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This article provides
an overview of basic electrical safety for individuals with little or limited
training or familiarity with electrical hazards. The concepts and principles
presented will help further an understanding of OSHA's electrical safety
standards for general industry, Title 29 Code of Federal Regulations (CFR),
Part 1910.302, Sub-part S-Design Safety Standards for Electrical Systems, and
1910.331 Electrical Safety-Related Work Practices Standard (1990). In general, OSHA'S
electrical standards are based on the National Fire Protection Associations'
Standard NFPA 70E, Electrical Safety Requirements for Employee Workplaces,
and in turn, from the National Electrical Code (NEC). OSHA's electrical
standards address concerns that electricity has long been recognized as a
serious workplace hazard, exposing employees to such dangers as electric
shock, electrocution, burns, fires, and explosions. In 1992, for example, the
Bureau of Labor Statistics reported that 6,210 work-related deaths occurred
in private sector workplaces employing 11 workers or more. Six percent of the
fatalities, or around 347 deaths, were the direct result of electrocutions at
work. What makes these statistics more tragic is that, for the most part,
these fatalities could have been easily avoided. OSHA'S electrical
standards help minimize these potential hazards by specifying safety aspects in
the design and use of electrical equipment and systems. The standards cover
only those parts of any electrical system that an employee would normally use
or contact. For example, the exposed and/or operating elements of an
electrical installation-lighting, equipment, motors, machines, appliances,
switches, controls, and enclosures-must be constructed and installed so as to
minimize workplace electrical dangers. For employers and
employees in the 25 states operating OSHA'S approved workplace safety and health
plans, their states may be enforcing standards and other procedures that
while "at least effective" federal standards are not always
identical to federal requirements. How Electricity Acts?
Electricity is
essential to modem life, both at home and on the job. Some employees work
with electricity directly, as is the case with engineers, electricians,
electronic technicians, and power line workers. Others, such as office
workers and sales-people, work with it indirectly. As a source of power,
electricity is accepted without much thought to the hazards encountered.
Perhaps because it has become such a familiar part of our surroundings, it
often is not treated with the respect it deserves. To handle electricity
safely, it is necessary to understand how it acts, how it can be directed,
what hazards it presents, and how these hazards can be controlled. Operating
an electric switch may be considered analogous to turning on a water faucet.
Behind the faucet or switch there must be a source of water or electricity,
with something to transport it, and with pressure to make it flow. In the
case of water, the source is a reservoir or pumping station; the
transportation is through pipes; and the force to make it flow is pressure,
provided by a pump. For electricity, the source is the power generating
station; current travels through electric conductors in the form of wires;
and pressure, measured in volts, is provided by a generator. Resistance to the flow
of electricity is measured in ohms and varies widely. It is determined by
three factors: the nature of the substance itself, the length and
cross-sectional area (size) of the substance, and the temperature of the
substance. Some substances, such
as metals, offer very little resistance to the flow of electric current and
are called conductors. Other substances, such as bakelite, porcelain,
pottery, and dry wood, offer such a high resistance that they can be used to
prevent the flow of electric current and are called insulators. Dry wood has a high
resistance, but when saturated with water its resistance drops to the point
where it will readily conduct electricity. The same thing is true of human
skin. When it is dry, skin
has a fairly high resistance to electric current; but when it is moist, there
is a radical drop in resistance. Pure water is a poor conductor, but small
amounts of impurities, such as salt and acid (both of which are contained in
perspiration), make it a ready conductor. When water is present either in the
environment or on the skin, anyone working with electricity should exercise
even more caution than they normally would. How Shocks Occur?
