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


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.


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.


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.


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.


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.


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


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.


Electrical Hazards

I. ShockII. BurnsIII. Falls

IV. FireV. Explosions


ELECTRIC SHOCK

• Electric shock occurs when the human body becomes part of the path through which current flows.

• The direct result can be electrocution.


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.


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 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)


Falls

• Involuntary muscle contractions can “throw” workers and cause falls.

• If working at elevation, the fall may cause serious injury or death.


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.


EXPLOSIONS

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


1. Grounding

2. Fuses and Circuit Breakers

3. Ground-Fault-Circuit-Interrupters

4. PPEs

5. Insulated Tools

6. Signs and Tags

Electrical Hazard Control


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

http://www.philmore-datak.com/images/8515P.jpg


Electrical System Grounding

• One conductor of the circuit is

intentionally grounded to earth

• Protects circuit from lightning, or other

high voltage contact


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


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.


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.


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.


• Live parts of electric equipment operating at

50 volts or more guarded against accidental

contact by approved cabinets

Guarding of live parts


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)


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.


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


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


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


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 Lecture 6 Electrical Safety Fall 2012/2013.

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Transcript of HU151 Lecture 6 Electrical Safety Fall 2012/2013.