safety basics tm ©2004 cooper bussmann b ussmann ® a wareness of s afety i ssues c ampaign...
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Safety BASICsTM
©2004 Cooper Bussmann©2004 Cooper Bussmann
Bussmann® Awareness of Safety Issues Campaign
Electrical Hazards
©2004 Cooper Bussmann
Electrical Hazards
What are the hazards as you approach
electrical equipment to perform work?
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Electrical Hazards
• Shock• Arc flash
– Heat– Fire
• Arc blast– Pressure– Shrapnel– Sound
Example of an arcing fault
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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
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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
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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.
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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
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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
burn
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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
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Electric Shock Injury
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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
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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
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BoltedShort Circuit
A B
Arcing Fault
A B
Current Thru Air
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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
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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
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• 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
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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
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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
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IEEE / PCIC Staged Arc Flash Test Set-up
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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
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Test 4 Still Photo
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Test 4 Still Photo
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Test 4 Still Photo
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Test 4 Still Photo
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Test 4 Still Photo
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Test 4 Still Photo
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Test 4 Still Photo
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>225oC/437oF
>225oC/437oF
Results: Test No.4
T1
T2
P1
T3
Sound
141.5 db @ 2 ft.
50oC/122oF
>2160 lbs/ft2
> Indicates Meter Pegged
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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. 3
601A.Class LCurrent Limiting Fuse
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Test 3 Still Photo
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Test 3 Still Photo
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Test 3 Still Photo
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Test 3 Still Photo
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> 175oC/347oF
Results: Test No.3
T1
T2
P1
Sound
133 db @ 2 ft.
62oC/143.6oF
504 lbs/ft2
T3(No Change
From Ambient)
> Indicates Meter Pegged
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22.6 KA SymmetricalAvailable Fault Current
@ 480V, 3 Phase
30A RK-1Current Limiting Fuse
Size 1 Starter
Test No. 1
601A.Class LCurrent LimitingFuse
Fault Initiated on Load Side of 30AFuse
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Test 1 Still Photo
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Test 1 Still Photo
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Test 1 Still Photo
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Test 1 Still Photo
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Results: Test No.1
T1
T2
P1
T3
Sound(No Change From Ambient)
(No Change From Ambient)
(No Change From Ambient)
(No Change From Ambient)
(No Change From Ambient)
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Non-Current Limiting
Reduced Fault Current
via Current-Limitation
Test 1
Test 4
Test 3
Current-Limitation: Arc Energy Reduction
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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
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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