- 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.
|
Guard
against contact with electrical voltages and control electrical
currents to create a safe work environment.
|
At about
1:45 a.m., two journeyman electricians began replacing bulbs and
making repairs on light fixtures in a spray paint booth at an
automobile assembly plant. The job required the two
electricians to climb on top of the booth and work from above.
The top of the booth was filled with pipes and ducts that restricted
visibility and movement. Flashlights were required.
The electricians started at opposite ends of the booth. One electrician
saw a flash of light, but continued to work for about 5 minutes,
then climbed down for some wire. While cutting the wire, he smelled
a burning odor and called to the other electrician. When no one
answered, he climbed back on top of the booth. He found his co-worker
in contact with a single-strand wire from one of the lights. Needle-nose
wire strippers were stuck in the left side of the victim’s
chest. Apparently, he had been stripping insulation from an improperly
grounded 530-volt, single-strand wire when he contacted it with
the stripper. In this case, the electricians knew they were working
on energized circuits. The breakers in the booth’s control
panel were not labeled and the lock used for lock-out/tag-out
was broken. The surviving electrician stated that locating the
means to de-energize a circuit often takes more time than the
actual job.
The electrician would be alive today if the following rules had
been observed.
- Always
shut off circuits—then test to confirm that they are
de-energized—before starting a job.
- Switchgear
that shuts off a circuit must be clearly labeled and easy
to access.
- Lock-out/tag-out
materials must always be provided, and lock-out/tag-out procedures
must always be followed
|
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.
 |
 |
| Always
test a circuit to make sure it is de-energized before working
on it. |
Lock-out/tag-out
saves lives. |
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. |
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. |
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.
|
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.
| Use the
right size and type of wire. |
| AWG—American
Wire Gauge—a measure of wire size |
 |
| Wires
come in different sizes. The maximum current each size can conduct
safely is shown. |
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
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.
|
fixed
wiring—the permanent wiring installed in homes and
other buildings
|
 |
| Nonmetalic
sheathing helps protect wires from damage. |
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.
|
flexible
wiring—cables with insulated and stranded wire that
bends easily
|
A 29-year-old
male welder was assigned to work on an outdoor concrete platform
attached to the main factory building. He wheeled a portable arc
welder onto the platform. Since there was not an electrical outlet
nearby, he used an extension cord to plug in the welder. The male
end of the cord had four prongs, and the female end was spring-loaded.
The worker plugged the male end of the cord into the outlet. He
then plugged the portable welder’s power cord into the female
end of the extension cord. At that instant, the metal case around
the power cord plug became energized, electrocuting the worker.
An investigation showed that the female end of the extension cord
was broken. The spring, cover plate, and part of the casing were
missing from the face of the female connector. Also, the grounding
prong on the welder’s power cord plug was so severely bent
that it slipped outside of the connection. Therefore, the arc
welder was not grounded. Normally, it would have been impossible
to insert the plug incorrectly. But, since the cord’s female
end was damaged, the “bad” connection was able to occur.
Do not let this happen to you. Use these safe practices:
- Thoroughly
inspect all electrical equipment before beginning work.
- Do
not use extension cords as a substitute for fixed wiring.
In this case, a weatherproof receptacle should have been installed
on the platform.
- Use
connectors that are designed to stand up to the abuse of the
job. Connectors designed for light-duty use should not be
used in an industrial environment.
|
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.
|
Don’t
use flexible wiring where it may get damaged.
|
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 current 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)
|
|
Wire
Size
|
Handles
up to
|
|
#10
AWG
|
30
amps
|
|
#12
AWG
|
25
amps
|
|
#14
AWG
|
18
amps
|
|
#16
AWG
|
13
amps
|
|
Remember:
The larger the gauge number, the smaller the wire!
|
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.
|
power—the
amount of energy used in a second, measured in watts
|
|
1 horsepower
= 746 watts.
|
|
Do
not use extension cords that are too long for the size of wire.
|
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.
|
Make
sure the path to ground is continuous.
|
Outlets
must be grounded properly.
|
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.
 |
|
This
exposed electrical equipment is guarded by an 8-foot fence.
|
|
guarding—a
covering or barrier that separates you from live electrical
parts
|
Use
covers to prevent accidental contact with electrical circuits.
|
A 20-year-old
male laborer was carrying a 20-foot piece of iron from a welding
shop to an outside storage rack. As he was turning a corner near
a bank of electrical transformers, the top end of the piece of
iron struck an uninsulated supply wire at the top of a transformer.
