Electrical
Safety
Safety and Health for Electrical Trades
Student Manual |
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Section
7
Safety Model Stage 3-Controlling Hazards: Safe Work Environment |
How Do You Control Hazards?
In order to control hazards, you must first create a safe work environment,
then work in a safe manner. Generally, it is best to remove the hazards
altogether and create an environment that is truly safe. When OSHA
regulations and the NEC are followed, safe work environments are created.
But, you never know when materials or equipment might
fail. Prepare yourself for the unexpected by using safe work practices.
Use as many safeguards as possible. If one fails, another may protect
you from injury or death.
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How Do You Create a Safe
Work Environment?
A safe work environment is created by controlling contact with electrical
voltages and the currents they can cause. Electrical currents need
to be controlled so they do not pass through the body. In addition
to preventing shocks, a safe work environment reduces the chance of
fires, burns, and falls.
You need to guard against contact with electrical voltages
and control electrical currents in order to create a safe work environment.
Make your environment safer by doing the following:
- Treat all conductors-even "de-energized"
ones-as if they are energized until they are locked out and tagged.
- Lock out and tag out circuits and machines.
- Prevent overloaded wiring by using the right size
and type of wire.
- Prevent exposure to live electrical parts by isolating
them.
- Prevent exposure to live wires and parts by using
insulation.
- Prevent shocking currents from electrical systems
and tools by grounding them.
- Prevent shocking currents by using GFCI's.
- Prevent too much current in circuits by using overcurrent
protection devices.
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Lock Out and Tag Out Circuits and Equipment
Create a safe work environment by locking out and tagging out circuits
and machines. Before working on a circuit, you must turn off the power
supply. Once the circuit has been shut off and de-energized, lock
out the switchgear to the circuit so the power cannot be turned back
on inadvertently. Then, tag out the circuit with an easy-to-see sign
or label that lets everyone know that you are working on the circuit.
If you are working on or near machinery, you must lock out and tag
out the machinery to prevent startup. Before you begin work, you must
test the circuit to make sure it is de-energized.
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Lock-Out/Tag-Out
Checklist
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Lock-out/tag-out is an essential safety procedure that protects
workers from injury while working on or near electrical circuits and
equipment. Lock-out involves applying a physical lock to the power
source(s) of circuits and equipment after they have been shut off
and de-energized. The source is then tagged out with an easy-to-read
tag that alerts other workers in the area that a lock has been applied.
In addition to protecting workers from electrical hazards, lock-out/tag-out
prevents contact with operating equipment parts: blades, gears, shafts,
presses, etc.
A worker was replacing a V-belt on a dust collector
blower. Before beginning work, he shut down the unit at the
local switch. However, an operator in the control room restarted
the unit using a remote switch. The worker's hand was caught
between the pulley and belts of the blower, resulting in cuts
and a fractured finger.
When performing lock-out/tag-out on machinery, you must
always lock out and tag out ALL energy sources leading to
the machinery.
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Also, lock-out/tag-out prevents the unexpected release
of hazardous gasses, fluids, or solid matter in areas where workers
are present.
An employee was cutting into a metal pipe using
a blowtorch. Diesel fuel was mistakenly discharged into the
line and was ignited by his torch. The worker burned to death
at the scene.
All valves along the line should have been locked out,
blanked out, and tagged out to prevent the release of fuel.
Blanking is the process of inserting a metal disk into the
space between two pipe flanges. The disk, or blank, is then
bolted in place to prevent passage of liquids or gasses through
the pipe.
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When performing lock-out/tag-out on circuits and
equipment, you can use the checklist below.
- Identify all sources of electrical
energy for the equipment or circuits in question.
- Disable backup energy sources
such as generators and batteries.
- Identify all shut-offs for each
energy source.
- Notify all personnel that equipment
and circuitry must be shut off, locked out, and tagged out. (Simply
turning a switch off is NOT enough.)
- Shut off energy sources and lock
switchgear in the OFF position. Each worker should apply
his or her individual lock. Do not give your key to anyone.
- Test equipment and circuitry to
make sure they are de-energized. This must be done by a qualified
person.*
- Deplete stored energy by bleeding,
blocking, grounding, etc.
