3.electrical safety.10

8/3/2019 3.Electrical Safety.10

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GeneralElectrical

Safety

The OSHA e-tool electrical safety presentation was used to create this presentation.


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Objectives

Describe how electricityworks.

Describe how shocks occur.

Describe how electricalcurrent affects the body.

Describe the most commonways individuals are injured

using electricity. Provide solutions to avoid

being injured while usingelectricity.


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Terms

amperes (amps, A)

cardiopulmonary

resuscitation (CPR)

copper circuit

conductivity

current

electrons energy

extension cord

ground circuit

ground fault circuit

interrupter (GFCI)

non-conductive materials

ohm Occupational Health and

Safety Administration

(OSHA)

reciprocal resistance

silver

voltage (volts, V)

watts (W)


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Definition: Electricity

Electricity is a form of energy due to a flowing

stream of electrons.

Electricity works because electric charges push and

pull on each other. There are two types of electric charges, positive charge

and negative charge.

Like charges repel.

This means that if you put two negative charges close

together and let them go, they would move apart.

http://science.howstuffworks.com/electricity.htm/printable Retrieved 8/28/09
http://science.howstuffworks.com/electricity.htm/printablehttp://science.howstuffworks.com/electricity.htm/printable


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Electrons

Electrons (negative charge) circulate in 3-dimensional ringsaround the nucleus (which includes positive-charged protons

and neutral-charged neutrons) of an atom.


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How Electricity Works

Example: A garden hose

Water moves fromhigh pressure

To low pressure

A similar thing occurs in an electrical wire

Current moves from high voltage to low voltage

Flow of Current

e e ee


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Terms

Voltage

Current

Circuit Resistance

Conductivity

Watts


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Terms: Voltage and current

Voltage is the rate at which energy is drawn froma source that produces a flow of electricity in a

circuit.

Voltage is the electrical pressure or potential in acircuit.

Voltage is expressed in volts (V).

The measure of electrical current in a circuit is in

amperes (amps).

Current is essentially how fast electrons are moving in a

circuit.

e e e e e e


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Term: Circuit

A circuit is an electrical

device that provides apath for electrical current

to flow.

A circuit must be completefor current to flow.


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Term: Electrical resistance

Electrical resistance is theopposition of a specificmaterial to the flow of

electric current. Resistance impedes the

electrical current.

Resistance is the result offriction.

Resistance is measured inohms.


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There is an equation for the

relationship between voltage, current,

and resistance.

V = IR (This [Ohms law] actually applies to directcurrent, but we will use it here to do somebasic problems.)

V = voltageI = current (amps)R = resistance (ohms)


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Solve for Voltage

V = IR

What is the voltage if there are 50 amps and

the resistance is 20 ohms? 50 amps x 20 ohms = 100 volts.

What is the voltage if there are 100 amps andthe resistance is 3 ohms?

100 amps x 3 ohms = 300 volts.


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Solve for I (amps) or R (resistance)

Voltage is 120 volts and resistance is 5 ohms.What are the amps.

I = V/R or V divided by R = I.

120 volts divided by 5 ohms = 24 amps.

Voltage is 210 volts and there are 21 amps ofcurrent. What is the resistance?

R = V/I or V divided by I = R. 210 volts divided by 21 amps = 10 ohms

resistance.


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Conductors

Good conductors have lowresistance.

Examples of good conductors aresilver and copper.

Water can be a conductor.

Water itself or water-containingcompartment as human cells,which are 70% water canconduct electricity.

This is a key aspect of electricalsafety.


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Another formula for electricity

Conductance is thereciprocal of resistance.

Conductance = Resistance

Electrical conductivity is ameasure of a material's

ability to conduct an electriccurrent.

1____

Note: The greater the resistance, the less the conductance, and vice versa.This should make intuitive sense, resistance and conductance being opposite

ways to denote the same essential electrical property.


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Watt

The watt (W) is a derived unit of power in the

International System of Units (SI).

A watt measures rate of energy conversion.

One watt is equivalent to 1 joule (J) or energy persecond.

By the definitions of electric potential (V) and

current (amp), work is done at a rate of one wattwhen one ampere flows through a potential

difference of one volt. [1W = 1V 1 Amp]


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Checking for an overload on a circuit

Add the individual power rating on the circuit

and divide by the voltage of the circuit.

If the sum exceeds the allowed current

(amperage), the line is overloaded.

Normal voltage in the U.S. on most electrical

systems is 120V.

An expected amperage might be 12 amps.


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Application

Calculate for the following: Flame photometer uses 400 W

Blood gas analyzer uses 300 W

Osometer uses 250 W

Chemistry analyzer uses 550 W TOTAL 1500 W

1W = 1V 1 Amp

1500 W = 120V x required amps

1500 divided by 120 = 12.5 amps

If the circuit only carries 12 amps, the circuit is overloaded at12.5 amps.


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You try one!

Calculate for the following:

Spectrophotometer uses 400 W

pH meteruses 300 W

Small water bath uses 130 W

Thermocycler uses 400 W

TOTAL 1230 W

1W = 1V 1 Amp

1230 W = 120V x required amps

1230 divided by 120 = __?__ amps

If the circuit carries 12 amps, is the circuit is overloaded?


