Introduction to Arc Flash - Revisions to the NFPA 70E, Electrically Safe Work Conditions, Reducing Arc Flash Hazard, Choosing Correct Personal Protection Equipment PPE, What Is An Arc Flash?
Many workers working on energised equipment are injured and/or killed each year. Several of these casualties are a result of arc flash.
Arc Flash is considered as one of the most destructive and dangerous instances when dealing with electrical wirings. A single occurrence can destroy metals and it has the ability to kill a person if not protected by Arc Flash Clothing. An arc flash can create an arc blast that can shatter anything because it is as hot as the as surface of the sun. This kind of heat can destroy metals instantly and completely burn a body beyond recognition.
Arc Flash ProtectionSerious injuries are caused by the arc flash:
Burns
Respiratory system damage
Hearing damage
Skin penetration from flying debris
Eye and face injuries
An arc flash may happen instantly and if the worker does not have the correct protection, they will already be dead when the arc flash hits them.
The use of Arc Flash Protective Equipment will lessen the damages caused by an arc flash because all of these equipments are solely made to withstand the heat.
Typical Arc Flash Clothing Applications
Working on electrical systems and switchrooms at 500 volts, live testing and proving dead on electrical systems, fitting and removal of LV-HV earths on electrical systems, working on panels/control circuits with exposed energised conductors, removal of bolted covers from energised electrical equipment, racking in/out of switchgear, racking in/out of starters and control gear, live testing and proving dead on electrical systems 11-33kV - T&D UK stock a broad range of Arc Flash Clothing and PPE.
Webinar - Electrical Arc Flash Hazards - Is your company in compliance?
Semelhante a Introduction to Arc Flash - Revisions to the NFPA 70E, Electrically Safe Work Conditions, Reducing Arc Flash Hazard, Choosing Correct Personal Protection Equipment PPE, What Is An Arc Flash?
Semelhante a Introduction to Arc Flash - Revisions to the NFPA 70E, Electrically Safe Work Conditions, Reducing Arc Flash Hazard, Choosing Correct Personal Protection Equipment PPE, What Is An Arc Flash? (20)
Introduction to Arc Flash - Revisions to the NFPA 70E, Electrically Safe Work Conditions, Reducing Arc Flash Hazard, Choosing Correct Personal Protection Equipment PPE, What Is An Arc Flash?
2. Worker Training of Electrical Hazards Including
Arc Flash SH-16614-07
This material was produced under grant number
SH-16614-07 from the Occupational Safety and
Health Administration, U.S. Department of
Labor. It does not necessarily reflect the views
or policies of the U.S. Department of Labor, nor
does mention of trade names, commercial
products, or organizations imply endorsement by
the U.S. Government.
2
3. Overview
•
•
•
•
•
•
•
•
•
3
Introduction
Revisions to the NFPA 70E
Electrically Safe Work Conditions
Energized Electrical Work Permit
Flash Protection Boundary and Limits of
Approach
NFPA 70E Boundaries and Spaces
Flash Protection Calculations
Choosing Correct PPE
Reducing the Arc Flash Hazard
4. Introduction
• What is Arc Flash?
– Arc flash results from an arcing fault, where
the electric arcs and resulting radiation and
shrapnel cause severe skin burns, hearing
damage, and eye injuries.
4
5. Introduction
Why are we so interested in Arc Flash now?
– Numerous workers are injured and/or killed
each year while working on energized
equipment. Many of these casualties are a
result of arc flash.
– Working on energized equipment has become
commonplace in many industries.
5
6. Introduction
Injuries that can result from an arc flash:
– Burns
– Respiratory system damage
– Hearing damage
– Skin penetration from flying debris
– Eye and face injuries
6
7. Introduction
Important Temperatures
Skin temperature for curable burn
Skin temperature causing cell death
Ignition of clothing
Burning clothing
Metal droplets from arcing
Surface of sun
Arc terminals
7
176°F
205°F
752°-1472°F
1472°F
1832°F
9000°F
35,000°F
8. Introduction
• A First Degree Burn is red and sensitive to
touch. There is minimal skin damage and
only the skin surface is involved.
Example: Sunburn
8
9. Introduction
• A Second Degree Burn involves the first
and second layers of skin. The skin
reddens intensely and blisters develop.
Severe pain and swelling occur and
chance for infection is present.
