Sunday, January 4, 2015

Basic circuit concepts : a simple electric circuit , open circuits and closed circuits , the schematic diagram , basic circuit notation , the short circuit , electrical safety , national electrical code ® and summary of basic circuit concepts

4–6 A SIMPLE ELECTRIC CIRCUIT

Figure 4–7 represents a simple, functional circuit. The necessary elements of such a circuit are described as source, load, conductors, and control. The first three of these terms are closely linked with the four important concepts discussed in Sections 4–2 to 4–5. The source represents the voltage that forces the electrons through the circuit. The battery, in this example, represents the source. DC electrical sources are very often color coded to designate polarity of the terminals, positive or negative. By convention (widespread agreement and usage) the negative terminal of a source will be identified by the color black and the positive terminal will be marked by red. The load (or load resistance) utilizes the electrical energy. The lamp represents the load. The conductors provide the path for the electron current; and the control is provided by the switch, which can be operated to turn the lamp on or off.

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4–7 OPEN CIRCUITS AND CLOSED CIRCUITS

Careful comparison of the two circuits shown in Figure 4–7 will reveal the differ- ence between open and closed circuits. You see, if electrons are to start moving and keep moving, a complete path for them to follow must exist. In other words, a closed circuit is required to allow the current to flow whenever electrical energy is needed by the load. Figure 4–7B represents this condition.

Now, let us look at Figure 4–7A. The switch has been opened. This simple switch is known as a knife switch. Its operation resembles that of a drawbridge. When the drawbridge is opened, the infinite resistance of the air between the switch contacts stops all current motion. This condition is known as an open circuit. This term applies to a circuit containing an open switch, a burned-out fuse, or any separation of wires that prevents current from flowing. If an appliance fails to operate when connected but does not blow a fuse, an open circuit in the device may be the problem.

4–8 THE SCHEMATIC DIAGRAM

Pictures of electric circuits and their components, such as that shown in Figure 4–7, are seldom used in the practice of electrical trades. It is more practical to convey ideas by means of graphic symbols known as schematic diagrams, or schematics. Figure 4–8 is a schematic representation of the circuit shown in Figure 4–7A. Compare the two drawings carefully. It is important that you learn how to draw and interpret such schematic diagrams. This type of skill requires some time, of course, and necessitates that you learn many graphic symbols. A chart of such symbols is shown in the Appendix, in Figures A–12 and A–13. It is suggested that you frequently refer to this list as you progress in your studies.

4–9 BASIC CIRCUIT NOTATION

In the preceding sections you became acquainted with some of the basic units of electrical measurement. At this time you should recall that the voltage (also called emf ) is measured in units called volts.

Let us assume that the battery symbol in Figure 4–8 represents a 12-volt car bat- tery. We might then use a circuit notation to show this specific value, as illustrated in Figure 4–9, where the notation reads E = 12 V. The letter E, in this case, stands for electromotive force (emf ).

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Alongside the wire in Figure 4–9, you see the notation I 5 3 A. The letter I stands for intensity and is used to represent current in electrical notations and formulas. Thus, since current is measured in amperes, or amps, the equation states that the current, I, in the circuit is equal to 3 amperes.

In the same circuit diagram, the symbol for a resistor has been used to replace the symbol of a lamp. This symbol is often used to indicate the general resistance of any load, without specifying its exact nature. The adjoining legend, R 5 4 Ω, means this load has a resistance of 4 ohms. The letter R represents resistance, and the word ohm is denoted by the Greek letter omega (Ω).

4–10 THE SHORT CIRCUIT

A short circuit is a parallel path of extremely low resistance, often caused accidentally; see Figures 4–10 and 4–11. In Figure 4–11, the frayed insulation on the lamp cord may permit the two wires to touch each other. If the wires do touch, they form a path of nearly 0 ohms resistance. As a result, a large amount of current appears in the wires leading to the place of contact. The wires can overheat and start a fire. To prevent such an outcome, fuses or circuit breakers are used in series with each house circuit. If a short circuit occurs, the excessive current melts the fuse wire or trips the circuit breaker, opening the circuit.

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4–11 ELECTRICAL SAFETY

All personnel working with electric circuits must be made aware of the special hazards they may encounter when they come in contact with live circuit components. A firm knowledge of electrical principles and safety practices will aid in combating irrational fear and help develop good safety attitudes.

Consider the following safety precautions:

1. Electrical shock can be harmful, if not fatal. Voltage levels are not the only deter- mining factor. One hundred twenty volts is sufficient to kill. On the other hand, one can experience high voltages, say 25,000 volts, without lasting damage. Instead, it is the effect of current through the body that is harmful. Generally, currents as low as

0.15 ampere are fatal.

2. Therefore, to avoid or minimize electrical shock, one should:

a. Never touch two wires at the same time.

b. Never touch one wire and ground (earth) at the same time.

3. Instead, be sure to:

a. Shut power off and, if possible, lock it out before working on a circuit.

b. Insulate the body from the ground.

c. Properly ground all metal enclosures of electrical equipment. A green-colored wire usually serves this purpose. This green wire is the equipment ground and attaches to the third prong of a male attachment cap (plug).

d. Discharge capacitors before touching their terminals.

e. Avoid use of metal measuring tapes in the vicinity of live conductors.

f. Use a type C extinguisher to combat electrical fires. Any other type of fire extinguisher may cause fatal shock.