Although the transformers were surrounded by a 6-foot fence, they
were about 3 feet taller than the fence enclosure. Each transformer
carried 4,160 volts.
When the iron hit the supply wire, the laborer was electrocuted.
A forklift operator heard the iron drop to the ground at about
8:46 a.m. and found the victim 5 minutes later. He was pronounced
dead on arrival at a local hospital.
- According
to OSHA, the enclosure around the transformers was too short.
The fence should have been at least 8 feet tall.
- The
company in this case did not offer any formal safety training
to its workers. All employers should develop safety and health
training programs so their employees know how to recognize
and avoid life-threatening hazards.
|
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.
- Provide guards
or barriers if live parts cannot be enclosed completely.
- 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.
 |
| This
cover cannot be removed without special tools. |
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.
A 29-year-old
male maintenance worker was found at 3:45 a.m. lying on his back
and convulsing. Beside him were an overturned cart and an electric
welding machine, both lying in a pool of water on the concrete
floor. Arcing was visible between the welding machine and the
floor. The worker was transported to the closest hospital, where
he was pronounced dead.
An examination of the welding machine showed that there were exposed
conductors in the machine’s cables. There were numerous cuts
and scrapes in the cables’ insulation. On other parts of
the machine, insulation was damaged or missing. Also, the machine
did not have a ground connection.
Investigators concluded that the maintenance worker was electrocuted
when he tried to turn off the welding machine, which was sitting
on the cart. The metal frame of the machine had become energized
due to the damaged insulation. When he touched the energized frame,
he completed the conducting path to ground. The current traveled
through his body to ground. Since he was probably standing in
water, the risk of a ground fault was even greater.
You must take steps to decrease such hazards in your workplace:
- Ground
circuits and equipment.
- Keep
all equipment in good operating condition with a preventive
maintenance program.
- Never
use electrical equipment or work on circuits in wet areas.
If you find water or dampness, notify your supervisor immediately.
|
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.
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.
 |
| Arc-fault
circuit breaker. |
Ground circuits and equipment
When an electrical system is not gro unded 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.
 |
| Ground
electrical devices. |
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.
 |
 |
| Grounding
rod in the earth. |
Grounding-type
receptacle. |
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.
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.
 |
| Portable
GFCI. |
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) atthe
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.
A
female assistant manager of a swim club was instructed to add
a certain chemical to the pool. She went down into the pump room,
barefoot. The room was below ground level, and the floor was covered
with water. She filled a plastic drum with 35-40 gallons of water,
then plugged a mixing motor into a 120-volt wall outlet and turned
on the motor. The motor would be used to mix the water and the
chemical, then the solution would be added to the pool. While
adding the chemical to the water in the drum, she contacted the
mixing motor with her left hand. Apparently, the motor had developed
a ground fault. Because of the ground fault, the motor was energized,
and she was electrocuted. A co-worker found the victim slumped
over the drum with her face submerged in water. The co-worker
tried to move the victim but was shocked. The assistant manager
was dead on arrival at a local hospital.
An investigation showed that the mixing motor was in poor condition.
The grounding pin had been removed from the male end of the power
cord, resulting in a faulty ground. The circuit was equipped with
a GFCI, but it was not installed properly. A properly wired and
functioning GFCI could have sensed the ground fault in the motor
and de-energized the circuit.
Take a look at what could have been done to prevent this death.
- The
employer should have kept the motor in better condition. Power
cords should be inspected regularly, and any missing prongs
should be replaced.
- All
pool-area electrical circuits should be installed by qualified
electricians.
- The
victim should have worn insulating boots or shoes since she
was handling electrical equipment.
- The
employer should have followed the law. The NEC requires that
all pool-associated motors have a permanent grounding system.
In this case, this regulation was not followed. Also, electrical
equipment is not permitted in areas without proper drainage.
- OSHA
requires employers to provide a work environment free of safety
and health hazards.
|
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, outdoor areas, crawl spaces,
unfinished basements, kitchens, and near wet bars.
 |
| Install
bonding jumpers around nonconductive material. |
| Use GFCI’s
to help protect people in damp areas. |
| bonding—joining
electrical parts to assure a conductive 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.
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.
 |
| Use overcurrent
protection devices (circuit breakers or fuses) in circuits. |
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 environ-ments. 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.
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.
|
Find
the cause of an overload
|
 |
|
Only
circuit breakers marked “SWD” should be used as switches.
|
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.