9
10. Introduction
• A Third Degree Burn causes charring of
skin and coagulation of blood vessels just
below the skin surface. All three layers of
skin are affected. Extensive scarring
usually results.
10
11. Introduction
• Skin damage will occur based on the intensity of
the heat generated by an electrical arc accident.
The heat reaching the skin of the worker is
dependant on the following three factors:
– Power of the arc at the arc location
– Distance of the worker to the arc
– Time duration of the arc exposure
11
12. Introduction
• The intent of NFPA 70E regarding arc
flash is to provide guidelines which will
limit injury to the onset of second degree
burns.
12
13. Introduction
Inhalation Injuries
In addition to burns, an arc flash can
cause inhalation injuries. More than a
hundred known toxic substances are
present in fire smoke. When inhalation
injuries are combined with external burns
the chance of death can increase
significantly.
13
14. Introduction
• The pressure of an arc blast is caused by
the expansion of the metal as it vaporizes
and the heating of the air by the arc
energy. This accounts for the expulsion of
molten metal up to 10 feet away.
• In addition, the sudden expansion of an arc
blast creates loud sounds that can cause
hearing damage.
14
15. Revisions To The NFPA 70E
As a result of the injuries
and deaths related to arc
flash, changes/additions
have been incorporated
into the National Fire
Protection Association
publication number 70E,
the most recent version
being NFPA 70E-2004.
15
19. Arc Flash Awareness
• NIOSH DVD:
Arc Flash Awareness
– Information and
discussion about arc
flash and comments
from workers injured
by an arc flash
19
20. Electrically Safe Work Conditions
• The equipment is not and cannot be energized:
To ensure an electrically safe work condition:
– Identify all power sources,
– Interrupt the load and disconnect power,
– Visually verify that a disconnect has opened the circuit,
– Locking out and tagging the circuit,
– Test for absence of voltage, and
– Ground all power conductors, if necessary.
20
21. Electrically Safe Work Conditions
• Lockout/Tagout
– A single qualified person de-energizing one
set of conductors.
– An unqualified person may never perform a
lockout/tagout, work on energized equipment,
or enter high risk areas.
21
22. Energized Electrical Work Permit
• When live parts over 50 volts are not
placed in an electrically safe work
condition it is considered energized
electrical work and must be down under a
written permit.
• Permit gives conditions and work practices
needed to protect employee from arc flash
or contact with live parts.
22
23. Energized Electrical Work Permit
An Energized Electrical Work Permit will include:
–
–
–
–
–
–
–
–
23
Circuit, equipment and location
Why working while energized.
Shock and arc flash hazard analysis
Safe work practices
Approach boundaries
Required PPE and tools
Access control
Proof of job briefing
24. Flash Protection Boundary and
Limits of Approach
• Definitions of Boundaries and Spaces
The closer you approach an exposed,
energized conductor or circuit part, the
greater the chance of an inadvertent
contact and the greater the injury that an
arc flash will cause. NFPA 70E-2004,
Annex C defines approach boundaries
and work spaces. The diagram on the next
slide illustrates these.
24
26. Flash Protection Boundary and
Limits of Approach
• Flash Protection Boundary
When an energized conductor is exposed,
you may not approach closer than the
flash boundary without wearing
appropriate personal protective clothing
and personal protective equipment.
26
27. Flash Protection Boundary and
Limits of Approach
• Flash Protection Boundary
IEEE defines “Flash Protection Boundary”
as: An approach limit at a distance from
live parts operating at 50 V or more that
are un-insulated or exposed within which a
person could receive a second degree
burn.
27
28. Flash Protection Boundary and
Limits of Approach
How Does Flash Protection Boundary Relate to
Working On Or Near Exposed Energized Parts?
• The radiant energy and molten material that is
released by an electric arc is capable of seriously
injuring or killing a human being at distances of up
to twenty feet.
The flash protection boundary is the closest
approach allowed by qualified or unqualified
persons without the use of arc flash PPE.
28
32. NFPA 70E Boundaries and Spaces
Good safety practices minimize risk:
• Switch remotely if possible.
• Standing aside and away as much as
possible during switching.
• Avoid leaning on or touching switchgear
and metallic surfaces.
• Use proper tools and PPE.
32
33. NFPA 70E Boundaries and Spaces
• NFPA 70E, Section 130.3(B) states:
• If work will be performed within the flash
protection boundary, the flash hazard
analysis shall determine, and the
employer shall document, the incident
energy exposure of the worker in
(cal/cm2).