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Answer

1230 divided by 120 = 10.25 amps

If the circuit carries 12 amps, is the circuit isoverloaded?

No, the circuit could handle this load.


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Role and advantage of circuit

breakers A circuit breaker is an

automatically-operated electrical

switch designed to protect an

electrical circuit from damage

caused by overload or short circuit. The basic function is to detect a

fault condition and, by interrupting

continuity, to immediately

discontinue electrical flow. When the overload or short circuit

situation is resolved, the circuit

breaker can be reset (flipped) to

restore normal function.

A


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Prevention of accidents


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Electrical Shocks

Electricity travels inclosed circuits through aconductor.

Shock results when thebody becomes part of theelectrical circuit.

Current enters the bodyat one point and leaves atanother.


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How Electrical Current Affects the Body

Current

(Amps)

Human Reaction

0.001 (1mA) Perception level. Just a faint tingle.

0.005 Slight shock felt; not painful but disturbing.Average individual can let go.

0.006-0.025(Women)

Painful shock, muscular control is lost.

0.009-0.030(Men)

This is called the freezing current or "let-go"range.

0.050-0.150 Extreme pain, respiratory arrest, severe muscularcontractions.

1 - 4.3 Ventricular fibrillation.

10 Cardiac arrest, severe burns and probabledeath.


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Involuntary muscle reaction

Muscles violently contract whenstimulated by excessive amounts ofelectricity.

These involuntary contractions can

damage muscles, tendons, andligament, and may even cause brokenbones.

If a victim is holding an electrocutingobject, hand muscles may contract

making it impossible to drop theobject.

Injury or death may result from a falldue to muscle contractions.


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Burns

The most common shock-related injury isa burn.Burns suffered in electrical incidents may

be one or more of the following three

types: electrical burns flash burns thermal contact burns.


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Electrical burns

Electrical burns cause

tissue damage, and are

the result of heat

generated by the flowof electric current

through the body.

Electrical burns are

serious injuries.

Immediate medical

attention is required.

Entrance wound

Exit wound


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Flash Burns

High temperaturesnear the bodyproduced by anelectric arc orexplosion cause arcor flash burns (alsoneed prompt medicalattention).

Image from anexplosion of aswitchbox.

http://www.osha.gov/SLTC/etools/construction/electrical_incidents/burn_examples.html#arc_burns Retreived 8/28/09
http://www.osha.gov/SLTC/etools/construction/electrical_incidents/burn_examples.htmlhttp://www.osha.gov/SLTC/etools/construction/electrical_incidents/burn_examples.html


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Thermal Contact Burns

Thermal contact

burns occur when

skin comes in

contact withoverheated

electric

equipment, or

when clothing isignited in an

electrical incident.



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Internal injuries

Our bodies use small electricalcurrents to transmit signals through

the nervous system and to contract

muscles.

Extra electrical current flowing

through the body can cause seriousdamage.

Medical problems can include internal

bleeding, tissue destruction, and

nerve or muscle damage. Internal injuries may not be

immediately apparent to the victim

or observers: however, left untreated,

the injuries can result in death.

Day 1

Surgery after several daysrevealed internal injuries.



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First aid for an

electrical burn

victim Electrical burns vary in severity depending upon:

how long the body was in contact with the electric current;

the strength of the current; the type of current;

the direction the current takes though the body.

Most severe burns are painless, due to the damaged

nerves.

Often these burns are deep.

Electrical burn wounds may look minor on the outside,

but could be severe on the inside.Retrieved 8/6/2010 from http://www.megavolt.co.il/Tips_and_info/first_aid.html
http://www.megavolt.co.il/Tips_and_info/first_aid.htmlhttp://www.megavolt.co.il/Tips_and_info/first_aid.html


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First aid for an

electrical fire

victim If the person is conscious and there are no signs of shock

(such as pale and clammy skin, weakness, bluish lips and

fingernails, having a rapid pulse, and decreasing alertness),begin treating the burned area.

Do not touch burns, break blisters, or remove burned

clothing.

Do not apply grease or oil to the burn. Cover the burn with a dry, sterile dressing.

Continue to keep the victim from getting chilled.

Seek medical help as soon as possible.Retrieved 8/6/2010 from http://www.megavolt.co.il/Tips_and


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Shock protection: Ground circuits

Ground circuits provide a path forstray current to pass directly to theground.

Ground circuits greatly reduce theamount of current passing throughthe body of a person in contact witha tool or machine that has anelectrical short.

Properly installed, the groundingconductor provides protection fromelectric shock.


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Ground Faults

Ground faults occur when

current flowing to the load (a

tool or appliance) does not

return by the prescribed route. A simple 120 V circuit, current

travels through the black

(ungrounded) wire to the loadand returns through the white

(grounded) wire.


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Ground fault protection

Ground-fault circuit interrupter (GFCI)works by comparing the amount ofcurrent going to and returning from theelectrical equipment along the

conductors. When the current different by 5 milliamps

(mA), the GFCI interrupts the currentwithin 1/40 second.

