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Part One. Basic Electrical Techniques

CHAPTER 1

Fundamentals

This manual emphasizes the constructional aspects of electric wiring. The term phase is used when referring to the angular displacement between two or more like quantities, either alternating electromotive force (EMF) or alternating current (AC). It is also used for distinguishing the different types of AC generators. For example, a machine designed to generate a single EMF wave is called a single-phase alternator, and one designed to generate two or more EMF waves is called a polyphase alternator.

USAGE

Power generators produce single- or three-phase voltages that can be used for electrical power systems at generated voltages or through transformer systems. Single-phase generators are normally used only for small lighting and single-phase motor loads. If the generated voltage is 120 volts, a two-wire system is used (see Table B-1[A]). In this situation, one conductor is grounded and the other is ungrounded, or hot. The generated single-phase voltage can be 240 volts. This voltage is normally used for larger single-phase motors. To provide power to lighting loads, the 240-volt phase is center-tapped to provide a three-wire, single-phase system (see Table B-1[B]). The center tap is the grounded neutral conductor. The voltage is 120 volts from this grounded conductor to either of the two ungrounded conductors. This is half of the total phase value. The voltage between the two ungrounded conductors is 240 volts. This system provides power for both lighting and single-phase, 240-volt motors.

The most common electrical system is the three-phase system. The generated EMFs are 120 degrees apart in phase. As shown in Table B-1 (C, D, E), three-phase systems may be carried by three or four wires. If connected in a delta , the common phase voltage is 240 volts. Some systems generate 480 or 600 volts. If the delta has a grounded center-tap neutral, then a voltage equal to half the phase voltage is available. If the phases are Y-connected, then the phase voltage is equal to 1.73 times the phase-to-neutral voltage. The most common electrical system found in the military is the three-phase, four-wire, 208/120-volt system.

Single-phase, three-wire systems and three-phase, four-wire systems provide voltages for both lighting and power loads. If the load between each of the three phases or between the two ungrounded conductors and their grounded center-tap neutral are equal, a balanced circuit exists. When this occurs, no current is flowing in the neutral conductor. Because of this, two ungrounded conductors and one grounded neutral may be used to feed two circuits. Thus, three conductors may be used where four are normally required.

Electric lamps for indoor lighting in the United States (US) generally operate at 100 to 120 volts from constant-potential circuits. Two- and three-wire distribution systems, either direct current (DC) or single-phase AC, are widely used for lighting installations.

These systems of distribution are capable of handling both lamp and motor loads -connected in parallel between the constant-potential lines. The three-wire system provides twice the potential difference between the outside wires as it does between either of the outside wires and the central or neutral wire. This system makes it possible to operate large motors at 240 volts, while lamps and smaller motors operate at 120 volts. When the load is unbalanced, a current in the neutral wire corresponds to the difference in current taken by the two sides. A balance of load is sought in laying out the wiring for lighting installations.

DRAWING SYMBOLS AND READING BLUEPRINTS

An electrician must be able to interpret simple blueprints, because construction orders are ordinarily in that form. He must be able to make simple engineering sketches that describe work for which he receives only verbal orders. TM 5-704 contains detailed information about engineering drawings.

SYMBOLS

The more common symbols and line conventions used in wiring plans are shown in Figures A-1 through A-6, pages A-1 through A-3. These symbols enable the electrician to determine the precise location of electrical equipment in a building by studying the drawing.

SCHEMATIC WIRING DIAGRAMS

Electrical plans show the items to be installed, their approximate location, and the circuits to which they are to be connected. A typical electrical plan for a post exchange is shown in Figure 1-1. The plan shows that the incoming service consists of three number (No) 8 wires and that two circuit-breaker panels are to be installed. Starting at the upper left, the plan shows that nine ceiling light outlets and two duplex wall outlets are to be installed in the bulk-storage area. The arrow designated B2 indicates that these outlets are to be connected to circuit 2 of circuit-breaker panel B. Note that three wires are indicated from this point to the double home-run arrows designated B1, B2. These wires are the hot wire from the bulk-storage area to circuit 2 of panel B, the hot wire from the administration area to circuit 1 of panel B, and a common neutral. The two hot conductors must be connected to different phases at the panel, thus allowing a cancellation of current in the neutral when both circuits are fully loaded. From the double arrowhead, these wires are run to the circuit-breaker panel without additional connections.

The wiring diagram shown in Figure 1-1 is the type used most frequently for construction drawings. Single lines indicate the location of wires connecting fixtures and equipment. Two conductors are indicated in a schematic diagram by a single line. If more than two wires are together, short parallel lines through the line symbols indicate the number of wires represented by the line. A dot placed at the point of intersection indicates connecting wires. No dot is used where wires cross without connecting.

Figure 1-1. Typical wiring diagram

You may encounter drawings in which the lines indicating the wiring have been omitted. This type of drawing shows only fixture and equipment symbols, and the electrician must determine the location of the actual wiring. Electrical drawings do not show any actual dimensions or dimension lines. Location dimensions and spacing requirements are given in the form of notes or follow the installation standards described in Chapter 3.