33
34. NFPA 70E Boundaries and Spaces
NFPA 70E, Section 130.3 (B) states:
• The incident energy exposure
level shall be based on the
working distance of the worker’s
face and chest areas from a
prospective arc source for the
specific task to be performed.
34
35. NFPA 70E Boundaries and Spaces
• NFPA 70E, Section 130.3(B) states:
Flame Resistant (FR) Clothing and Personal
Protective Equipment (PPE) shall be used by
the employee based upon the incident energy
exposure associated with the specific task.
35
36. Flash Protection Calculations
• The Incident Energy and Flash Protection
Boundary can be calculated in an Arc
Flash Hazard Analysis.
• There are two methods:
– NFPA 70E-2004, Annex D
– IEEE Std 1584TM
36
37. Flash Protection Calculations
Step 1
• Collect the System Installation Equipment
Data
Step 2
• Determine the Power System’s Modes of
Operation
– Normal operation, tie switched closed, dual
feeds
– Perform analysis for worst case condition
37
38. Flash Protection Calculations
Step 3
• Determine the Bolted Fault Currents
– Find symmetrical RMS current and X/R ratio at each point of
concern.
•
•
•
38
Theoretically worst case fault magnitude
Determines equipment interrupting ratings
Impedance at fault location is considered
to be zero ohms
39. Flash Protection Calculations
Step 4
• Determine the Arc Fault Currents
– The arc fault current for each location where
an arc flash hazard exists and the portion of
the current that flows through the closest
upstream device that will clear this fault must
be determined
39
40. Flash Protection Calculations
Arcing Fault Current is fault current flowing through
an electrical arc plasma.
•
•
Faults which are not bolted
Poor electrical connection
between conductors can cause
arcing
• Arcing results in tremendous
heat (35,000)
40
41. Flash Protection Calculations
Step 5
• From the Protective Device Characteristics,
Find the Arcing Duration
– The total clearing time of the fault will
determine the “time” factor in the incident
energy equation.
41
42. Flash Protection Calculations
CURRENT IN AMPERES
The fault clearing time is
determined from the
Coordination Study’s
Time Current Curves.
1000
T4
T4
100
The total clearing time of
the primary fuse for a
secondary side fault is 1
second.
T4
1 sec
M2
1
T4 Main Phase
T4 - T5 Phase
TX Inrush
0.10
0.01
42
TIME IN SECONDS
HMCP 250 A
100 hp O/L
10
0.5 1
10
100
MCC Fdr Phase
1K
10K
T4 arc flash.tcc Ref. Voltage: 480 Current Scale x10^2 T4 arc flash.drw
43. Flash Protection Calculations
Step 6
• Record the System Voltages and
Equipment Classes
– For each bus or arc hazard location
Step 7
• Determine Working Distances
– Arc flash protection is always based on the
incident energy to a person’s face and body at
the working distance
43
44. Flash Protection Calculations
Step 8
• Determine Incident Energy
– This is best done using a software package.
• Calculating incident energy requires the
following parameters:
– Max. bolted 3-ph fault current available at the
equipment
– Total protective upstream device clearing time max
fault current
– Distance of worker from the arc
44
45. Flash Protection Calculations
Step 9
• Determine the Flash Protection Boundary
for All Equipment
– The incident energy for the flash-protection
boundary must be set at the minimum energy
beyond which a second degree burn could
occur - 1.2 cal/cm2
45
47. Flash Protection Calculations
The estimated incident energy for an arc in
open air is:
EMA = 5271DA-1.9593 tA[0.0016F 2-0.0076F+0.8938]
EMA=maximum open arc incident energy (cal/cm 2)
DA=distance from arc electrodes (inches)
tA=arc duration (seconds)
F=bolted fault current in kA (16kA-50kA)
47
48. Flash Protection Calculations
The estimated incident energy for an arc in
a box is:
EMB = 1038.7DB-1.4738 tA[0.0093F2-0.3453F+5.9675]
EMB=max 20 in. cubic box incident energy (cal/cm 2)
DB=distance from arc electrodes (inches) for 18 in.
and greater
tA=arc duration (seconds)
F=bolted fault current in kA (16kA-50kA)
48
49. Flash Protection Calculations
• Test results have shown that the incident
energy for an open air arc is
approximately inversely proportional to the
distance squared.
