NEW SCIENCE
FIRE SAFETY
ARTICLE
MITIGATING
WORKPLACE ARC
FLASH RISKS
DECEMBER 2014
UL.COM/NEWSCIENCE
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FIRE SAFETY
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NEW SCIENCE FIRE SAFETY / MITIGATING WORKPLACE ARC FLASH RISKS
2
WHY MITIGATING WORKPLACE ARC FLASH RISKS MATTERS
One of the most serious electrical hazards in the workplace is a burn or other injury
caused by an accidental arc flash — a flashover of electric current that leaves its
intended path and travels through the air from one conductor to either another
conductor or to ground.1 Mitigating the risk of arc flash hazards in the workplace is
critically important because there are five to ten arc flash incidents each day in the U.S.
alone,2 resulting in 7,000 burn injuries and more than 2,000 workers admitted to burn
centers for treatment of severe arc flash burns each year.3 Due to the dangerous amount
of energy typically released by an arc flash, these incidents also risk worker fatality.4
CONTEXT
The release of energy from an arc flash incident can be immense, producing
extremely dangerous levels of heat up to 35,000º F.5 An arc flash can also produce
intense light, and the blast effects of the arc can produce sound that can reach 140dB
(about as loud as a gunshot), pressure (the shock wave impact from an explosion)
that can rrange from less than one to more than 10 pounds per square inch (equivalent
to 144 to more than 1,440 pounds per square foot), and shrapnel that is often in the
form of molten metal.6
Many factors can cause an arc flash in an energized circuit, including dust, dropped
tools, accidental touching, condensation, material failure corrosion and faulty
installation.7 When an arc flash occurs, a worker’s exposure to hazards is dependent on
three primary factors: working distance, or workers’ proximity to the source of the arc;
the available incident energy; and the clearing time of overcurrent protective devices,
the interval covering the detection of an excessive current until the device interrupts
or breaks the current.8 Electrical incidents occur relatively infrequently — representing
a comparatively small percentage of total work-related health and safety incidents9
— and arc flash injuries are a subset of these; however, even when nonfatal, it is fairly
common for arc flash victims to never regain their prior quality of life.10
Photo courtesy of IEEE/NFPA Collaboration
on Arc Flash Research.
Mitigating the risk of arc flash hazards
in the workplace is critically important
because there are five to ten arc flash
incidents each day in the U.S. alone.2
Although arc flash incidents represent a serious safety hazard to workers in specific
contexts, the study of this phenomenon does not lend itself to a conventional
approach. UL typically investigates and tests electrical equipment based on foreseeable
normal and abnormal use conditions. However, arc flash hazards usually involve
equipment that has been improperly used or not properly maintained, or unforeseen
hazards such as a wrench or other metal tool that has been accidentally dropped
across a live electrical bus. Because of this complexity, a new approach was required.11
NEW SCIENCE FIRE SAFETY / MITIGATING WORKPLACE ARC FLASH RISKS
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WHAT DID UL DO?
In conjunction with the Institute of Electrical and Electronics Engineers (IEEE) and the
National Fire Protection Association (NFPA), UL was a principal sponsor of the IEEE/
NFPA Arc Flash Collaborative Research Project. UL was also represented in the project’s
Technical Advisory Group (TAG) along with other representatives from the principal
sponsors and global experts on arc flash hazard research. The project was designed to
better understand arc flash phenomena and associated hazards, with a core research
focus on gaining insights into and measurements of the thermal effects of an arc flash.
The research was also designed to study the nonthermal effects of an arc flash —
specifically blast pressure, sound and light — hazards that had not been fully
studied before.12
The collaborative research project was intended to address potential limitations of the
previous measurement practices and tests found in IEEE 1584TM 2002 that had been
brought into question. Specifically, although the 1584 model was shown to fit the data
well, concern has arisen on how well the 1584 standard can predict the arc current and
the incident energy associated with “real-world” arcing faults. In addition, the research
sought to understand and address the potential nonthermal hazards of an arc blast.13
The IEEE 1584 Guide for Performing Arc-Flash Hazard Calculations was first published
in 2002 for the purpose of presenting methods for calculating arc flash incident energy
and protection boundaries in electrical systems to which workers might be exposed.
