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HU151 Lecture 6. Electrical Safety Fall 2012/2013. Outline. Classification of Exposure Electrical Hazards Electrical Hazard Control. Classification of Exposure. High Voltage >600 volts: typically associated with “outdoor” electrical transmission.

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HU151 Lecture 6

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Hu151 lecture 6

HU151Lecture 6

Electrical Safety

Fall 2012/2013


Outline

Outline

Classification of Exposure

Electrical Hazards

Electrical Hazard Control

HU151_ Lect7_ Electrical


Classification of exposure

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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Hu151 lecture 6

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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Hu151 lecture 6

How Human Nerve Cells Transmit Signals.

Dendrites

Synapse

Axon

Na+

Na+

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.

K+

Na+

K+

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Hu151 lecture 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.

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Conductivity of human body

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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Hu151 lecture 6

Effects of Mains Derived Current

on the Human Body.

As current increases the effects

get more severe.

Tingly feeling

Perception

Can not let go Current

Interruption of Normal Cardiac Function

Stimulates muscles

And you cant let go

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Hu151 lecture 6

Effects of Mains Derived Current

on the Human Body.

Ventricular Fibrillation:I > 50 mA

Can’t let go:I > 5 mA

Tingling SensationI > 0.5 mA

For comparison two 60 Watt light bulbs draw a total of 1 ampere of current.

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Electrical hazards

Electrical Hazards

Shock

Burns

Falls

Fire

Explosions

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Electric shock

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

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:

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

Arc Flash and Arc Blasts

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Arc blast

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

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 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

  • Explosions occur when electricity provides a source of ignition for an explosive mixture in the atmosphere.

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Hu151 lecture 6

Electrical Hazard Control

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Electrical hazard control

Electrical Hazard Control

  • Grounding

  • Fuses and Circuit Breakers

  • Ground-Fault-Circuit-Interrupters

  • PPEs

  • Insulated Tools

  • Signs and Tags

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Grounding

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

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

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

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Fuses and circuit breakers

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

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

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

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

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 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

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

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

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

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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Hu151 lecture 6

End

HU151_ Lect7_ Electrical


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