- Apply a tag to alert other workers
that an energy source or piece of equipment has been locked out.
- Make sure everyone is safe and accounted for before
equipment and circuits are unlocked and turned back on. Note that
only a qualified person may determine when it is safe to re-energize
circuits.
*OSHA defines a "qualified
person" as someone who has received mandated training on
the hazards and on the construction and operation of equipment
involved in a task. |
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Control Inadequate
Wiring Hazards
Electrical hazards result from using the wrong size or type of wire.
You must control such hazards to create a safe work environment. You
must choose the right size wire for the amount of current expected
in a circuit. The wire must be able to handle the current safely.
The wire's insulation must be appropriate for the voltage and tough
enough for the environment. Connections need to be reliable and protected.
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Control Hazards of
Fixed Wiring
The wiring methods and size of conductors used in a system depend
on several factors:
- Intended use of the circuit system
- Building materials
- Size and distribution of electrical load
- Location of equipment (such as underground burial)
- Environmental conditions (such as dampness)
- Presence of corrosives
- Temperature extremes
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Fixed, permanent wiring is better than extension cords,
which can be misused and damaged more easily. NEC requirements for
fixed wiring should always be followed. A variety of materials can
be used in wiring applications, including nonmetallic sheathed cable
(Romex®), armored cable, and metal and plastic conduit. The choice
of wiring material depends on the wiring environment and the need
to support and protect wires.
Aluminum wire and connections should be handled with
special care. Connections made with aluminum wire can loosen due to
heat expansion and oxidize if they are not made properly. Loose or
oxidized connections can create heat or arcing. Special clamps and
terminals are necessary to make proper connections using aluminum
wire. Antioxidant paste can be applied to connections to prevent oxidation.
Control Hazards of Flexible
Wiring
Use flexible wiring properly
Electrical cords supplement fixed wiring by providing the flexibility
required for maintenance, portability, isolation from vibration, and
emergency and temporary power needs.
Flexible wiring can be used for extension cords or power
supply cords. Power supply cords can be removable or permanently attached
to the appliance.
DO NOT use flexible wiring in situations where frequent
inspection would be difficult, where damage would be likely, or where
long-term electrical supply is needed. Flexible cords cannot be used
as a substitute for the fixed wiring of a structure. Flexible cords
must not be . . .
- Run through holes in walls, ceilings, or floors;
- Run through doorways, windows, or similar openings
(unless physically protected);
- Attached to building surfaces (except with a tension
take-up device within 6 feet of the supply end);
- Hidden in walls, ceilings, or floors; or
- Hidden in conduit or other raceways.
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Use the Right Extension
Cord
The size of wire in an extension cord must be compatible with the
amount of current
the cord will be expected to carry. The amount of current depends
on the equipment plugged into the extension cord. Current ratings
(how much current a device needs to operate) are often printed on
the nameplate. If a power rating is given, it is necessary to divide
the power rating in watts by the voltage to find the cur-rent rating.
For example, a 1,000-watt heater plugged into a 120-volt circuit will
need almost 10 amps of current. Let's look at another example: A 1-horsepower
electric motor uses electrical energy at the rate of almost 750 watts,
so it will need a minimum of about 7 amps of current on a 120-volt
circuit. But, electric motors need additional current as they startup
or if they stall, requiring up to 200% of the nameplate current rating.
Therefore, the motor would need 14 amps.
Add to find the total current needed to operate all the appliances
supplied by the cord. Choose a wire size that can handle the total
current.
American Wire Gauge
(AWG)
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Wire
size |
Handles
up to |
#10 AWG
#12 AWG
#14 AWG
#16 AWG |
30 amps
25 amps
18 amps
13 amps |
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The length of the extension cord also needs to be considered
when selecting the wire size. Voltage drops over the length of a cord.
If a cord is too long, the voltage drop can be enough to damage equipment.
Many electric motors only operate safely in a narrow range of voltages
and will not work properly at voltages different than the voltage
listed on the nameplate. Even though light bulbs operate (somewhat
dimmer) at lowered voltages, do not assume electric motors will work
correctly at less-than-required voltages. Also, when electric motors
start or operate under load, they require more current. The larger
the size of the wire, the longer a cord can be without causing a voltage
drop that could damage tools and equipment.
