Introducing Electricity and Electrical Safety · The electricity generated in the power plants is distributed through high-power transmission lines on tall towers. The electrical

Introducing Electricity andElectrical Safety

IMAGINE OUR WORLD without electricity.There would be no iPods, cell

phones, computers, video games,televisions, or numerous other deviceswe often take for granted. Generatingelectricity, distributing it, andharnessing it for multiple uses is critical for our society. Thisunit looks at the basics of electricity and electrical safety.

Objective:

� Explain the basics of electricity and electrical safety.

Key Terms:

�

Electricity andElectrical Safety

Electricity is the flow of electrons in a

conductor. The electrons must have a path

to and from their source. This path is called

a circuit. Different kinds of electrical

devices are used as parts of the circuit. These

devices perform a variety of functions, such

as turning the electricity off and on and pro-

viding electricity to lights or appliances.

E-unit: Introducing Electricity and Electrical Safety

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alternatingcurrent (AC)

amperagecircuitcircuit breakerconductorscurrent

direct current(DC)

electricityfuseGFCI (ground-

fault circuitinterrupter)

insulators

kilowattsNational Electrical

Code (NEC)ohmsovercurrentphaseresistanceservice entrance

service panelsingle-phasestaticthree-phasevoltagevoltage dropwatts

FIGURE 1. Electricity is the flow of electrons.

TYPES OF ELECTRICITY

Two kinds of electricity are static and current. Static is made of electrons that do not

move. An example is the shock received between two people who touch after walking on

woolen carpet in cold weather. Current is made of moving electrons. This is the type used in

our work and daily lives. Current electricity may be in the form of direct current or alternating

current. Direct current (DC) flows in only one direction. DC is usually generated by bat-

tery-base electrical systems and used in the electrical systems of internal combustion engines

or in flashlight batteries. Alternating current (AC) reverses the direction of flow many times

each second. AC is the type used in homes and factories.

ELECTRICAL SERVICE

Electrical service begins with a power source,

which is usually a large generating plant with a sys-

tem of lines to get the electricity to the user. A gener-

ator may be used in case of a power outage, when a

loss of power may result in the loss of life, crops, etc.

Conductors and Insulators

The materials needed to provide service include

conductors and insulators. Conductors are materi-

als that transmit electricity. They are wires that are




FIGURE 2. Copper wires (conductors) are enclosed

in plastic (an insulator).

EXPLORING OUR WORLD…

SCIENCE CONNECTION: Electricity Facts

The source of electricity is atoms. Every atom has at least one electron that travels around

the nucleus of the atom at great speed. When electrons are forced to flow from atom to atom,

they create electricity.

Most electricity is generated in large power plants using coal, oil, natural gas, nuclear energy,

or hydropower. Increasingly electricity is being produced with geothermal energy, wind power,

biomass, and solar energy. The various energy sources provide the power to turn turbines inside

generators. The shafts of the spinning turbines turn electromagnets that are surrounded by

heavy coils of copper wire. In the process, a magnetic field is created, which causes the elec-

trons in the copper wire to jump from atom to atom.

The electricity generated in the power plants is distributed through high-power transmission

lines on tall towers. The electrical current must often travel long distances to get where it is

needed. As it travels, it loses some of its strength. Transformers along the way restore lost

power.

Electricity travels at a fast 186,000 miles per second.

generally made of copper or aluminum. Conductors are used to connect a source to devices

using electricity. Other materials, known as insulators, which are not good conductors of

electricity, are used to confine electricity to the path from its source to the device being pow-

ered. Common insulators are rubber, plastic, and glass.

Path of Electricity

Service is provided to homes, businesses, and

other small users of electricity by three wires.

Two of the wires are “hot,” each carrying 120

volts. The other wire is “neutral,” providing the

return path for electricity. These wires are con-

nected to a service entrance, which is where

the electricity enters a building.

A meter at the service entrance measures the

electricity being used. The service entrance is

grounded with a wire connected to a ground rod

driven several feet into the earth. It is needed to

provide a return path to the ground and to carry

stray electrical current out of the system.