DRAWING NOTES

A list of drawing notes is ordinarily provided on a schematic wiring diagram to designate special wiring requirements and to indicate building conditions that alter standard installation methods.

COLOR CODING

Standard color coding requires that a grounded or neutral conductor be identified by an outer color of white or neutral gray for No 6 wire or smaller. Larger conductors can be identified either by an outer identification of white or neutral gray or by white markings at the terminals. The ungrounded conductors of a circuit should be identified with insulation colored black, red, and blue and used in that order in two-, three-, or four-wire circuits, respectively. All circuit conductors of the same color must be connected to the same hot feeder conductor throughout the installation. A grounding conductor, used solely for grounding purposes, should be bare or have a green covering.

SPLICES

A spliced wire must be as good a conductor as a continuous conductor. Splices should be avoided whenever possible, but they are permitted anywhere if they are located inside an electrical box. The best wiring practice (including open wiring systems) is to run continuous wires from the service box to the outlets.

SOLDERLESS

Connectors (Figure 1-2) are sometimes used in place of splices because they are easier to install. Since heavy wires are difficult to splice and solder properly, split-bolt connectors are commonly used for wire joints. Solderless connectors, popularly called wire nuts, are used for connecting small-gauge and fixture wires. One design consists of a funnel-shaped, metal spring insert that is molded into a plastic shell; the other type has a removable insert that contains a setscrew to clamp the wires. In either design, the plastic shell is screwed onto the insert to cover the joint.

Follow the steps below to connect a wire nut:

Step 1. Strip off about 1 inch of insulation from the ends of the wires that you are going to join. Twist the stripped ends clockwise at least one and one-half turns.

Step 2. Snip 3/8 to 1/2 inch off the twisted wires so that the ends are even.

Step 3. Screw the wire nut on clockwise.

SOLDERED

When a solderless connector is not used, the splice must be soldered before it is considered to be as good as the original conductor. The primary requirements for obtaining a good solder joint are a clean soldering iron, a clean joint, and a nonacid flux. These requirements can be satisfied by using pure rosin on the joint or by using a rosin-core solder. To ensure a good solder joint, apply the electric-heated or copper soldering iron to the joint until the joint melts the solder by its own heat.

TAPED

Use plastic tape to insulate splices for temporary or expedient wiring. On a two--conductor cable, separate the two legs. Secure the tape on one leg, tape the first leg, close the legs together, and tape the wire splice past the end. Adequately cover all bare copper. Apply three layers for voltages up to 600 volts. Half lap the tape (overlap by half the width of the tape) for padded mechanical protection.

INSULATION AND WIRE CONNECTIONS

When attaching a wire to a switch or an electrical device or when splicing a wire to another wire, remove the wire insulation to bare the copper conductor. Make the cut at an angle to the conductor to avoid nicking and weakening the wire. After removing the protective insulation, scrape or thoroughly sand the conductor to remove all traces of insulation and oxide from the wire.

To attach the trimmed wire to the terminal, always insert the wire loop under the terminal screw (Figure 1-3), so that tightening the screw tends to close the loop. When -correctly inserted, the loop brings the wire insulation ends close to the terminal.

JOB SEQUENCE

SCOPE

The installation of interior wiring is generally divided into two major divisions called roughing-in and finishing. Roughing-in is the installation of outlet boxes, cable wire, and conduit. Finishing includes the installation of switches, receptacles, covers, and fixtures and the completion of the service. Other trades use the interval between these two work periods for plastering, enclosing walls, finishing walls, and trimming.

ROUGHING-IN

Step 1. Mounting outlet boxes. This step can be expedited if the locations of all boxes are first marked on the studs and joists of the building. Some boxes have special brackets for mounting on the building members. All boxes that are to be concealed must be installed so that the forward edge or plaster ring of the boxes will be flush with the finished walls.

Step 2. Circuiting and installing wire, cable, or conduit. This involves drilling and cutting the building members to allow for the passage of the conductor or its protective covering. For surface-type wiring, this includes installing conduit, cable, and surface-type boxes and covers on a finished wall. The production-line method of first drilling the holes for all runs (installations between boxes) at one time and then -installing all of the wire, cable, or conduit will expedite the job.

Step 3. Pulling wires in conduit between boxes. This step, which can be included as the first step in the finishing phase, requires care in handling of the wires to prevent marring the finished wall or floor surfaces.

FINISHING

Step 1. Splicing the joints in the outlet and junction boxes and connecting the grounding wires. This step ensures the proper installation of leads to the terminals of switches, ceiling and wall outlets, and fixtures.

Step 2. Attaching the devices and their cover plates to the boxes. This step ensures that the service-entrance cable and fusing or circuit-breaker panels are connected and the circuits are fused. The fixtures are generally supported by the use of special mounting brackets called fixture studs or hickeys.

Step 3. Testing for proper circuiting. The final step in the wiring of any building requires the testing of all outlets by inserting a test prod or test lamp, operating all switches in the building, and loading all circuits to ensure that the proper circuiting has been installed.