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Busbar impedance calculation
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SafetyBASICsTM
2004 Cooper Bussmann2004 Cooper Bussmann
BussmannAwarenessof Safety IssuesCampaign
Electrical Hazards
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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 electrocutioneach 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 500Foot 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 W
Ohms law: I = V / R amps= 480 volts / 1000 W= 0.48 amps (480 mA)
Product safety standardsconsider 5 mAto be the safe upper limit for childrenand adults
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Electric Shock
mA Affect on person
0.5 - 3 - Tingling sensations
3 - 10 - Muscle contractions and pain10 - 40 - Let-go threshold
30 - 75 - Respiratory paralysis
100 - 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 PotentialCurrent pass ing thro ugh the hear t and lungs is the most ser ious
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Electric Shock Injury
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Arc Flash
As much as 80% of all electrical injuries are
burnsresulting from an arc-flash and ignition
of flammable clothing
Arc temperature can reach 35,000F - 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 timesits original volume
when vaporized Heat from the arc, causes air to expand, in the
same way that thunder is created from a lightningstrike
This may result in a violent explosion of circuitcomponents and thrown shrapnel
The blast can destroy structures, knock workersfrom ladders, or across the room
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BoltedShort Circuit
A B
ArcingFault
A B
Current
Thru Air
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Electric Arc
Copper Vapor:Solid to Vapor
Expands 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
Heatburns & ignition of material
Arc temperature of 35,000oF
Molten metal, copper vapor, heated air
Second degree burn threshold:
80oC / 175
oF (0.1 sec), 2nddegree burn
Third degree burn threshold:
96oC / 205oF (0.1 sec), 3rddegree 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 fromladder/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 runAll 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 Symmetrical
Available Fault Current
@ 480V, 3 Phase
Fault Initiated on
Line Side of 30A
Fuse
30A RK-1
Current Limiting Fuse
Size 1 Starter
Test No. 46 cycle STD640A OCPD
Non Current Limiting
with 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/437
oF
>225oC/
437oF
Results: Test No.4
T1T2
P1
T3
Sound141.5 db @ 2 ft.
50oC/122
oF
>2160 lbs/ft2
> Indicates Meter Pegged
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22.6 KA Symmetrical
Available Fault Current
@ 480V, 3 Phase
Fault Initiated on
Line Side of 30A
Fuse30A RK-1
Current Limiting Fuse
Size 1 Starter
Test No. 3601A.
Class L
Current 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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> 175
o
C/347oF
Results: Test No.3
T1
T2
P1
Sound
133 db @ 2 ft.
62oC/143.6
oF
504 lbs/ft2
T3(No Change
From Ambient)
> Indicates Meter Pegged
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22.6 KA Symmetrical
Available Fault Current
@ 480V, 3 Phase
30A RK-1
Current Limiting
Fuse
Size 1 Starter
Test No. 1601A.
Class L
Current Limiting
Fuse
Fault Initiated on
Load Side of 30A
Fuse
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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 ChangeFrom 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 bodyArcing faults can generate enormous
amounts of energy
Injuries from arcing faults are a result ofthe 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 tosignificantly reduce the energy released
during arcing faults
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