The grounding path for extension cords must be kept
intact to keep you safe. A typical extension cord grounding system
has four components:
- a third wire in the cord, called a ground wire;
- a three-prong plug with a grounding prong on one
end of the cord;
- a three-wire, grounding-type receptacle at the other
end of the cord; and
- a properly grounded outlet.
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Control Hazards
of Exposed Live Electrical Parts: Isolate Energized Components
Electrical hazards exist when wires or other electrical parts are
exposed. These hazards need to be controlled to create a safe work
environment. Isolation of energized electrical parts makes them
inaccessible unless tools and special effort are used. Isolation
can be accomplished by placing the energized parts at least 8 feet
high and out of reach, or by guarding. Guarding is a type of isolation
that uses various structures-like cabinets, boxes, screens, barriers,
covers, and partitions-to close-off live electrical parts.
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Take
the following precautions to prevent injuries from contact with live parts:
- Immediately report exposed live parts to a supervisor
or teacher. As a student, you should never attempt to correct the condition
yourself without supervision.
- Use covers, screens, or partitions for guarding that
require tools to remove them.
- Replace covers that have been removed from panels,
motors, or fuse boxes.
- Even when live parts are elevated to the required
height (8 feet), care should be taken when using objects (like metal rods
or pipes) that can contact these parts.
- Close unused conduit openings in boxes so that foreign
objects (pencils, metal chips, conductive debris, etc.) cannot get inside
and damage the circuit.
Control Hazards of Exposure
to Live Electrical Wires: Use Proper Insulation
Insulation is made of material that does not conduct electricity (usually
plastic, rubber, or fiber). Insulation covers wires and prevents conductors
from coming in contact with each other or any other conductor. If conductors
are allowed to make contact, a short circuit is created. In a short circuit,
current passes through the shorting material without passing through a load
in the circuit, and the wire becomes overheated. Insulation keeps wires
and other conductors from touching, which prevents electrical short circuits.
Insulation prevents live wires from touching people and animals, thus protecting
them from electrical shock.
Insulation helps protect wires from physical damage and conditions
in the environment. Insulation is used on almost all wires, except some
ground wires and some high-voltage transmission lines. Insulation is used
internally in tools, switches, plugs, and other electrical and electronic
devices.
Special insulation is used on wires and cables that are used
in harsh environments. Wires and cables that are buried in soil must have
an outer covering of insulation that is flame-retardant and resistant to
moisture, fungus, and corrosion.
In all situations, you must be careful not to damage
insulation while installing it. Do not allow staples or other supports to
damage the insulation. Bends in a cable must have an inside radius of at
least 5 times the diameter of the cable so that insulation at a bend is
not damaged. Extension cords come with insulation in a variety of types
and colors. The insulation of extension cords is especially important. Since
extension cords often receive rough handling, the insulation can be damaged.
Extension cords might be used in wet places, so adequate insulation is necessary
to prevent shocks. Because extension cords are often used near combustible
materials (such as wood shavings and sawdust) a short in an extension cord
could easily cause arcing and a fire.
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Insulation on individual wires 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 gray insulation. The ungrounded conductors, or "hot"
wires, may be any color other than green, white, or gray. They are
usually black or red.
Conductors
and cables must be marked by the manufacturer to show the following:
- Maximum voltage capacity,
- AWG size,
- Insulation-type letter, and
- The manufacturer's name or trademark.
Control hazards
of shocking currents
Ground circuits and equipment
When an electrical system is not grounded properly, a hazard exists.
This is because the parts of an electrical wiring system that a person
normally touches may be energized, or live, relative to ground. Parts
like switch plates, wiring boxes, conduit, cabinets, and lights need
to be at 0 volts relative to ground. If the system is grounded improperly,
these parts may be energized. The metal housings of equipment plugged
into an outlet need to be grounded through the plug.