• Enclosing a 3-ph arc in a box can increase
the incident energy from 1.5 to 3 times
depending upon the arc parameters and
box dimensions when compared to an
open air arc with the same parameters.
49
50. Flash Protection Calculations
There are resources on the
internet to assist in calculations:
•
http://
www.littelfuse.com/arccalc/calc.html
•
http://www.pnl.gov/contracts/esh-procedures/forms/sp00e230.xls
•
http://
www.bussmann.com/arcflash/index.aspx
50
52. Choosing Correct PPE
Section 130.7(A) states that employees
working in areas where there are electric
hazards shall be provided with, and shall
use, protective equipment that is designed
and constructed for the specific part of the
body to be protected and for the work to
be performed.
52
53. Choosing Correct PPE
• Personal Protective Equipment, PPE, for
the arc flash is the last line of defense.
• It is not intended to prevent all injuries, but
is intended to mitigate the impact of an arc
flash, should one occur.
53
54. Choosing Correct PPE
•
After the Arc-Flash Hazard Analysis has
been performed, PPE is selected as
follows:
Clothing’s ATPV or EBT (in cal/cm2)
>
Calculated Hazard Level (in cal/cm2)
*ATPV can be obtained from clothing manufacturer
54
55. Choosing Correct PPE
ATPV - Arc Thermal Performance
Exposure Value
EBT - Breakopen Threshold Energy
Rating
Calculated Hazard Level - Incident
Energy in cal/cm2
55
59. Choosing Correct PPE
• Specialized Arc-Flash
Protection Equipment:
Face Shield -- Attaches to
Hard Hat
• Use: Hazard/Risk
Category 2
59
60. Choosing Correct PPE
• Specialized Arc-Flash
Protection Equipment:
Gloves and Leather
Protectors,
(ATPV Values not
Established for Rubber)
• Use: Hazard/Risk
Category 2, 3, and 4
for the Leather
Protectors
60
61. Choosing Correct PPE
• NFPA 70E, Section 130.7(C)(9)(a) states:
– When selected in lieu of the flash hazard
analysis of 130.3(A), Table 130.7(C)(9)(a)
shall be used to determine the hazard/risk
category for each task.
• NFPA 70E, Section 130.7(C)(10) states:
– Once the Hazard/Risk Category has been
identified, Table 130.7(C)(10) shall be used to
determine the required personal protective
equipment (PPE) for the task.
61
62. Choosing Correct PPE
The tables in NFPA 70E-2004 provide the
simplest methods for determining PPE
requirements. They provide instant
answers with almost no field data. The
tables provide limited application and are
conservative for most applications.
*These tables are not intended as a substitution for an arc hazard
analysis, but only as a guide.
62
67. Choosing Correct PPE
• NFPA 70E-2004, Section 130.7(C), Table
130.7(C)(9)(a) lists common work tasks
with respective Hazard/Risk category of
each task.
• After the Hazard Risk Category has been
determined from Table 130.7(C)(9)(a),
then Table 130.7(C)(10) is used to
determine the Protective Clothing and
Personal Protective Equipment required
for the task.
67
68. Choosing Correct PPE
Hazard/
Risk
Category
V-rated
V-rated
Gloves
Tools
CB or fused switch operation with enclosure doors closed
0
N
N
Reading a panel meter while operating a meter switch
0
N
N
CB or fused switch operation with enclosure doors open
1
N
N
Work on energized parts, including voltage testing
2*
Y
Y
Task (Assumes Equipment Is Energized, and Work Is Done
Within the Flash Protection Boundary)
68
69. Choosing Correct PPE
• NFPA 70E, Table 130.7(C)(11) lists the
characteristics and degrees of protection
for various Flame Resistant (FR) clothing
systems.
69
70. Choosing Correct PPE
NFPA 70E Table 130.7(C)(11) Typical Protective Clothing Systems
Clothing Description (Number of
clothing layers is given in
parenthesis)
Total
Weight
oz/yd2
Hazard
Risk
Category
Minimum Arc Thermal
Performance Exposure
Value (ATPV)* or
Breakdown Threshold
Energy (EBT)* Rating of
PPE
cal/cm2
0
Untreated Cotton (1)
4.5 - 7
N/A
1
FR shirt and FR pants (1)
4.5 - 8
5
2
Cotton underwear plus FR shirt and
FR pants (2)
9 - 12
8
3
Cotton underwear plus FR shirt and
FR pants plus FR coverall (3)
16 - 20
25
4
Cotton underwear plus FR shirt and
FR pants plus double layer switching
coat and pants (4)
24-30
40
* ATPV is defined as the incident energy that would just cause the onset of a second
degree burn.
