is connected to ground and to the frame of the applicance or tool.
 
Aboard ship, neither side is grounded; therefore, both sides are considered the "fine," or both are hot. The black or the white conductor may be connected to either line, since there is no difference. The green conductor is connected to ground. Ground aboard ship is the ship's hull.
 
The purpose of the ground wire (green) is to prevent an electrical shock to the operator in case there is an electrical short to the frame of the appliance or tool.
 
Q5.    What markings are found on spaghetti sleeving?
 
Q6.    What is the purpose of the green conductor in a power tool or electrical appliance cable?
 

ELECTRICAL DIAGRAMS

 
It is absolutely essential that personnel in the electrical or electronic ratings be able to "read" (interpret) various types of electrical diagrams. Personnel working in these ratings commonly refer to all electrical diagrams as "schematics." This term is not correct, however. A schematic is a specific type of diagram with characteristics of its own and with a specific purpose. Each of the various diagrams discussed in this chapter has a specific purpose and distinguishing features that set it apart from the others. The diagrams discussed may be used for the following purposes:
 
·   To learn a specific system operation
 
·   To locate the components of a system
 
 

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Figure 3-6. - Pictorial diagram of automotive starter and ignition systems.

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ISOMETRIC DIAGRAM
 
The purpose of an isometric diagram is to assist you in locating a component within a system. If you do not know where to look for a component, the isometric diagram is of considerable value to you. This type of diagram shows you the outline of a ship, airplane, or piece of equipment. Within the outline are drawn the various components of a system in their respective locations. The isometric diagram also shows the interconnecting cable runs between these components.
 
Figure 3-7 is an isometric diagram of portions of the same automobile starting and lighting systems discussed in the pictorial diagram (figure 3-6). The battery, starter, and other components can now be seen, each in its actual location within the automobile.
 

Figure 3-7. - Isometric diagram.

 
BLOCK DIAGRAM
 
A block diagram is used primarily to present a general description of a system and its functions. This type of diagram is generally used in conjunction with text material. A block diagram shows the major components of a system and the interconnections of these components. All components are shown in block form, and each block is labeled for identification purposes.
 
The block diagram shown in figure 3-8 is an illustration of an automobile's electrical power, starting, and ignition systems. It must be emphasized that the following explanation is primarily for the purpose of assisting you in learning to "read" or interpret a block diagram. The explanation of the functions of the automobile power, starting, and ignition systems is of secondary importance. By tracing from component to component in the block diagram and following the explanation, you are given a general description of the system functions. In addition, you should be able to understand the arrangement of the components in a block diagram.
 
 

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Figure 3-8. - Block diagram.

 
The battery is the initial source of power for the starter and ignition systems. The starter is turned by power from the battery when the ignition switch is turned to the START position. Power is also supplied, through the ignition switch, to the coil. From the coil, power is supplied to the distributor and finally to the spark plugs for ignition.
 
Once the engine is running, the starter is no longer required. The running engine acts as the prime mover for the alternator. (This is accomplished through a belt and pulley system attached to the engine's crankshaft.) The alternator now takes over as the power supplier for the ignition system. It supplies power through the ignition switch to the coil, from the coil to the distributor, and finally from the distributor to the spark plugs. At the same time, the alternator supplies power back through the voltage regulator to the battery for charging purposes. This completes the cycle until the engine is shut down and started again.
 
Note that the engine is not shown in the block diagram as the prime mover for the alternator. The engine is a mechanical rather than an electrical function. The illustrated block diagram is of the electrical system only. There are block diagrams that show strictly mechanical components or both mechanical and electrical components.
 
SINGLE-LINE DIAGRAM
 
The single-line diagram is used basically for the same purpose as the block diagram. When used with text material, it gives you a basic understanding of the functions of the components of a system.
 
There are two major differences between the single-line diagram and the block diagram. The first difference is that the single-line diagram uses symbols, rather than labeled blocks, to represent components. Second, the single-line diagram shows all components in a single line (figure 3-9). There are no interconnections shown for selected components as were shown on the block diagram (for example, alternator to voltage regulator and back to the battery). The single-line diagram is very simplified and should be used primarily to learn (in very broad terms) the function of each of the various components as a part of the total system.
 
 

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Figure 3-9. - Single-line diagram.

 
Q7.    What type of electrical diagram is used to identify the components of a system?
 
Q8.    What type of diagram is used to find the location of a component?
 
Q9.    What types of diagrams are the most convenient from which to learn the basic Functions of a circuit?
 
SCHEMATIC DIAGRAM
 
The schematic diagram shows, by means of graphic symbols, the electrical connections and functions of a specific circuit arrangement. The schematic diagram is used to trace the circuit and its functions without regard to the actual physical size, shape, or location of the component devices or parts. The schematic diagram is the most useful of all the diagrams in learning overall system operation.
 
Figure 3-10  is a schematic diagram of an automobile electrical system. The automobile electrical system uses the frame of the automobile as a conductor. The frame is called the ground side. Figure 3-10 shows all the electrical components grounded on one side. The negative side of the battery is also grounded. Therefore, the frame is the negative conductor of the system. The opposite side of each of the components is connected through switches to the positive side of the battery. For the purpose of teaching schematic reading, we will discuss only the lighting system and engine instruments.
 
 

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NEETS Table of Contents

• Introduction to Matter, Energy, and Direct Current
• Introduction to Alternating Current and Transformers
• Introduction to Circuit Protection, Control, and Measurement
• Introduction to Electrical Conductors, Wiring Techniques, and Schematic Reading
• Introduction to Generators and Motors
• Introduction to Electronic Emission, Tubes, and Power Supplies
• Introduction to Solid-State Devices and Power Supplies
• Introduction to Amplifiers
• Introduction to Wave-Generation and Wave-Shaping Circuits
• Introduction to Wave Propagation, Transmission Lines, and Antennas
• Microwave Principles
• Modulation Principles
• Introduction to Number Systems and Logic Circuits
• Introduction to Microelectronics
• Principles of Synchros, Servos, and Gyros
• Introduction to Test Equipment
• Radio-Frequency Communications Principles
• Radar Principles
• The Technician's Handbook, Master Glossary
• Test Methods and Practices
• Introduction to Digital Computers
• Magnetic Recording
• Introduction to Fiber Optics