A service panel follows the meter. It

houses the circuit breakers for the system and is

used to distribute the power to individual cir-

cuits throughout the system. When a circuit uses

too much electricity, an overcurrent causes a cir-

cuit breaker to trip, shutting down the power to

that circuit. An overcurrent exists when the

current flow in a circuit exceeds the amperage

rating of the circuit’s conductors,

load(s), or other device(s). The

excessive heat caused by an

overcurrent may burn or damage

a conductor’s insulation and cause

a fire. A circuit breaker is a

heat-sensitive switch, which auto-

matically trips when the electric-

ity demand is so great that it

causes the temperature in the

conductor to get too hot. In some

cases, a system uses a fuse rather

than a circuit breaker. Fuses have

metal links that melt when the

current flow is too great. The size




FIGURE 3. Wires deliver electricity to homes and

businesses.

FIGURE 4. The service panel houses circuit breakers.

of circuit breaker or fuse is determined by the size of

wire used, which in turn is determined by the antici-

pated load of a circuit.

A GFCI (ground-fault circuit interrupter) is

type of breaker that is extremely sensitive to circuit

imbalances. It is designed to protect people who are

using electrical devices near damp areas, such as in

bathrooms, in kitchens, outside, etc.

Circuits

Circuits are formed by wires, appliances, and other

devices. Two wires are necessary to have a flow of

current. One wire, known as the “hot” conductor,

carries the electrical current from the source to the

device, while the other wire, known as the “neutral”

conductor, provides a return of the electrical current.

Circuits with two wires are known as 120-volt cir-

cuits, and those with three wires as 240-volt circuits.

Lighting and appliances use 120-volt circuits, whereas

equipment and appliances with greater demand, such

as clothes dryers, electric ranges, and heaters or air-

conditioners, use 240-volt circuits.

Electrical Wiring Guidelines

Guidelines for electrical wiring have been established by the National Electrical Code

(NEC) to ensure the safe use of electricity. They provide numerous rules for safe electrical

installations. Local governments may also have codes that apply to the installation of wires, to

appliances, and to other uses of electricity.

TERMINOLOGY

Various terms are important in understanding electricity. Some of those terms follow.

Voltage is the pressure in a circuit that causes the electrons or current to flow. It is some-

times called electromotive force (EMF). A volt is the unit by which electrical pressure is mea-

sured with a voltmeter. When electricity must be carried a long distance through wires, there

will be a decrease in voltage, referred to as voltage drop. Voltage drop occurs because of

resistance in the conductors.

Amperage is the amount of electrical current flowing past a point in a circuit. Amperage is

measured with an ammeter.

Watts are a measure of electrical power. Electrical power is work being done by current

(amperage) under pressure (voltage) in getting the electrons through the resistance of wires




FIGURE 5. A GFCI is extremely sensitive to circuit

imbalances.

and machines back to the generator. Units of 1,000 watts are called kilowatts. The relation-

ship between watts (P), amps (I), and volts (E) is P = I × E.

Resistance is the tendency of the wire to resist the flow of electrons or current through

the wire. Within a circuit, electrical resistance is dependent upon size, length, and the material

of the conductor. Smaller diameter wire will have more resistance than larger. The longer the

wire in the circuit, the more resistance. Finally, compared with other metals, silver, copper,

and aluminum offer the least resistance to the flow of an electrical current. Resistance is mea-

sured in ohms. The relationship of ohms (R), volts (E), and amps (I) is E = I × R.

Direct current (DC) flows in one direction in a circuit. It is often used in automobiles and

tractors. Nearly all the electrical current produced for home and farm use in the United States

is 60-cycle alternating current (AC). The direction of flow of AC is reversed by a generator 120

times per second. Each pair of reversals or one back-and-forth motion is called a cycle. Thus,

there are 60 cycles per second.

Phase is a timed source of electricity through a conductor. Single-phase is current from

one source with three wires: one hot, one neutral, and one ground. Three-phase is actually

three single phases combined. The three are combined to give equally spaced peak voltages.

Three hot wires and a fourth neutral wire or just three hot wires may by used, depending on

the system design.

SAFETY PRACTICES

Electricity is a very safe and economical source of power. However, injury and loss of life

can occur quickly when electricity is improperly used. Property can be destroyed by electrical

failures and fires. Some electrical safety practices are listed here.

� Avoid damp working areas. Never handle electrical equipment with wet hands or whilestanding in a wet or damp place.

� Protect each circuit. Be certain that each circuit is protected with either a circuit breakeror a fuse of proper amperage.

