HU151 Lecture 6 Electrical Safety Fall 2012/2013.
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Transcript of HU151 Lecture 6 Electrical Safety Fall 2012/2013.
HU151Lecture 6
Electrical Safety
Fall 2012/2013
HU151_ Lect7_ Electrical 2
Outline• Classification of Exposure
• Electrical Hazards
• Electrical Hazard Control
HU151_ Lect7_ Electrical 3
Classification of ExposureHigh 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.
HU151_ Lect7_ Electrical 4
How Human Nerve Cells Transmit Signals.
DendritesAxon
Synapse
K+
Na+
Na+
The Axon maintains a chemicalbalance with more potassium ionsinside the cell and sodiom ionsoutside the cell.
HU151_ Lect7_ Electrical 5
How Human Nerve Cells Transmit Signals.
DendritesAxon
Synapse
K+
Na+
Na+Na+
K+
When signal is transmitted the myelinsheet changes so that the sodiumand potassium ions change places.
This results in an electrical change inthe cell and this in turn causes thenext section of myelin to change.
HU151_ Lect7_ Electrical 6
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.
HU151_ Lect7_ Electrical 7
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.
HU151_ Lect7_ Electrical 8
Effects of Mains Derived Current on the Human Body.
Perception
Can not let go Current
Interruption of Normal Cardiac Function
As current increases the effectsget more severe. Tingly feeling
Stimulates musclesAnd you cant let go
HU151_ Lect7_ Electrical 9
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.
HU151_ Lect7_ Electrical 10
Electrical Hazards
I. ShockII. BurnsIII. Falls
IV. FireV. 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.
HU151_ Lect7_ Electrical 12
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.
HU151_ Lect7_ Electrical 13
Arc Flash and Arc Blasts
• Arc Flash:– 80%-Burns due to
ignition of clothing– Temperature-35,000 F– Fatal Burns-10 ft.– Molten metal
• Arc Blast:– Pressure Wave– Heat – Molten metal– Destruction of
structures and life
HU151_ Lect7_ Electrical 14
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)
HU151_ Lect7_ Electrical 15
Falls
• Involuntary muscle contractions can “throw” workers and cause falls.
• If working at elevation, the fall may cause serious injury or death.
HU151_ Lect7_ Electrical 16
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.
HU151_ Lect7_ Electrical 17
EXPLOSIONS
• Explosions occur when electricity provides a source of ignition for an explosive mixture in the atmosphere.
HU151_ Lect7_ Electrical 18
HU151_ Lect7_ Electrical 19
1. Grounding
2. Fuses and Circuit Breakers
3. Ground-Fault-Circuit-Interrupters
4. PPEs
5. Insulated Tools
6. Signs and Tags
Electrical Hazard Control
HU151_ Lect7_ Electrical 20
Grounding
• Grounding is a method of protecting from electric shock.
• It offers low resistance path that has sufficient
current-carrying capacity to prevent the build-up 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
fault occur
HU151_ Lect7_ Electrical 23
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.
HU151_ Lect7_ Electrical 24
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.
HU151_ Lect7_ Electrical 25
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.
HU151_ Lect7_ Electrical 26
• Live parts of electric equipment operating at
50 volts or more guarded against accidental
contact by approved cabinets
Guarding of live parts
HU151_ Lect7_ Electrical 27
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)
HU151_ Lect7_ Electrical 28
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.
HU151_ Lect7_ Electrical 29
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
HU151_ Lect7_ Electrical 30
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
HU151_ Lect7_ Electrical 31
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
HU151_ Lect7_ Electrical 32
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!
HU151_ Lect7_ Electrical 33