Electrical
Safety
Safety and Health
for Electrical Trades
Student Manual |
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Section 1
Electricity Is Dangerous
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Whenever you work with power tools or on electrical
circuits there is a risk of electrical hazards, especially electrical
shock. Anyone can be exposed to these hazards at home or at work.
Workers are exposed to more hazards because job sites can be cluttered
with tools and materials, fast-paced, and open to the weather. Risk
is also higher at work because many jobs involve electric power tools.
Electrical trades workers must pay special attention to electrical
hazards because they work on electrical circuits. Coming in contact
with an electrical voltage can cause current to flow through the body,
resulting in electrical shock and burns. Serious injury or even death
may occur. As a source of energy, electricity is used without much
thought about the hazards it can cause. Because electricity is a familiar
part of our lives, it often is not treated with enough caution. As
a result, an average of one worker is electrocuted on the job every
day of every year! Electrocution is the third leading cause of
work-related deaths among 16- and 17-year-olds, after motor vehicle
deaths and workplace homicide. Electrocution is the cause of
12% of all workplace deaths among young workers.
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This manual will present many topics. There
are four main types of electrical injuries: electrocution (death due
to electrical shock), electrical shock, burns, and falls. The dangers
of electricity, electrical shock, and the resulting injuries will be
discussed. The various electrical hazards will be described. You
will learn about the safety model, an important tool for recognizing,
evaluating, and controlling hazards. Important definitions and notes
are shown in the margins. Practices that will help keep you safe and
free of injury are emphasized. To give you an idea of the hazards caused
by electricity, case studies about real-life deaths will be described.
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How is an
Electrical Shock Received?
An electrical shock is received when electrical current
passes through the body. Current will pass through the body in a variety
of situations. Whenever two wires are at different voltages, current
will pass between them if they are connected. Your body can connect
the wires if you touch both of them at the same time. Current will
pass through your body.
In most household wiring, the black wires and the red
wires are at 120 volts. The white wires are at 0 volts because they
are connected to ground. The connection to ground is often through
a conducting ground rod driven into the earth. The connection can
also be made through a buried metal water pipe. If you come in
contact with an energized black wire-and you are also in contact with
the neutral white wire-current will pass through your body. You will
receive an electrical shock.
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You can even receive
a shock when you are not in contact with an electrical ground. Contact
with both live wires of a 240-volt cable will deliver a shock. (This
type of shock can occur because one live wire may be at +120 volts while
the other is at -120 volts during an alternating current cycle-a difference
of 240 volts.). You can also receive a shock from electrical components
that are not grounded properly. Even contact with another person who
is receiving an electrical shock may cause you to be shocked. |
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Summary of Section 1
You will receive an electrical shock if a part of your body completes
an electrical circuit by...
- Touching a live wire and an electrical ground, or
- Touching a live wire and another wire at a different voltage.
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Section 2
Dangers of Electrical Shock
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The severity of injury from electrical shock depends
on the amount of electrical current and the length of time the current
passes through the body. For example, 1/10 of an ampere (amp) of electricity
going through the body for just 2 seconds is enough to cause death.
The amount of internal current a person can withstand and still be
able to control the muscles of the arm and hand can be less than 10
milliamperes (milliamps or mA). Currents above 10 mA can paralyze
or "freeze" muscles. When this "freezing" happens,
a person is no longer able to release a tool, wire, or other object.
In fact, the electrified object may be held even more tightly, resulting
in longer exposure to the shocking current. For this reason, hand-held
tools that give a shock can be very dangerous. If you can't let go
of the tool, current continues through your body for a longer time,
which can lead to respiratory paralysis (the muscles that control
breathing cannot move). You stop breathing for a period of time. People
have stopped breathing when shocked with currents from voltages as
low as 49 volts. Usually, it takes about 30 mA of current to cause
respiratory paralysis.
Currents greater than 75 mA may cause ventricular fibrillation (very
rapid, ineffective heartbeat). This condition will cause death within
a few minutes unless a special device called a defibrillator is used
to save the victim. Heart paralysis occurs at 4 amps, which means
the heart does not pump at all. Tissue is burned with currents greater
than 5 amps. 2
The table shows what usually happens for a range of
currents (lasting one second) at typical household voltages. Longer
exposure times increase the danger to the shock victim. For example,
a current of 100 mA applied for 3 seconds is as dangerous as a current
of 900 mA applied for a fraction of a second (0.03 seconds). The muscle
structure of the person also makes a difference. People with less
muscle tissue are typically affected at lower current levels. Even
low voltages can be extremely dangerous because the degree of injury
depends not only on the amount of current but also on the length of
time the body is in contact with the circuit.