Building codes require GFCI in specificareas, especially bathrooms, kitchens, orother areas likely to be wet.


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Portable GFCI

Portable GFCI should be used on all120 V, single phase, 15 and 20 amperereceptacles for temporary use of

power tools. Plug end (lower in this image) directly

into power source, not into anextension cord.

Follow manufacturersrecommendations.


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Ground faults

A persons body can act as a pathto ground when a fault occurs.

Dangers include frayed wires and

faulty electrical equipment.

Moisture increases the dangerbecause of the conductivity ofwater.

A person working in a wetlocation was fatally electrocuteddue to the lack of insulation onthis wire.

http://www.osha.gov/SLTC/etools/construction/electrical_incidents/burn_examples.html#arc_burnsRetreived 8/28/09


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Power strips and

surge protectors Multioutlet surge protection can be used to

supply power to equipment that needs surgeprotection, but not used to provide more outletsdue to the lack of permanent wiring.

These types of extension cords are inappropriatefor laboratory equipment. Cords that are not 3-wire-types.

Cords that have been modified.

Use of inappropriate extension cords arecommon Occupational Health and SafetyAdministration (OSHA) violations.


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Avoiding hazards

Ground all power supplysystems, electrical circuits,and electrical equipment.

Do not remove groundpins/prongs from cord- andplug-connected equipmentor extension cords.

Use double-insulated tools. Ground all exposed metal

parts of equipment.
http://www.osha.gov/SLTC/etools/construction/images/electrical_panel.jpg


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Improper use of flexible cords

These cords are

improperly wired directlyto the electrical circuit,

are not protected by aGFCI, and

are two-wire cords that

are not grounded and notrated for hard- or extra-hard service.
http://www.osha.gov/SLTC/etools/construction/images/temp_wiring01.jpg


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Flexible Cord Safe Practice

Visually inspect all electrical equipment beforeuse.

Remove any equipment with frayed cords, missingground prongs, cracked tool casings, etc. from

service. Apply a warning tag to any defective tool and do

not use it until it has been properly repaired. Remove cord by pulling on plug, not on the cord.
http://www.osha.gov/SLTC/etools/construction/images/doublestrainrelief.jpghttp://www.osha.gov/SLTC/etools/construction/images/electrical_panel.jpg


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Flexible Cord Safe Practices

Only use factory-assembled cordsets.

Use only extension cords thathave a ground wire (3-wire type).

Use only cords, connectiondevices, and fittings that areequipped with strain relief.

Protect flexible cords fromdamage.

Secure cords to prevent injuries.


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Power lines

Avoid contact. Maintain a 10 ft. distance froman energized power line.

Use non-conductive

(fiberglass) ladders and othertools.

Locate power lines

Prior to digging, call 1-800-

662-4111. Have power lines de-energized

and grounded before workbegins.


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Humans and electrical

conductivity

Water is a conductor of electrical current.

Human skin is relatively resistant to current in dryconditions.

Volts/ohms equals current. (An ohm is a unit of electricalresistance equal to the resistance between two points on aconductor when a potential difference of one volt betweenthem produces a current of one ampere

120/100,000 ohms = 1 mA.

Barely perceptible current When skin is wet, the resistance to current drops

dramatically. 120/1000 ohms = 120 mA.

Sufficient current to cause ventricular fibrillation.

Low voltage electrocution is more likely in wet conditions.


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First aid for an

electrical shockvictim

DO NOT Touch the victim if power (current) is still on.

Step in electrified water.

Do not move person after removed from current, especially if

head or neck injury is possible Get near a person being electrocuted by high voltage current.

Current can arc as far a 7 meters.

IN THIS CASE DO CALL FOR IMMEDIATE HELP.

Retrieved 8/6/2010 from http://www.megavolt.co.il/Tips_and_info/first_aid.html


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First aid for an

electrical shock

victim

DO Turn off power if possible.

Use a non-conductive material (wooden pole) to push the

current away or dry ropes to pull person free of contact

with current.

Administer CPR if the person is not breathing.

Stay until help arrives.

Retrieved 8/6/2010 from http://www.megavolt.co.il/Tips_and_info/first_aid.html


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Fire extinguisher for anelectrical fire

Many fire extinguishers are a

combination of types A, B, and

C suitable for fires caused by

Wood, paper, trash (A)

Flammable liquids (B)

Electrical equipment fires (C).

Fire extinguishers available in

Southern Miss teaching areas

are rated for A, B, and C fires.


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Laboratory Safety Manual

Page 9 of the Manualincludes safety rules for

the Department of medical Technology at the

University of Southern Mississippi


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OSHA e-Tool

OSHAs e-tool can be viewed online at:

www.osha.gov/SLTC/etools/construction/electrical_incidents/mainpage.html
http://www.osha.gov/SLTC/etools/construction/electrical_incidents/mainpage.htmlhttp://www.osha.gov/SLTC/etools/construction/electrical_incidents/mainpage.htmlhttp://www.osha.gov/SLTC/etools/construction/electrical_incidents/mainpage.htmlhttp://www.osha.gov/SLTC/etools/construction/electrical_incidents/mainpage.html

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