* EBT is defined as the highest incident energy which did not cause FR fabric breakopen
and did not exceed the second degree burn criteria. E BT is reported when ATPV cannot
be measured due to fabric breakopen.
70
71. Choosing Correct PPE
The equations in NFPA 70E-2004 provide
more accurate methods than tables for
determining PPE requirements. System
data and studies are required. The
equations are based upon limited fuse and
circuit breaker data.
71
72. Choosing Correct PPE
• Remember: PPE is the last line of
defense. PPE cannot prevent all
injuries and will only lessen the
impact of an arc flash. In many cases
the use of PPE has saved lives or
prevented serious injury.
72
73. Reducing The Arc Flash Hazard
OSHA 1910.333 severely limits the
situations in which work is performed
on or near equipment or circuits that
are or may be energized.
73
74. Reducing The Arc Flash Hazard
EQUIPMENT ALTERNATIVES
• Metal-Clad Switchgear
Structural design reduces the
possibility of arcing faults within the
enclosure.
74
75. Reducing The Arc Flash Hazard
EQUIPMENT ALTERNATIVES
• Arc Resistant Switchgear
EEMAC Standard G14-1 defines the
requirements for arc resistant
switchgear. Includes robust design
and pressure relief vents.
75
76. Reducing The Arc Flash Hazard
EQUIPMENT ALTERNATIVES
• Current-Limiter Power Circuit Breakers
Reduces the clearing time which
reduces the incident energy.
76
77. Reducing The Arc Flash Hazard
EQUIPMENT ALTERNATIVES
• Current-Limiting Reactors
Reduces the magnitude of fault
current which reduces the incident
energy.
77
78. Reducing The Arc Flash Hazard
EQUIPMENT ALTERNATIVES
• Zone Selective Interlocking of Circuit
Breakers
Deactivates the preset delay on the
circuit breaker closest to the fault,
which then trips with no intentional
delay.
78
79. Reducing The Arc Flash Hazard
Whatever the analysis method or
proposed method of solution, each
work task must be analyzed assuming
worst case conditions.
79
80. Reference Materials
•
•
•
•
•
•
•
•
80
Standard for Electrical Safety in the Workplace, NFPA 70E 2004 Edition
Controlling Electrical Hazards. OSHA Publication 3075, (2002). Also available as a 350 KB PDF,
71 pages. Provides a basic overview of electrical safety on the job, including information on how
electricity works, how to protect against electricity, and how OSHA can help.
Electrical Safety: Safety and Health for Electrical Trades Student Manual. US Department of
Health and Human Services (DHHS), National Institute for Occupational Safety and Health
(NIOSH), Publication No. 2002-123, (2002, January), 1.7 MB PDF, 88 pages. This student
manual is part of a safety and health curriculum for secondary and post-secondary electrical
trades courses. It is designed to engage the learner in recognizing, evaluating, and controlling
hazards associated with electrical work. http://www.cdc.gov/niosh/pdfs/02-123.pdf
Electrocutions Fatality Investigation Reports. National Institute for Occupational Safety and
Health (NIOSH) Safety and Health Topic. Provides information regarding hundreds of fatal
incidents involving electrocutions investigated by NIOSH and state investigators
Working Safely with Electricity. OSHA Fact Sheet, 353 KB PDF, 2 pages. Provides safety
information on working with generators, power lines, extension cords, and electrical equipment.
http://www.osha.gov/OshDoc/data_Hurricane_Facts/elect_safety.pdf
Lockout/Tagout. OSHA Fact Sheet, (2002), 212 KB PDF, 2 pages. A 92 KB PDF (Spanish
version) is also available.
http://www.cdc.gov/nasd/docs/d001501-d001600/d001514/d001514.html
Lockout-Tagout Interactive Training Program. OSHA. Includes selected references for training
and interactive case studies. http://www.osha.gov/dts/osta/lototraining/index.htm
NIOSH Arc Flash Awareness, NIOSH Publication No. 2007-116D
NIOSH Suggested Discussion Questions:
If one of the incidents in the video had happened to you, how would it have changed your life or the life of your family?
