introducing electricity and electrical safety · the electricity generated in the power plants is...
TRANSCRIPT
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.
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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
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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
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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
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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.
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� 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
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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
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