safety basics tm ©2004 cooper bussmann b ussmann ® a wareness of s afety i ssues c ampaign...

Safety BASICsTM

Bussmann® Awareness of Safety Issues Campaign

Electrical Hazards

What are the hazards as you approach

electrical equipment to perform work?

Electrical Hazards

• Shock• Arc flash

– Heat– Fire

• Arc blast– Pressure– Shrapnel– Sound

Example of an arcing fault

I = V / Z

• What happens with shock?

• What happens when there is a fault?

• What is the difference between a short-circuit and an arcing fault?

Basic Electrical Theory

Electric Shock

• Over 30,000 non-fatal electrical shock accidents occur each year

• Over 600 people die from electrocution each year

• Electrocution remains the fourth (4th) highest cause of industrial fatalities

• Most injuries and deaths could be avoided

Human Resistance Values

Resistance (ohms)

Condition Dry Wet

Finger touch 40,000 to 1,000,000 4,000 to 15,000

Hand holding wire 15,000 to 50,000 3,000 to 6,000

Finger-thumb grasp 10,000 to 30,000 2,000 to 5,000

Hand holding pliers 5,000 to 10,000 1,000 to 3,000

Palm touch 3,000 to 8,000 1,000 to 2,000

Hand around 1-1/2 inch pipe 1,000 to 3,000 500 to 1,500

Two hands around 1-1/2 inch pipe 500 to 1,500 250 to 750

Hand immersed 200 to 500

Foot immersed 100 to 300

Human body, internal, excluding skin

200 to 1,000

This table was compiled from data developed by Kouwenhoven and Milnor.

Electric Shock

Human body resistance (hand to hand) across the body is about 1000

Ohms law: I = V / R amps = 480 volts / 1000

= 0.48 amps (480 mA)

Product safety standards consider 5 mA to be the safe upper limit for children and adults

Electric Shock

mA Affect on person 0.5 - 3 - Tingling sensations 3 - 10 - Muscle contractions and pain 10 - 40 - “Let-go” threshold 30 - 75 - Respiratory paralysis100 - 200 - Ventricular fibrillation200 - 500 - Heart clamps tight 1500 + - Tissue and organs start to

Electric Current Pathways

(A) Touch Potential (B) Step Potential (C and D) Touch / Step Potential

Current passing through the heart and lungs is the most serious

Electric Shock Injury

Arc Flash

As much as 80% of all electrical injuries are burns resulting from an arc-flash and ignition of flammable clothing

Arc temperature can reach 35,000°F - this is four times hotter than the surface of the sun

Fatal burns can occur at distances over 10 ft

Over 2000 people are admitted into burn centers each year with severe electrical burns

Arc Blast

An arc fault develops a “pressure wave”Sources of this blast include:

– Copper expands 67,000 times its original volume when vaporized

– Heat from the arc, causes air to expand, in the same way that thunder is created from a lightning strike

This may result in a violent explosion of circuit components and thrown shrapnel

The blast can destroy structures, knock workers from ladders, or across the room

BoltedShort Circuit

Arcing Fault

Current Thru Air

Electric Arc

Copper Vapor:Solid to VaporExpands by67,000 times

Intense Light

Hot Air-Rapid Expansion

35,000 °F

Pressure Waves

Sound Waves

Molten Metal

Shrapnel

Personnel Hazards Associated With Arc Flash & Arc Blast

• Heat – burns & ignition of material– Arc temperature of 35,000oF– Molten metal, copper vapor, heated air

• Second degree burn threshold:– 80oC / 175oF (0.1 sec), 2nd degree burn

• Third degree burn threshold:– 96oC / 205oF (0.1 sec), 3rd degree burn

• Intense light– Eye damage, cataracts

• Pressures from expansion of metals & air

• Eardrum rupture threshold: – 720 lbs/ft2

• Lung damage threshold:– 1728 - 2160 lbs/ft2

• Shrapnel • Flung across room or from

ladder/bucket

Personnel Hazards Associated With Arc Flash & Arc Blast

Flash protection boundaries and incident energy exposure calculations both dependent upon:

Duration of arc-fault or time to clear• Speed of the overcurrent protective device

Arc-fault current magnitude• Available fault current

• Current-limitation can reduce

Overcurrent Protection Role

IEEE / PCIC & NFPA 70E

• Ad Hoc Safety Subcommittee– Users– Consultants– Manufacturers– Medical experts

• Following are some of the tests run– All of the devices used for this testing were

applied according to their listed ratings

IEEE / PCIC Staged Arc Flash Test Set-up

22.6 KA SymmetricalAvailable Fault Current

@ 480V, 3 Phase

Fault Initiated on Line Side of 30AFuse

30A RK-1Current Limiting Fuse

Size 1 Starter

Test No. 46 cycle STD

640A OCPDNon Current Limitingwith Short Time Delay

Set @ 6 cycle opening

Test 4 Still Photo

>225oC/437oF

Results: Test No.4

141.5 db @ 2 ft.

50oC/122oF

>2160 lbs/ft2

> Indicates Meter Pegged

@ 480V, 3 Phase

Fault Initiated on Line Side of 30AFuse

Size 1 Starter

Test No. 3

601A.Class LCurrent Limiting Fuse

Test 3 Still Photo

> 175oC/347oF

Results: Test No.3

133 db @ 2 ft.

62oC/143.6oF

504 lbs/ft2

T3(No Change

From Ambient)

> Indicates Meter Pegged

@ 480V, 3 Phase

Size 1 Starter

Test No. 1

601A.Class LCurrent LimitingFuse

Fault Initiated on Load Side of 30AFuse

Test 1 Still Photo

Results: Test No.1

Sound(No Change From Ambient)

(No Change From Ambient)

Non-Current Limiting

Reduced Fault Current

via Current-Limitation

Test 1

Test 4

Test 3

Current-Limitation: Arc Energy Reduction

Summary

• Shock, arc flash and arc blast are the three recognized electrical hazards

• Shock injuries result from electrical current flowing through the body

• Arcing faults can generate enormous amounts of energy

• Injuries from arcing faults are a result of the tremendous heat and pressure generated

Summary

• Overcurrent protective devices have an impact on the two most important variables of arc flash hazards:– Time (speed of the OCPD)– Fault current magnitude (current-limitation

may help reduce)• Current-limitation may be able to

significantly reduce the energy released during arcing faults

safety basics tm ©2004 cooper bussmann b ussmann ® a wareness of s afety i ssues c ampaign...

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