4. At times it may be necessary to test live circuits in operation. Under such conditions it is advisable to put one hand in your pocket or behind your back. This prevents contact between two hot wires or between one hot wire and ground. Remove metal jewelry, such as rings and watches.

5. Hot soldering irons can cause severe burns. Be sure hot soldering irons have sufficient time to cool before you store them.

6. Hot solder splatters easily. Keep this in mind before you shake your soldering tool.

Molten solder on someone’s body may cause serious injury.

7. Wear safety glasses for the following activities:

a. Using rotating power tools, such as grinders and drills

b. Chiseling or chipping

c. Handling chemical electrolytes

8. When charging batteries, remember that hydrogen gas is highly volatile. A small electrical spark from a wire is sufficient to start an explosion.

9. Ventilate batteries when you are charging them. Be sure to loosen the cell caps to allow gases to escape.

10. To prevent oxidation, make electrical connections and splices well and be sure they are tight. Poor connections introduce resistance at the junction, which inevitably causes heat and fire hazards.

11. Do not walk away from a machine that is in motion. When shutting down a machine, stay with it until it has completely stopped.

12. Report all injuries to your instructor, regardless of how slight or unimportant you think they might be.

13. Some hand tools, such as pliers or screwdrivers, have insulated handles. Such insulation may not always be sufficient to protect you from accidental shock. You may have to take additional safety precautions.

14. If contact is made with metal parts, causing a short circuit, live wires may cause serious burns and blinding flashes.

15. Some electrical components, especially resistors, develop considerable heat. Allow them to cool before touching.

There are a few more specialized safety precautions that will be pointed out to you as you progress in your studies.

4–12 NATIONAL ELECTRICAL CODE ®

The foremost guide to electrical safety is the National Electrical Code® (NEC®). The Code® is a compilation of recommendations and regulations for the safeguarding of people and property with relation to electricity.

This book, first printed in 1897, has been revised every three years. These revisions have been sponsored since 1911 by the National Fire Protection Association. Electrical fires due to inadequate or overloaded wiring systems present real hazards that prompted the original group effort of insurance and architectural interests to develop the Code® in the first place. The book deals with rules and regulations pertaining to the safe installation of electrical conductors and equipment in private and public buildings.

Every state has governmental bodies that have legal jurisdiction over enforcement of the Code® . All cities and townships throughout the country have offices of building safety with inspectors who enforce the provisions of the Code®. Local authorities may waive specific requirements in the Code® or permit alternate methods as long as the desired objectives are achieved within safety guidelines. Many local codes are more stringent in setting minimum standards. Where such codes exist they supersede the provision of the NEC®.

To become a licensed electrician demands a thorough understanding of the codes as well as passing licensing exams. Licensing procedures are generally administered by the office of electrical inspection at local city of township halls.

SUMMARY

• The four key concepts of an electric circuit are voltage, current, resistance, and watts.

• Voltage may be thought of as a pressure (or force) that exists between two points of different potential.

• Voltage is also known as electromotive force (emf ).

• Voltage is measured in units called volts.

• Electron current is a motion of free electrons from negative to positive. This is the standard adopted for this book.

• Conventional current is said to flow from positive to negative.

• Current is measured in units called amperes.

• When measuring amperes, we measure the rate of flow of electrons (1 ampere = 1 coulomb per second).

• The effect of the electrical impulse travels with the speed of light, while individual electrons move relatively slowly.

• The opposition to current flow is called resistance.

• Resistance is measured in units called ohms.

• Resistance always generates heat when a current flows through it.

• Electricity is a form of pure energy that is converted to some other form, usually referred to as electrical power.

• Electrical power is measured in watts.

• An electric circuit consists of a load connected to a source by means of conductors. An boptional control is often added to such a circuit.

• Schematic diagrams are the blueprints of the electrical trades.

• Closed circuits provide a complete path for the electrons.

• There can be no current flow in an open circuit.

• Short circuits are caused when a low-resistance path bypasses the load.

• Short circuits cause excessive current flow and create potential fire hazards.

• Fuses and circuit breakers are used to protect against excessive currents.

• Electrical safety rules must be followed to ensure the safety of personnel and equipment.

• The NEC® governs the safety standards for electrical installations throughout the United States.

• Local codes may supersede the NEC® with more stringent requirements.

• Licensing procedures for electricians require extensive knowledge of the Code®.

Achievement Review

1. What is the difference between the negative pole and the positive pole of a battery or power source?

2. What colors are used to identify (a) the negative pole and (b) the positive pole?

3. What is voltage? Define the word.

4. What are some other names for voltage?

5. What letter symbol is used for voltage?

6. Define the word current.

7. What letter symbol is used for current?

8. Explain the difference between conventional current flow and electron current flow.

9. What is an ohm, and what is its letter symbol?

10. What is the unit of measurement for current?

11. What kind of instrument is used for measuring resistance?

12. What are amperemeters used for?

13. What is an emf? Explain.

14. How fast does the electron current travel?

15. What is meant by potential or potential difference?

16. Draw a simple schematic with a DC source of 6 volts connected to a 1.5-ohm load. Indicate a current of 4 A flowing, as well as the voltage and resistance values.

17. Explain the purpose of fuses and circuit breakers.

18. Explain how fuses differ from circuit breakers.

19. Voltage, current, resistance, and watts are the basic concepts underlying all electrical and electronic principles.

a. Write brief but concise definitions for the following terms: voltage, current, resistance, and power.

b. Complete the chart below.

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