TM
Safety BASICs
Electrical Hazards
Bussmann® Awareness of Safety Issues Campaign
©2004 Cooper Bussmann
Electrical Hazards
What are the hazards as you approach
electrical equipment to perform work?
©2004 Cooper Bussmann
Electrical Hazards
• Shock
• Arc flash
– Heat
– Fire
• Arc blast
– Pressure
– Shrapnel
– Sound
Example of an arcing fault
©2004 Cooper Bussmann
Basic Electrical Theory
I=V/Z
• What happens with shock?
• What happens when there is a fault?
• What is the difference between a shortcircuit and an arcing fault?
©2004 Cooper Bussmann
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
©2004 Cooper Bussmann
Human Resistance Values
Resistance (ohms)
Condition
Dry
Wet
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
500 to 1,500
250 to 750
Finger touch
Two hands around 1-1/2 inch pipe
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.
©2004 Cooper Bussmann
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 standards consider 5 mA to
be the safe upper limit for children and
adults
©2004 Cooper Bussmann
Electric Shock
mA
0.5 - 3
3 - 10
10 - 40
30 - 75
100 - 200
200 - 500
1500 +
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Affect on person
- Tingling sensations
- Muscle contractions and pain
- “Let-go” threshold
- Respiratory paralysis
- Ventricular fibrillation
- Heart clamps tight
- Tissue and organs start to burn
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
©2004 Cooper Bussmann
Electric Shock Injury
©2004 Cooper Bussmann
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
©2004 Cooper Bussmann
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
©2004 Cooper Bussmann
Bolted
Short Circuit
Arcing
Fault
Current
Thru Air
A
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B
A
B
Electric Arc
35,000 °F
Molten Metal
Pressure Waves
Sound Waves
Shrapnel
Copper Vapor:
Solid to Vapor
Expands by
67,000 times
Hot Air-Rapid Expansion
Intense Light
©2004 Cooper Bussmann
Personnel Hazards Associated
With Arc Flash & Arc Blast
• Heat – burns & ignition of material
o
– Arc temperature of 35,000 F
– Molten metal, copper vapor, heated air
• Second degree burn threshold:
o
o
– 80 C / 175 F (0.1 sec), 2nd degree burn
• Third degree burn threshold:
o
o
– 96 C / 205 F (0.1 sec), 3rd degree burn
• Intense light
– Eye damage, cataracts
©2004 Cooper Bussmann
Personnel Hazards Associated
With Arc Flash & Arc Blast
• 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
©2004 Cooper Bussmann
Overcurrent Protection Role
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
©2004 Cooper Bussmann
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
©2004 Cooper Bussmann
IEEE / PCIC Staged Arc Flash Test
Set-up
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Test No. 4
22.6 KA Symmetrical
Available Fault Current
@ 480V, 3 Phase
6 cycle STD
Set @ 6 cycle opening
640A OCPD
Non Current Limiting
with Short Time Delay
Fault Initiated on
Line Side of 30A
Fuse
30A RK-1
Current Limiting Fuse
Size 1 Starter
©2004 Cooper Bussmann
©2004 Cooper Bussmann
Test 4 Still Photo
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Test 4 Still Photo
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Test 4 Still Photo
©2004 Cooper Bussmann
Test 4 Still Photo
©2004 Cooper Bussmann
Test 4 Still Photo
©2004 Cooper Bussmann
Test 4 Still Photo
©2004 Cooper Bussmann
Test 4 Still Photo
©2004 Cooper Bussmann
Results: Test No.4
Sound
141.5 db @ 2 ft.
P1
T2
>2160 lbs/ft2
o
o
>225 C/437 F
T3
o
o
50 C/122 F
T1
o
>225 C/
o
437 F
> Indicates Meter Pegged
©2004 Cooper Bussmann
Test No. 3
22.6 KA Symmetrical
Available Fault Current
@ 480V, 3 Phase
601A.
Class L
Current Limiting Fuse
Fault Initiated on
Line Side of 30A
Fuse
30A RK-1
Current Limiting Fuse
Size 1 Starter
©2004 Cooper Bussmann
©2004 Cooper Bussmann
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
©2004 Cooper Bussmann
Results: Test No.3
Sound
133 db @ 2 ft.
T2
o
o
62 C/143.6 F
T3
(No Change
From Ambient)
P1
504 lbs/ft2
T1
o
> 175 C/
o
347 F
> Indicates Meter Pegged
©2004 Cooper Bussmann
Test No. 1
22.6 KA Symmetrical
Available Fault Current
@ 480V, 3 Phase
601A.
Class L
Current Limiting
Fuse
30A RK-1
Current Limiting
Fuse
Fault Initiated on
Load Side of 30A
Fuse
©2004 Cooper Bussmann
Size 1 Starter
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Test 1 Still Photo
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Test 1 Still Photo
©2004 Cooper Bussmann
Test 1 Still Photo
©2004 Cooper Bussmann
Test 1 Still Photo
©2004 Cooper Bussmann
Results: Test No.1
Sound
(No Change
From Ambient)
T2
(No Change
From Ambient)
T3
(No Change
From Ambient)
©2004 Cooper Bussmann
P1
(No Change
From Ambient)
T1
(No Change
From Ambient)
Current-Limitation: Arc Energy
Reduction
Test 4
Non-Current Limiting
Test 3
Reduced Fault Current
via Current-Limitation
©2004 Cooper Bussmann
Test 1
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
©2004 Cooper Bussmann
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
©2004 Cooper Bussmann