Have you ever experienced an arc flash? What happened? Did you change any safety practices in your work after the incident? What were they?
What are some behaviors that you can change to help prevent an arc flash incident to yourself and/or to your fellow electricians?
What PPE is recommended for your task? What are some of the excuses electricians use for not wearing appropriate PPE?
In, general, as electricians become more experienced, do you think their safety habits change? How?
After viewing this video, which, if any, of your safety work habits would you like to change?
Do you have any suggestions for your supervisor that you feel could help prevent an arc flash incident from happening to you or any of your co-workers?
What suggestions do you have for your company to help prevent arc flash incidents?
Arc Flash Awareness: DHHS(NIOSH) Publication No. 2007-116D
Ordering Information: Copies of NIOSH documents are available from:
NIOSH-Publications Dissemination
4676 Columbia Parkway
Cincinnati, OH 45226-1998
Fax: 513-533-8573, Phone: 1-800-35-NIOSH
Email: pubstaft@cdc.gov, website: www.cdc.gov/niosh
The NEC warning label should remind a qualified worker who intends to open the equipment for analysis or work that a serious hazard exists and that the worker should follow appropriate work practices and wear appropriate PPE for the specific hazard (a non qualified worker must not be opening the equipment).
The NEC 2002, section 110.16 requirement for labeling does not call for any quantification of the incident energy, however, some companies are choosing to provide more information on the arc flash label as shown above.
The manufacturer of the equipment is normally not aware of the site specific configuration of the power system at the point where the equipment will be connected or aware of the site specific tasks that will be evaluated.
The installer or the owner of the equipment must label the equipment.
Arc-flash protection is always based on the incident energy level on the person’s face and body at the working distance, not the incident energy on the hands or arms. The degree of injury in a burn depends on the percentage of a person’s skin that is burned. The head and body are a large percentage of total skin surface area and injury to these areas is much more life threatening than burns on the extremities.
Data required for this study is similar to data collected for typical short-circuit and protective device coordination studies, however it also requires information on low-voltage distribution and control equipment plus its feeders and large branch circuits. This information is used to determine the time of arc duration from the protective device time current curves.
Find the symmetrical root-mean-square (RMS) bolted fault current and X/R ratio at each point of concern for the 1.5 to 4 cycle case. This includes all locations where people could be working. It is important to include all cables. Conservatively high fault levels do not necessarily increase safety, they may reduce it. Don’t add a safety factor. Higher current can give shorter arc fault duration and the wrong energy value.
Lower fault currents often persist longer than higher currents because of the characteristic shape of most protective-device time-current curves.
Common causes of bolted faults:
1. Re-energizing without removing temporary grounding.
2. Incorrect connection of a parallel run of cables to a motor terminal box. Instead of connecting a-a, b-b, c-c; connections are a-b, a-b, c-c.
The arc fault current for each location where an arc flash hazard exists and the portion of that current that flows through the closest upstream protective device that will clear this fault must be determined.
The arc fault current depends primarily on the bolted fault current. The bolted fault current in the protective device can be found from the short-circuit study by looking at the detailed bus fault printout or looking at the fault levels that are “one-bus-away”. It is important to separate fault contributions from all other sources that will not flow through the protective device that will clear the fault. This includes alternate feeder and downstream motor fault contributions. The arc fault currents can then be calculated. The calculated arc fault current will be lower than the bolted fault current due to arc impedance, especially for applications less than 1000 V.
There are two types of protective devices in power distribution systems. These are fuses or circuit breakers. For fuses, the manufacturer’s time-current curves may include both melting and clearing time. If so, use the clearing time. If they show only the average melt time, add to that time 15% for times up to 0.03 seconds, and 10% above 0.03 seconds to determine total clearing time. For Current Limiting Fuses (CLF), use the maximum clearing time from the fuse time current curve if fault current is not in the current limiting range. The current limit point is at the 0.01 sec point on the fuse curve. For one to two times the limit point in amps, use 0.008 seconds (1/2 cycle). For over two times the limit point, use 0.004 seconds (1/4 cycle).
For circuit breakers with integral trip units, the manufacturer’s time-current curves include both the tripping time and the clearing time. The total clearing time as shown by the top band of the curve is the time value that should be used.