Much of the guide information was developed from test programs designed at the
time to validate empirically based physical model equations. The NFPA 70E® Standard
for Electrical Safety in the Workplace, which is intended to provide a practical safe
working environment for electrical workers and other employees, relies on IEEE 1584
and similar documents for information on arc flash boundaries and incident energy
levels. Addressing gaps in IEEE 1584 was, therefore, critical to enhancing the electrical
safety of workers.14
When an arc flash occurs, a worker’s
exposure to hazards is dependent on
the working distance — or a worker’s
proximity to the source of the arc —
the available incident energy, and the
clearing time of overcurrent protective
devices.
Phase I of the arc flash research began in late 2008 and continued through 2009. For
this phase, UL provided technical assistance with the actual laboratory tests, design
and construction of special measurement calorimeters and procurement of data
acquisition equipment.15 The Phase I testing was designed to check the following
among test labs using identical test protocols and arrangement:
• Instrumentation functionality and sensitivity
• Overall measurement accuracy
• Repeatability of experiments
• Consistency of test results16
NEW SCIENCE FIRE SAFETY / MITIGATING WORKPLACE ARC FLASH RISKS
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During Phase I, there were approximately 251 AC arcing tests conducted with a total of
64 unique combinations of test conditions that encompassed two supply voltages, two
bolted fault current levels, two electrode-gap widths, two electrode orientations, two
scheduled test durations and four configurations. Before each scheduled testing, the
test conditions and drawing specifications were shared with each lab and discussed to
ensure that all testing was performed using similar test setups, ranging from system
voltage and electrode gap width to arc duration and configurations.
Phase I concluded with an evaluation of the capabilities, limitations and data
measurement abilities of each of the test labs.17
Phase II of the testing began in 2010, and continued through 2013. This phase included
more than 1,500 tests at additional voltages and currents, ranging as high as 13.8 kV
and 63 kA, and included measurement of the pressure, sound and light given off by an
arc blast.18 Coupled with the 200 tests conducted during Phase I and an additional 100
special tests, the research encompassed more than 1,800 tests in total.19 Key findings
from the research include the following:
A. Voltage, Current and Power
Phase arc currents decrease after the first cycle and then increase as an arc stabilizes;
such waveforms are common for lower bolted fault current levels. An arc’s ability to
stabilize and to sustain is primarily an issue for lower voltages (480 V and less) and
depends on several factors, including the bolted fault current, electrode gap width and
configuration (presence of an enclosure as well as dimensions and interior spacing).20
UL was a principal sponsor of the IEEE/
NFPA Arc Flash Collaborative Research
Project that was designed to better
understand arc flash phenomena and
its associated hazards.
B. Incident Energy Comparison
During an arc test, the calorimeters used to measure incident energies show a
temperature rise, which is converted to incident energy. The calorimeters were
arranged in three rows at heights of four and a half, five and five and a half feet off the
ground to provide an approximate representation of the torso, arms and face areas
of a worker. For equation development for incident energy is based on the maximum
energy associated with the highest temperature rise experienced by any single
calorimeter during an arc test. However, the overall and row average incident energies
provide much insight into understanding the heat flow and heat levels experienced.
Based on the test results of Phases I and II, the following factors have been found to
impact the level of incident energy (IE):
• Bolted fault current level
• Duration of the arc
• Electrode orientation/presence of an enclosure
NEW SCIENCE FIRE SAFETY / MITIGATING WORKPLACE ARC FLASH RISKS
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• Calorimeter arrangement, height and measurement distance
• Voltage level
• Gap width between electrodes
• Distance between electrode and back panel
• Dimensions of the metal enclosure21
C. Nonthermal Hazards
The nonthermal hazards of an arc flash include blast pressure, shrapnel, sound, toxic
gases and light. Blast pressure and shrapnel can seriously injure or kill anyone in the
vicinity of an arcing fault. The sound created by the blast can cause permanent hearing
loss, and the intense visible and ultraviolet light given off by the blast can produce
temporary or permanent blindness.22
D. Pressure Measurement
When an arcing fault is initiated, the gases expand rapidly in the vicinity of the arc. A
high-pressure front is created as the expanding gases compress the surrounding air.