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Grounding is connecting an electrical system to the earth with a
wire. Excess or stray current travels through this wire to a grounding
device (commonly called a "ground") deep in the earth. Grounding
prevents unwanted voltage on electrical components. Metal plumbing
is often used as a ground. When plumbing is used as a grounding conductor,
it must also be connected to a grounding device such as a conductive
rod. (Rods used for grounding must be driven at least 8 feet into
the earth.) Sometimes an electrical system will receive a higher voltage
than it is designed to handle. These high voltages may come from a
lightning strike, line surge, or contact with a higher-voltage line.
Sometimes a defect occurs in a device that allows exposed metal parts
to become energized. Grounding will help protect the person working
on a system, the system itself, and others using tools or operating
equipment connected to the system. The extra current produced by the
excess voltage travels relatively safely to the earth.
Grounding creates a path for currents produced by unintended voltages
on exposed parts. These currents follow the grounding path, rather
than passing through the body of someone who touches the energized
equipment. However, if a grounding rod takes a direct hit from a lightning
strike and is buried in sandy soil, the rod should be examined to
make sure it will still function properly. The heat from a lightning
strike can cause the sand to turn into glass, which is an insulator.
A grounding rod must be in contact with damp soil to be effective.
Leakage current occurs when an electrical current escapes from its
intended path. Leakages are sometimes low-current faults that can
occur in all electrical equipment because of dirt, wear, damage, or
moisture. A good grounding system should be able to carry off this
leakage current. A ground fault occurs when current passes through
the housing of an electrical device to ground. Proper grounding protects
against ground faults. Ground faults are usually caused by misuse
of a tool or damage to its insulation. This damage allows a bare conductor
to touch metal parts or the tool housing.
When you ground a tool or electrical system, you create a low-resistance
path to the earth (known as a ground connection). When done properly,
this path has sufficient current-carrying capacity to eliminate voltages
that may cause a dangerous shock.
Grounding does not guarantee that you will not be shocked, injured,
or killed from defective equipment. However, it greatly reduces the
possibility.
Equipment needs to be grounded under any of these circumstances:
- The equipment is within 8 feet vertically and 5 feet horizontally
of the floor or walking surface.
- The equipment is within 8 feet vertically and 5 feet horizontally
of grounded metal objects you could touch.
- The equipment is located in a wet or damp area and is not isolated.
- The equipment is connected to a power supply by cord and plug
and is not double-insulated.
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Use
GFCI's
The use of GFCI's has lowered the number of electrocutions dramatically.
A GFCI is a fast-acting switch that detects any difference in current between
two circuit conductors. If either conductor comes in contact-either directly
or through part of your body-with a ground (a situation known as a ground
fault), the GFCI opens the circuit in a fraction of a second. If a current
as small as 4 to 6 mA does not pass through both wires properly, but instead
leaks to the ground, the GFCI is tripped. The current is shut off.
There is a more sensitive kind of GFCI called an isolation
GFCI. If a circuit has an isolation GFCI, the ground fault current passes
through an electronic sensing circuit in the GFCI. The electronic sensing
circuit has enough resistance to limit current to as little as 2 mA, which
is too low to cause a dangerous shock.
GFCI's are usually in the form of a duplex receptacle. They
are also available in portable and plug-in designs and as circuit breakers
that protect an entire branch circuit. GFCI's can operate on both two- and
three-wire ground systems. For a GFCI to work properly, the neutral conductor
(white wire) must (1) be continuous, (2) have low resistance, and (3) have
sufficient current-carrying capacity.
GFCI's help protect you from electrical shock by continuously monitoring
the circuit. However, a GFCI does not protect a person from line-to-line
hazards such as touching two "hot" wires (240 volts) at the same
time or touching a "hot" and neutral wire at the same time. Also
be aware that instantaneous currents can be high when a GFCI is tripped.
A shock may still be felt. Your reaction to the shock could cause injury,
perhaps from falling.
Test GFCI's regularly by pressing the "test" button.
If the circuit does not turn off, the GFCI is faulty and must be replaced.
The NEC requires that GFCI's be used in these high-risk
situations:
- Electricity is used near water.
- The user of electrical equipment is grounded (by touching grounded
material).
- Circuits are providing power to portable tools or outdoor receptacles.
