Transcript Electrical Safety

HU151
Lecture 6
Electrical Safety
Fall 2012/2013
Outline
• Classification of Exposure
• Electrical Hazards
• Electrical Hazard Control
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Classification of Exposure
High Voltage
• >600 volts: typically associated with “outdoor”
electrical transmission.
Accounts for 60% of electrocutions (OSHA).
Low Voltage:
• <600 volts: typically associated with “indoor”
electrical service.
Accounts for 32% of electrocutions (OSHA).
Low voltage does not imply safe voltage.
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How Human Nerve Cells Transmit Signals.
Dendrites
Synapse
Axon
Na+
The Axon maintains a chemical
balance with more potassium ions
inside the cell and sodiom ions
outside the cell.
K+
Na+
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How Human Nerve Cells Transmit Signals.
Dendrites
Synapse
Axon
When signal is transmitted the myelin
sheet changes so that the sodium
and potassium ions change places.
This results in an electrical change in
the cell and this in turn causes the
next section of myelin to change.
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Na+
Na+
K+
K+
Na+
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External Electrical Stimulation of Human Nerve Cells
Electricity flowing through the human body
can cause enough of a change in
the electrical environment around a nerve cell to stimulate it.
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Conductivity of Human Body
• Electric current seeks the path of least
resistance to the ground.
• Human tissues and body fluids are
relatively good conductors because of
high aqueous-electrolyte content.
• If a person touches an energized bare
wire or faulty equipment, electricity will
instantly pass through the body to the
ground, causing potentially fatal shock.
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Effects of Mains Derived Current
on the Human Body.
As current increases the effects
get more severe.
Tingly feeling
Perception
Stimulates muscles
And you cant let go
Can not let go Current
Interruption of Normal Cardiac Function
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Effects of Mains Derived Current
on the Human Body.
Ventricular Fibrillation:
I > 50
mA
Can’t let go:
I> 5
mA
Tingling Sensation
I > 0.5 mA
For comparison two 60 Watt light bulbs draw a total of 1 ampere of current.
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Electrical Hazards
I. Shock
II. Burns
III. Falls
IV. Fire
V. Explosions
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ELECTRIC SHOCK
• Electric shock occurs when the human
body becomes part of the path through
which current flows.
• The direct result can be electrocution.
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Electrical Burns
• Most common nonfatal electrical injury.
• Types:
– Internal: “deep tissue”.
– Skin: “entry” and “exit” points.
– Arc: “flash” burns from heat and radiant
energy.
• Common sites of visible skin burns are the
hands and feet.
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Arc Flash and Arc Blasts
• Arc Flash:
• Arc Blast:
– 80%-Burns due to
ignition of clothing
– Temperature-35,000 F
– Fatal Burns-10 ft.
– Molten metal
–
–
–
–
Pressure Wave
Heat
Molten metal
Destruction of
structures and life
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Arc Blast
• Cause
– Short Circuit caused by working on
energized equipment (Dropped Tool)
– Occurs in milliseconds
– Temp: 30,000 degrees
– Air expands very violently (Excessive
pressure)
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Falls
• Involuntary muscle
contractions can
“throw” workers and
cause falls.
• If working at
elevation, the fall
may cause serious
injury or death.
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Wiring Fires
• Wiring often fails due to:
–
–
–
–
–
faulty installation,
overloading,
physical damage,
aging and
deterioration by chemical action, heat, moisture and
weather.
• Such wiring should be replaced and new circuits
installed.
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EXPLOSIONS
• Explosions occur when electricity provides
a source of ignition for an explosive
mixture in the atmosphere.
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Electrical Hazard Control
1.
2.
3.
4.
5.
6.
Grounding
Fuses and Circuit Breakers
Ground-Fault-Circuit-Interrupters
PPEs
Insulated Tools
Signs and Tags
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Grounding
• Grounding is a method of protecting from
electric shock.
• It offers low resistance path that has sufficient
current-carrying capacity to prevent the buildup of hazardous voltages.
• Two Types
– System Grounding
– Equipment Grounding
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Electrical System Grounding
• One conductor of the circuit is
intentionally grounded to earth
• Protects circuit from lightning, or other
high voltage contact
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Equipment Grounding
• All metal frames & enclosures of equipment are
grounded by a permanent connection or bond
• The equipment grounding conductor provides a
path for dangerous fault current to return to the
system ground at the supply source should a
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fault occur
Fuses and Circuit Breakers
– Each circuit must be protected by a fuse
or circuit breaker that will blow or “trip”
when its safe carrying capacity is
surpassed.
– If a fuse blows or circuit breaker trips
repeatedly while in normal use (not
overloaded), check for shorts and
other faults in the line or devices.
– Do not resume use until the trouble is
fixed.
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Ground-Fault Circuit-Interrupters
(GFCI’s)
• GFCI’s are designed to detect any
leakage of current in an electrical
circuit.
• GFCI’s turn off or “trip” the circuit
whenever the leakage is greater than
5 mA.
• For comparison two 60 Watt light
bulbs draw a total of 1 ampere of
current.
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Types of GFCI’s
1. A GFCI receptacle used in place of
standard receptacle.
2. A portable GFCI plugs into a standard
receptacle.
3. A GFCI circuit breaker combines leakage
current detection with the function of a
circuit breaker.
• Whenever working in a wet area, or
outdoors, employees should use one of
these types of GFCI’s.
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Guarding of live parts
• Live parts of electric equipment operating at
50 volts or more guarded against accidental
contact by approved cabinets
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Extension Chords
• Use extension cords only
when necessary and make
sure they are heavy
enough for the job.
• Avoid creating an “octopus”
by inserting several plugs
into a multi-plug outlet
connected to a single wall
outlet. (CDC)
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Extension Chords (2)
• Extension cords should only
be used on a temporary basis
in situations where fixed wiring
is not feasible.
• If it is necessary to use an
extension cord, never run it
across walkways or aisles.
– It causes a potential tripping
hazard.
– It wears down the insulation.
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Electrical Protective Equipment
• Employees working in areas where there are
potential electrical hazards shall be provided
with, and shall use, electrical protective
equipment that is appropriate for the specific
parts of the body to be protected and for the
work to be performed
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Insulated Tools
• When working near exposed energized
conductors or circuit parts, each employee
shall use insulated tools or handling equipment
if the tools or handling equipment might make
contact with such conductors or parts
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Safeguards for personnel
protection
• The following alerting techniques shall be used
to warn and protect employees from hazards
which could cause injury due to electric shock,
burns, or failure of electric equipment parts:
– Safety signs and tags
– Barricades
– Attendants
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Lockout/Tagout
 Your personal lock and
personal danger tag is what
protects you from systems
being re-energized while you
are working on them.
 You are the only person
authorized to remove them
except under specially
controlled conditions.
 If you don’t install them,
you are not protected!
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