LOW VOLTAGE DOES NOT MEAN LOW HAZARD!
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Effects of Electrical Current*
on the Body 3 |
Sometimes high voltages lead to additional injuries.
High voltages can cause violent muscular contractions. You may lose
your balance and fall, which can cause injury or even death if you fall
into machinery that can crush you. High voltages can also cause severe
burns (as seen on pages 9 and 10).
At 600 volts, the current through the body may be as great as 4 amps,
causing damage to internal organs such as the heart. High voltages also
produce burns. In addition, internal blood vessels may clot. Nerves
in the area of the contact point may be damaged. Muscle contractions
may cause bone fractures from either the contractions themselves or
from falls. |
A severe shock can cause much more damage to the
body than is visible. A person may suffer internal bleeding and destruction
of tissues, nerves, and muscles. Sometimes the hidden injuries caused
by electrical shock result in a delayed death. Shock is often only the
beginning of a chain of events. Even if the electrical current is too
small to cause injury, your reaction to the shock may cause you to fall,
resulting in bruises, broken bones, or even death.
The length of time of the shock greatly affects the
amount of injury. If the shock is short in duration, it may only be
painful. A longer shock (lasting a few seconds) could be fatal if
the level of current is high enough to cause the heart to go into
ventricular fibrillation. This is not much current when you realize
that a small power drill uses 30 times as much current as what will
kill. At relatively high currents, death is certain if the shock is
long enough. However, if the shock is short and the heart has not
been damaged, a normal heartbeat may resume if contact with the electrical
current is eliminated. (This type of recovery is rare.)
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The amount of current passing through
the body also affects the severity of an electrical shock. Greater voltages
produce greater currents. So, there is greater danger from higher voltages.
Resistance hinders current. The lower the resistance (or impedance in
AC circuits), the greater the current will be. Dry skin may have a resistance
of 100,000 ohms or more. Wet skin may have a resistance of only 1,000
ohms. Wet working conditions or broken skin will drastically reduce
resistance. The low resistance of wet skin allows current to pass into
the body more easily and give a greater shock. When more force is applied
to the contact point or when the contact area is larger, the resistance
is lower, causing stronger shocks.
The path of the electrical current through the body affects the severity
of the shock. Currents through the heart or nervous system are most
dangerous. If you contact a live wire with your head, your nervous system
will be damaged. Contacting a live electrical part with one hand-while
you are grounded at the other side of your body-will cause electrical
current to pass across your chest, possibly injuring your heart and
lungs.
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There have been cases where an arm or leg is severely
burned by high-voltage electrical current to the point of coming off,
and the victim is not electrocuted. In these cases, the current passes
through only a part of the limb before it goes out of the body and into
another conductor. Therefore, the current does not go through the chest
area and may not cause death, even though the victim is severely disfigured.
If the current does go through the chest, the person will almost surely
be electrocuted. A large number of serious electrical injuries involve
current passing from the hands to the feet. Such a path involves both
the heart and lungs. This type of shock is often fatal. |
Summary of Section 2
The danger from electrical shock depends on...
- the amount of the shocking current
through the body,
- the duration of the shocking current
through the body, and
- the path of the shocking current through
the body.
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Section
3
Burns Caused by Electricity |
The most common shock-related, nonfatal injury is a burn. Burns caused
by electricity may be of three types: electrical burns, arc burns,
and thermal contact burns. Electrical burns can result when a
person touches electrical wiring or equipment that is used or maintained
improperly. Typically, such burns occur on the hands. Electrical burns
are one of the most serious injuries you can receive. They need to
be given immediate attention. Additionally, clothing may catch fire
and a thermal burn may result from the heat of the fire.
Arc-blasts occur when powerful, high-amperage currents arc through
the air. Arcing is the luminous electrical discharge that occurs when
high voltages exist across a gap between conductors and current travels
through the air. This situation is often caused by equipment failure
due to abuse or fatigue. Temperatures as high as 35,000°F have
been reached in arc-blasts.
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There
are three primary hazards associated with an arc-blast.
(1) Arcing gives off thermal radiation (heat) and intense light, which can
cause burns. Several factors affect the degree of injury, including skin
color, area of skin exposed, and type of clothing worn. Proper clothing,
work distances, and overcurrent protection can reduce the risk of such a
burn.