� Ground each circuit properly. Each circuit must have a ground (neutral) wire and agrounding wire to be properly grounded.

� Use ground-fault circuit interrupters (GFCIs). To protect the operator who works out-side or in damp locations, make sure the electrical source is protected by a ground-faultcircuit interrupter.

� Ground electrical equipment. All 120-volt electrical equipment should be equipped witha three-prong grounding-type plug or be double insulated. Never cut off a groundingprong just to make the connection work.

� Disconnect the main switch. Before making any repairs on an electrical circuit, alwaysmake certain the current has been disconnected to that circuit at the circuit breaker.

� Correct the source of trouble. Before resetting a circuit breaker or replacing a blown fuse,correct the cause of the trouble. Repair or replace any equipment that gives a shock whenit is used.




� Purchase safe equipment. Select portable electrical equipment that is grounded with athree-prong plug or is double insulated. Look for the “UL” label, indicating that theequipment has been tested and approved by Underwriters Laboratories, Inc.

� Review the local electrical code. When rewiring a building, follow the local electricalcode.

� Seek professional help instead of using trial-and-error methods when electrical devices donot function properly.

� Avoid plumbing hazards. Do not locate switches or light fixtures near plumbing fixtures.

� Inspect and repair cords periodically. Inspect all extension cords and electrical appliancecords periodically for exposed wires, faulty plugs, poor insulation, and loose connections.Correct all hazards found on electrical cords.

� Open circuits with switches. Never pull a plug from an outlet while the equipment is inoperation. This creates an arc and will eventually foul the plug or the outlet, which cancause electrical shock or a possible fire.

� Never make temporary repairs. Make sure all repairs are as good as new. When splicingwires, be sure all strands are twisted together, the connections are strong, and the splice isfully insulated.

� Use electrical cords safely.Do not hang electrical cordson nails or run them underrugs or around pipes. Avoidusing extension cords as per-manent wiring installations.

� Do not overload circuits.When new equipment isinstalled, make sure it is pro-tected by a circuit of properamperage rating.

� Unplug electrical tools. Donot leave a tool plugged inwhen it is not in use, unlessit is designed for continuousoperation.

Summary:

� Electricity is the flow of electrons in a conductor. The path through which elec-trons flow is called a circuit. Two kinds of electricity are static and current. Currentelectricity may be in the form of direct current or alternating current.

Electrical service begins with a power source, which is usually a large generatingplant with a system of lines to get the electricity to the user. Materials needed toprovide service include conductors and insulators. Service is provided to users of




FIGURE 6. Electricity warning signs.

electricity by three wires. A meter at the service entrance measures the electricitybeing used. A service panel houses the circuit breakers for the system and distrib-utes power to individual circuits throughout the system.

Various terms are important in understanding electricity. Some of those terms arevoltage, amperage, watts, resistance, ohms, and phase.

Injury and loss of life can occur quickly when electricity is improperly used. Prop-erty can be destroyed by electrical failures and fires.

Checking Your Knowledge:

� 1. What is electricity?

2. How do direct and alternating current compare?

3. How does electricity get from the power source to the user?

4. What are the meanings of voltage, amperage, watts, resistance, ohms, and phase?

5. What are some electrical safety practices?

Expanding Your Knowledge:

� Conduct an electrical safety check in your home. Look for overloaded outlets, oldcords, and similar hazards. Take steps to correct the problems.

Web Links:

� Basic Electricity

http://www.reprise.com/host/electricity/

Electricity Explained

http://www.eia.doe.gov/energyexplained/index.cfm?page=electricity_home

Fundamentals of Electricity

http://cipco.apogee.net/foe/fbbi.asp

Electricity

http://www.ndt-ed.org/EducationResources/HighSchool/Electricity/hs_elec_index.htm

Electrical Safety

http://ehs.okstate.edu/modules/electric/index.htm




http://www.reprise.com/host/electricity/

http://www.eia.doe.gov/energyexplained/index.cfm?page=electricity_home

http://cipco.apogee.net/foe/fbbi.asp

http://ehs.okstate.edu/modules/electric/index.htm

http://www.ndt-ed.org/EducationResources/HighSchool/Electricity/hs_elec_index.htm

Documents

Introducing Electricity and Electrical Safety · The electricity generated in the power plants is distributed through high-power transmission lines on tall towers. The electrical