Electricity travels in
closed circuits, and its normal route is through a conductor. Electric shock
occurs when the body becomes a part of the electric circuit. The current must
enter the body at one point and leave at another. Electric shock normally
occurs in one of three ways. Individuals-while in contact with the ground-
must come in contact with both wires of the electric circuit, one wire of an
energized circuit and the ground, or a metallic part that has become
"hot" by contact with an energized conductor. The metal parts of
electric tools and machines may become energized if there is a break in the
insulation of the tool or machine wiring. The worker using these tools and
machines is made less vulnerable to electric shock when there is a
low-resistance path from the metallic case of the tool or machine to the
ground. This is done through the use of an equipment grounding conductor- a
low-resistance wire that causes the unwanted current to pass directly to the
ground, thereby greatly reducing the amount of current passing through the
body of the person in contact with the tool or machine. If the equipment
grounding conductor has been properly installed, it has a low resistance to
ground, and the worker is protected. Severity of the Shock
The severity of the
shock received when a person becomes a part of an electric circuit is
affected by three primary factors: the amount of current flowing through the
body (measured in amperes), the path of the current through the body, and the
length of time the body is in the circuit. Other factors that may affect the
severity of shock are the frequency of the current, the phase of the heart
cycle when shock occurs, and the general health of the person. The effects of electric
shock depend upon the type of circuit, its voltage, resistance, current,
pathway through the body, and duration of the contact. Effects can range from
a barely perceptible tingle to immediate cardiac arrest. Although there are
no absolute limits or even known values that show the exact injury from any
given current, the table shows the general relationship between the degree of
injury and amount of current for a 60-cycle hand-to-foot path of one second's
duration of shock. The table also
illustrates that a difference of less than 100 milliamperes exists between a
current that is barely perceptible and one that can kill. Muscular
contraction caused by stimulation may not allow the victim to free himself or
herself from the circuit, and the increased duration of exposure increases
the dangers to the shock victim. For example, a current of 100 milliamperes
for 3 seconds is equivalent to a current of 900 milliamperes applied for .03
seconds in causing ventricular fibrillation. The so-called low voltages can
be extremely dangerous because, all other factors being equal, the degree of
injury is proportional to the length of time the body is in the circuit. LOW
VOLTAGE DOES NOT IMPLY LOW HAZARD! A severe shock can
cause considerably more damage to the body than is visible. For example, a
person may suffer internal hemorrhages and destruction of tissues, nerves,
and muscles. In addition, shock is often only the beginning in a chain of
events. The final injury may well be from a fall, cuts, burns, or broken
bones. |
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Effects of Electric
Current in the Human Body |
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Current |
Reaction |
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1 Milliampere |
Perception level. Just
a faint tingle. |
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5 Milliamperes |
Slight shock felt; not
painful but disturbing. |
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6-25 Milliamperes
(women) |
Painful shock, muscular
control is lost. |
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9-30 Milliamperes (men) |
This is called the
freezing current or "let-go" range. |
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50-150 Milliamperes |
Extreme pain,
respiratory arrest, severe muscular contractions.* |
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1,000-4,300
Milliamperes |
Ventricular
fibrillation. (The rhythmic pumping action of the heart ceases.) Muscular
contraction and nerve damage occur. Death is most likely. |
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10,000-Milliamperes |
Cardiac arrest, severe
burns and probable death. |
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*If the extensor
muscles are excited by the electric shock, the person may be thrown away from
the circuit. Burns and Other Injuries
The most common
shock-related injury is a burn. burns suffered in electrical accidents may be
of three types: electrical burns, arc burns, and thermal contact burns. Electrical burns are
the result of the electric current flowing through tissues or bone. Tissue
damage is caused by the heat generated by the current flow through the body.
Electrical burns are one of the most serious injuries you can receive and
should be given immediate attention. Arc or flash burns, on
the other hand, are the result of high temperatures near the body and are
produced by an electric arc or explosion. They should also be attended to promptly.