For relay operated circuit breakers, the relay curves show only the relay operating time in the time-delay region. For relays operating in their instantaneous region, allow 16 milliseconds on 60 Hz systems for operation. The circuit breaker opening time must be added to the relay time to give a total clearing time.
<number>
Equation 1 provides smaller values of the flash boundary because actual transformer ratings and power system impedances are used.
Equation must not be applied without an accurate up-to-date short circuit analysis at the point of exposure.
For transformers rated below .75 MVA, multiply transformer MVA by 1.25.
For each bus, document the system voltage and the class of equipment. For example, record whether the equipment is metal-enclosed switchgear or metal-clad switchgear.
IEEE 1584, Table - Classes of equipment and typical bus gaps
Arc-flash protection is always based on the incident energy level on the person’s face and body at the working distance, not the incident energy on the hands or arms. The degree of injury in a burn depends on the percentage of a person’s skin that is burned. The head and body are a large percentage of total skin surface area and injury to these areas is much more life threatening than burns on the extremities.
The equations in IEEE 1584-2002 are available in spreadsheet format. These equations have been incorporated into the packages of power system software vendors.
lg is the log10
Ia is arcing current in kA
K is -0.153 for open configurations, and
is -0.097 for box configurations
Ibf is bolted fault current for 3-ph symmetrical RMS in kA
V is system voltage in kV
G is the gap between conductors in mm
NFPA equations for “Open Air” and “Arc in a Box” are valid for 600 V or below.
NFPA equations for “Open Air” and “Arc in a Box” are valid for 600 V or below.
A simplified two-category approach is found in NFPA 70E-2000, Table F-1 of Part II, Appendix F NFPA. This table assures adequate PPE for electrical workers within facilities with large and diverse electrical systems. The clothing listed in Table F-1 fulfills the minimum FR clothing requirements of NFPA 70E-2000, Tables 3-3.9.1 and 3-3.9.2 NFPA and should be used with the other PPE appropriate for the Hazard/ Risk Category that is found in of NFPA 70E-2000, Table 3-3.9.2 NFPA.
A simplified two-category approach is found in NFPA 70E-2000, Table F-1 of Part II, Appendix F NFPA. This table assures adequate PPE for electrical workers within facilities with large and diverse electrical systems. The clothing listed in Table F-1 fulfills the minimum FR clothing requirements of NFPA 70E-2000, Tables 3-3.9.1 and 3-3.9.2 NFPA and should be used with the other PPE appropriate for the Hazard/ Risk Category that is found in of NFPA 70E-2000, Table 3-3.9.2 NFPA.
A simplified two-category approach is found in NFPA 70E-2000, Table F-1 of Part II, Appendix F NFPA. This table assures adequate PPE for electrical workers within facilities with large and diverse electrical systems. The clothing listed in Table F-1 fulfills the minimum FR clothing requirements of NFPA 70E-2000, Tables 3-3.9.1 and 3-3.9.2 NFPA and should be used with the other PPE appropriate for the Hazard/ Risk Category that is found in of NFPA 70E-2000, Table 3-3.9.2 NFPA.
A simplified two-category approach is found in NFPA 70E-2000, Table F-1 of Part II, Appendix F NFPA. This table assures adequate PPE for electrical workers within facilities with large and diverse electrical systems. The clothing listed in Table F-1 fulfills the minimum FR clothing requirements of NFPA 70E-2000, Tables 3-3.9.1 and 3-3.9.2 NFPA and should be used with the other PPE appropriate for the Hazard/ Risk Category that is found in of NFPA 70E-2000, Table 3-3.9.2 NFPA.
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1910.333(a)(1)
"Deenergized parts." Live parts to which an employee may be exposed shall be deenergized before the employee works on or near them, unless the employer can demonstrate that deenergizing introduces additional or increased hazards or is infeasible due to equipment design or operational limitations. Live parts that operate at less than 50 volts to ground need not be deenergized if there will be no increased exposure to electrical burns or to explosion due to electric arcs.
Note 1: Examples of increased or additional hazards include interruption of life support equipment, deactivation of emergency alarm systems, shutdown of hazardous location ventilation equipment, or removal of illumination for an area.
Note 2: Examples of work that may be performed on or near energized circuit parts because of infeasibility due to equipment design or operational limitations include testing of electric circuits that can only be performed with the circuit energized and work on circuits that form an integral part of a continuous industrial process in a chemical plant that would otherwise need to be completely shut down in order to permit work on one circuit or piece of equipment.