(2) A high-voltage arc can produce a considerable pressure wave blast. A
person 2 feet away from a 25,000-amp arc feels a force of about 480 pounds
on the front of the body. In addition, such an explosion can cause serious
ear damage and memory loss due to concussion. Sometimes the pressure wave
throws the victim away from the arc-blast. While this may reduce further
exposure to the thermal energy, serious physical injury may result. The
pressure wave can propel large objects over great distances. In some cases,
the pressure wave has enough force to snap off the heads of steel bolts
and knock over walls.
(3) A high-voltage arc can also cause many of the copper and aluminum components
in electrical equipment to melt. These droplets of molten metal can be blasted
great distances by the pressure wave. Although these droplets harden rapidly,
they can still be hot enough to cause serious burns or cause ordinary clothing
to catch fire, even if you are 10 feet or more away.
Electrical Fires
Electricity is one of the most common causes
of fires and thermal burns in homes and workplaces.
Defective or misused electrical equipment is a major cause of electrical
fires. If there is a small electrical fire, be sure to use only a
Class C or multipurpose (ABC) fire extinguisher, or you might make
the problem worse. All fire extinguishers are marked with letter(s)
that tell you the kinds of fires they can put out. Some extinguishers
contain symbols, too.
The letters and symbols are explained below (including suggestions
on how to remember them)
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A (think: Ashes) = paper, wood, etc. |
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B (think: Barrel) = flammable liquids |
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C (think: Circuits) = electrical fires |
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Here are a couple of fire extinguishers at a worksite. Can you tell
what types of fires they will put out?
However, do not try to put out fires unless you have received
proper training. If you are not trained, the best thing you can do
is evacuate the area and call for help.
Thermal burns may result if an explosion occurs when electricity ignites
an explosive mixture of material in the air. This ignition can result
from the buildup of combustible vapors, gasses, or dusts. Occupational
Safety and Health Administration (OSHA) standards, the NEC, and other
safety standards give precise safety requirements for the operation
of electrical systems and equipment in such dangerous areas. Ignition
can also be caused by overheated conductors or equipment, or by normal
arcing at switch contacts or in circuit breakers.
Summary
of Section 3
Burns are the most common injury caused by electricity. The three
types of burns are . . .
- electrical burns,
- arc burns, and
- thermal contact burns.
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First
Aid Fact Sheet |
What Should I Do If a Co-Worker
Is Shocked or Burned by Electricity?
Shut off the electrical current if the victim is still in contact
with the energized circuit. While you do this, have someone else call
for help. If you cannot get to the switchgear quickly, pry the victim
from the circuit with something that does not conduct electricity
such as dry wood. Do not touch the victim yourself if he or she
is still in contact with an electrical circuit! You do not want to
be a victim, too!
Do not leave the victim unless there is absolutely no
other option. You should stay with the victim while Emergency Medical
Services (EMS) is contacted. The caller should come back to you afterwards
to verify that the call was made. If the victim is not breathing,
does not have a heartbeat, or is badly injured, quick response by
a team of emergency medical technicians (EMT's) or paramedics gives
the best chance for survival.
Learn first aid and CPR now!
Once you know that electrical current is no longer flowing
through the victim, call out to the victim to see if he or she is
conscious (awake). If the victim is conscious, tell the victim not
to move. It is possible for a shock victim to be seriously injured
but not realize it. Quickly examine the victim for signs of major
bleeding. If there is a lot of bleeding, place a cloth (such as a
handkerchief or bandanna) over the wound and apply pressure. If the
wound is in an arm or leg and keeps bleeding a lot, gently elevate
the injured area while keeping pressure on the wound. Keep the victim
warm and talk to him or her until help arrives.
If the victim is unconscious, check for signs of breathing.
While you do this, move the victim as little as possible. If the victim
is not breathing, someone trained in CPR should begin artificial breathing,
then check to see if the victim has a pulse. Quick action is essential!
To be effective, CPR must be performed within 4 minutes of the shock.
If you are not trained in CPR or first aid, now is the
time to get trained-before you find yourself in this situation! Ask
your instructor or supervisor how you can become certified in CPR.
You also need to know the location of (1) electricity shut-offs ("kill
switches"), (2) first-aid sup-plies, and (3) a telephone so you
can find them quickly in an emergency.
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