Finally, thermal
contact burns are those normally experienced when the skin comes in contact
with hot surfaces of overheated electric conductors, conduits, or other
energized equipment. Additionally, clothing may be ignited in an electrical
accident and a thermal burn will result. All three types of burns may be
produced simultaneously. Electric shock can also
cause injuries of an indirect or secondary nature in which involuntary muscle
reaction from the electric shock can cause bruises, bone fractures, and even
death resulting from collisions or falls. In some cases, injuries caused by
electric shock can be a contributory cause of delayed fatalities. In addition to shock
and burn hazards, electricity poses other dangers. For example, when a short
circuit occurs, hazards are created from the resulting arcs. If high current
is involved, these arcs can cause injury or start a fire. Extremely
high-energy arcs can damage equipment, causing fragmented metal to fly in all
directions. Even low-energy arcs can cause violent explosions in atmospheres
that contain flammable gases, vapors, or combustible dusts. Preventing Electrical Hazards
Electrical accidents
appear to be caused by a combination of three possible factors- unsafe
equipment and/or installation, workplaces made unsafe by the environment, and
unsafe work practices. There are various ways of protecting people from the
hazards caused by electricity. These include: insulation, guarding,
grounding, electrical protective devices, and safe work practices. Insulation One way to safeguard
individuals from electrically energized wires and parts is through
insulation. An insulator is any material with high resistance to electric
current. Insulators-such as
glass, mica, rubber, and plastic-are put on conductors to prevent shock,
fires, and short circuits. Before employees prepare to work with electric
equipment, it is always a good idea for them to check the insulation before
making a connection to a power source to be sure there are no exposed wires. The
insulation of flexible cords, such as extension cords, is particularly
vulnerable to damage. The insulation that
covers conductors is regulated by Subpart S of 29 Code of Federal
Regulations (CFR) Part 1910.302, Design Safety Standards for Electrical
Systems, as published in the Federal Register on January 16, 1981.
Subpart S generally
requires that circuit conductors (the material through which current flows)
be insulated to prevent people from coming into accidental contact with the
current. Also, the insulation should be suitable for the voltage and existing
conditions, such as temperature, moisture, oil, gasoline, or corrosive fumes.
All these factors must be evaluated before the proper choice of insulation
can be made. Conductors and cables
are marked by the manufacturer to show the maximum voltage and American Wire
Gage size, the type letter of the insulation, and the manufacturer's name or
trademark. Insulation is often color coded. In general, insulated wires used
as equipment grounding conductors are either continuous green or green with
yellow stripes. The grounded conductors that complete a circuit are generally
covered with continuous white or natural gray-colored insulation. The
ungrounded conductors, or "hot wires," may be any color other than
green, white, or gray. They are often colored black or red. Guarding Live parts of electric
equipment operating at 50 volts or more must be guarded against accidental
contact. Guarding of live parts may be accomplished by: · location in a
room, vault, or similar enclosure accessible only to qualified persons; · use of
permanent, substantial partitions or screens to exclude unqualified persons; · location on a
suitable balcony, gallery, or platform elevated and arranged to exclude unqualified
persons; or · elevation of 8
feet (2.44 meters) or more above the floor. Entrances to rooms and
other guarded locations containing exposed live parts must be marked with
conspicuous warning signs forbidding unqualified persons to enter. Indoor electric wiring
more than 600 volts and that is open to unqualified persons must be made with
metal-enclosed equipment or enclosed in a vault or area controlled by a lock.
In addition, equipment must be marked with appropriate caution signs. Grounding Grounding is another
method of protecting employees from electric shock; however, it is normally a
secondary protective measure. The "ground" refers to a conductive
body, usually the earth, and means a conductive connection, whether
intentional or accidental, by which an electric circuit or equipment is
connected to earth or the ground plane. By "grounding" a tool or
electrical system, a low-resistance path to the earth is intentionally
created. When properly done, this path offers sufficiently low resistance and
has sufficient current carrying capacity to prevent the buildup of voltages
that may result in a personnel hazard. This does not guarantee that no one
will receive a shock, be injured, or be killed. It will, however,
substantially reduce the possibility of such accidents - especially when used
in combination with other safety measures discussed in this booklet. There are two kinds of
grounds required by Design Safety Standards for Electrical Systems (Subpart
S). One of these is called the "service or system ground." In this
instance, one wire-called "the neutral conductor" or "grounded
conductor" - is grounded. In an ordinary low-voltage circuit, the white
(or gray) wire is grounded at the generator or transformer and again at the
service entrance of the building. This type of ground is primarily designed
to protect machines, tools, and insulation against damage. To offer enhanced
protection to the workers themselves, an additional ground, called the
"equipment ground," must be furnished by providing another path
from the tool or machine through which the current can flow to the ground.