- Temporary wiring or extension cords are used.
- Specifically, GFCI's must be installed in bathrooms, garages,
out-door areas, crawl spaces, unfinished basements, kitchens, and
near wet bars.
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Bond
Components to Assure Grounding Path
In order to assure a continuous, reliable electrical path to ground, a bonding
jumper wire is used to make sure electrical parts are connected. Some physical
connections, like metal conduit coming into a box, might not make a good
electrical connection because of paint or possible corrosion. To make a
good electrical connection, a bonding jumper needs to be installed.
A metal cold water pipe that is part of a path to ground may
need bonding jumpers around plastic antivibration devices, plastic water
meters, or sections of plastic pipe. A bonding jumper is made of conductive
material and is tightly connected to metal pipes with screws or clamps to
bypass the plastic and assure a continuous grounding path. Bonding jumpers
are necessary because plastic does not conduct electricity and would interrupt
the path to ground.
Additionally, interior metal plumbing must be bonded to the
ground for electrical service equipment in order to keep all grounds at
the same potential (0 volts). Even metal air ducts should be bonded to electrical
service equipment.
Control Overload
Current Hazards
When
a current exceeds the current rating of equipment or wiring, a hazard exists.
The wiring in the circuit, equipment, or tool cannot handle the current
without heating up or even melting. Not only will the wiring or tool be
damaged, but the high temperature of the conductor can also cause a fire.
To prevent this from happening, an overcurrent protection device (circuit
breaker or fuse) is used in a circuit. These devices open a circuit automatically
if they detect current in excess of the current rating of equipment or wiring.
This excess current can be caused by an overload, short circuit, or high-level
ground fault.
Overcurrent protection devices are designed to protect equipment
and structures from fire. They do not protect you from electrical shock!
Overcurrent protection devices stop the flow of current in a circuit when
the amperage is too high for the circuit. A circuit breaker or fuse will
not stop the relatively small amount of current that can cause injury or
death. Death can result from 20 mA (.020 amps) through the chest (see Section
2). A typical residential circuit breaker or fuse will not shut off the
circuit until a current of more than 20 amps is reached!
But overcurrent protection devices are not allowed in areas
where they could be exposed to physical damage or in hazardous environments.
Overcurrent protection devices can heat up and occasionally arc or spark,
which could cause a fire or an explosion in certain areas. Hazardous environments
are places that contain flammable or explosive materials such as flammable
gasses or vapors (Class I Hazardous Environments), finely pulverized flammable
dusts (Class II Hazardous Environments), or fibers or metal filings that
can catch fire easily (Class III Hazardous Environments). Hazardous environments
may be found in aircraft hangars, gas stations, storage plants for flammable
liquids, grain silos, and mills where cotton fibers may be suspended in
the air. Special electrical systems are required in hazardous environments.
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If an overcurrent protection
device opens a circuit, there may be a problem along the circuit. (In
the case of circuit breakers, frequent tripping may also indicate that
the breaker is defective.) When a circuit breaker trips or a fuse blows,
the cause must be found.
A circuit breaker is one kind of overcurrent protection
device. It is a type of automatic switch located in a circuit. A circuit
breaker trips when too much current passes through it. A circuit breaker
should not be used regularly to turn power on or off in a circuit,
unless the breaker is designed for this purpose and marked "SWD"
(stands for "switching device").
A fuse is another type of overcurrent protection device.
A fuse contains a metal conductor that has a relatively low melting
point. When too much current passes through the metal in the fuse,
it heats up within a fraction of a second and melts, opening the circuit.
After an overload is found and corrected, a blown fuse must be replaced
with a new one of appropriate amperage.
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Summary of Section 7
Control contact with electrical voltages and control electrical currents
to create a safe work environment.
- Lock out and tag out circuits and machines.
- Prevent overloaded wiring by using the right size and type of
wire.
- Prevent exposure to live electrical parts by isolating them.
- Prevent exposure to live wires and parts by using insulation.
- Prevent shocking currents from electrical systems and tools by
grounding them.
- Prevent shocking currents by using GFCI's.
- Prevent too much current in circuits by using overcurrent protection
devices.
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