This additional ground safeguards the electric equipment operator in the
event that a malfunction causes the metal frame of the tool to become
accidentally energized. The resulting heavy surge of current will then
activate the circuit protection devices and open the circuit. Circuit
Protection Devices Circuit protection
devices are designed to automatically limit or shut off the flow of
electricity in the event of a ground-fault, overload, or short circuit in the
wiring system. Fuses, circuit breakers, and ground-fault circuit interrupters
are three well-known examples of such devices. Fuses and
circuit-breakers are over-current devices that are placed in circuits to monitor
the amount of current that the circuit will carry. They automatically open or
break the circuit when the amount of current flow becomes excessive and
therefore unsafe. Fuses are designed to melt when too much current flows
through them. Circuit breakers, on the other hand, are designed to trip open
the circuit by electro-mechanical means. Fuses and circuit
breakers are intended primarily for the protection of conductors and
equipment. They prevent over-heating of wires and components that might otherwise
create hazards for operators. They also open the circuit under certain
hazardous ground-fault conditions. The ground-fault
circuit interrupter, or GFCI, is designed to shutoff electric power within as
little as 1/40 of a second. It works by comparing the amount of current going
to electric equipment against the amount of current returning from the
equipment along the circuit conductors. If the current difference exceeds 6
milliamperes, the GFCI interrupts the current quickly enough to prevent electrocution.
The GFCI is used in high-risk areas such as wet locations and construction
sites. Safe Work
Practices Employees and others
working with electric equipment need to use safe work practices. These
include: deenergizing electric equipment before inspecting or making repairs,
using electric tools that are in good repair, using good judgment when
working near energized lines, and using appropriate protective equipment.
Electrical safety-related work practice requirements are contained in Subpart
S of 29 CFR Part 1910, in Sections 1910.331-1910.335. Training To ensure that they use
safe work practices, employees must be aware of the electrical hazards to
which they will be exposed. Employees must be trained in safety-related work
practices as well as any other procedures necessary for safety from
electrical hazards. Deenergizing
Electrical Equipment. The accidental or unexpected sudden starting of electrical
equipment can cause severe injury or death. Before ANY inspections or repairs
are made -- even on the so-called low-voltage circuits-the current must be
turned off at the switch box and the switch padlocked in the OFF position. At
the same time, the switch or controls of the machine or other equipment being
locked out of service must be securely tagged to show which equipment or
circuits are being worked on. Maintenance employees
should be qualified electricians who have been well instructed in lockout
procedures. No two locks should be alike; each key should fit only one lock,
and only one key should be issued to each maintenance employee. If more than
one employee is repairing a piece of equipment, each should lock out the
switch with his or her own lock and never permit anyone else to remove it.
The maintenance worker should at all times be certain that he or she is not
exposing other employees to danger. Overhead
Lines If work is to be
performed near overhead power lines, the lines must be deenergized and
grounded by the owner or operator of the lines, or other protective measures
must be provided before work is started. Protective measures (such as
guarding or insulating the lines) must be designed to prevent employees from
contacting the lines. Unqualified employees
and mechanical equipment must stay at least 10 feet (3.05 meters) away from
overhead power lines. If the voltage is more than 50,000 volts, the clearance
must be increased by 4 inches (10 centimeters) for each additional 10,000
volts. When mechanical
equipment is being operated near over-head lines, employees standing on the
ground may not contact the equipment unless it is located so that the
required clearance cannot be violated even at the maximum reach of the
equipment. Protective Equipment. Employees whose occupations
require them to work directly with electricity must use the personal
protective equipment required for the jobs they perform. This equipment may
consist of rubber insulating gloves, hoods, sleeves, matting, blankets, line
hose, and industrial protective helmets. Tools. To maximize his or her own
safety, an employee should always use tools that work properly. Tools must be
inspected before use, and those found questionable, removed from service and
properly tagged. Tools and other equipment should be regularly maintained.
Inadequate maintenance can cause equipment to deteriorate, resulting in an
unsafe condition. Tools that are used by
employees to handle energized conductors must be designed and constructed to
withstand the voltages and stresses to which they are exposed. Good Judgment. Perhaps the single most
successful defense against electrical accidents is the continuous exercising
of good judgment or common sense. All employees should be thoroughly familiar
with the safety procedures for their particular jobs. When work is performed on
electrical equipment, for example, some basic procedures are: · Have the
equipment deenergized. · Ensure that the
equipment remains deenergized by using some type of lockout and tag
procedure. · Use insulating
protective equipment. · Keep a safe
distance from energized parts.
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