The severity of the blast pressure depends on the initial peak pressure, the duration of
the overpressure, the distance of individuals from the incident location, and the degree
of focus due to the presence of a confined area or walls. Blast pressures are greater
when the explosion occurs indoors, particularly in small, enclosed rooms in which the
pressure wave can reflect off the walls.
Individuals may be injured or killed by blast pressures through three mechanisms:
• Injuries that directly result from the pressure wave striking the body are known as
primary blast injuries. Air- and fluid-filled organs, such as the lungs, gastrointestinal
tract and middle ear, are susceptible to primary injuries. Primary blast injuries can
cause concussions or mild traumatic brain injury even if they do not actually involve
a direct blow to the head.
• Secondary injuries result from flying debris propelled by the blast wind. Shrapnel
wounds can occur anywhere, including the eyes and head.
• Tertiary injuries result from the individual being thrown by the blast wind.
Individuals may be injured by a fall or by being propelled into a wall or equipment.
More than one blast injury may be sustained, and damage to one organ often
affects other organs.
NEW SCIENCE FIRE SAFETY / MITIGATING WORKPLACE ARC FLASH RISKS
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Arc blast pressures have been measured or estimated using pressure sensors, a
pendulum and high-speed video. Recording accurate measurements using traditional
direct pressure sensors is challenging due to the high magnetic flux and high
temperature plasma gas given off during an arc event. Based on the two consecutive
high-speed video frames (1000 frames per second) showing the movement of the
arc cloud and air, the estimated pressure reached 1.7 psi (245 lb/ft2) at the opening of
the enclosure. The actual pressure was affected by air temperature and composition.
Furthermore, the calorimeters blocked some airflow and influenced measurement.
Although project testing is now complete, measuring pressure accurately was a
significant challenge, and additional testing in the future may be warranted.23
E. Sound Level Measurement
Blast pressure frequently injures the ear. The initial positive air pressure may cause
lesions on the eardrum and internal ear; it may also dislocate or interrupt the chain
of auditory ossicles or rupture fenestra (membranes). The OSHA Code of Federal
Regulations 1910.95(b)(2), states, “Exposure to impulsive or impact noise should not
exceed 140 dB peak sound pressure level.” When the potential peak sound pressure is
140 dB or greater, individuals should wear personal hearing protection devices (PHPDs)
to reduce the exposure level to the recommended OSHA limits. Using a protective
hood may also attenuate the sound pressure level. Personal protective equipment (PPE)
categories are based on incident energy, a summation of heat flux over time. Because
peak sound pressure is linked to the initial formation of the arc, the PPE categories are
not an effective method for assessing sound hazards. In medium-voltage arc tests, the
peak sound pressures, measured at a distance of three meters from the electrodes,
ranged from 150 to 170 dB, in excess of federal standards.24
The arc flash research project helped
to develop a scientific relationship
between the electrical characteristics
and hazard characteristics of an arcing
fault or arc flash incident.
F. Light Measurement
The wavelengths of visible light lie between 400 and 700 nm. It has been reported
that the light radiated by an arc flash covers part of the ultraviolet region and is
predominantly in the range of 200 to 600 nm. Flash blindness is the temporary loss
of vision when the retina receives an excess of thermal energy — but less energy
than required to cause a burn. A reduction in visual acuity can last a few minutes or
a few days. The contributing factors are glare, afterimage and the bleaching of the
photochemical substances within the rods and cones of the retina. Glare is an excess
of light that hinders vision; even after the light source is no longer present, scotomatic
glare from intense light can cause a reduction in the sensitivity of the retina.
Both a spectrometer and a light sensor were employed during the arc flash testing
to measure illumination levels in lux (lumen/m2). Neutral density filters were used
to attenuate the light to levels falling within the measurable ranges of the device
NEW SCIENCE FIRE SAFETY / MITIGATING WORKPLACE ARC FLASH RISKS
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ratings. The frequency response of the neutral density filters was characterized in
the laboratory. The light measured through the neutral density filters was calibrated
to their actual values. Sample illumination measurements show that the light
intensity increases at closer measurement distances and for larger bolted fault
currents. Additionally, for the test configurations, the larger electrode-gap widths also
significantly increased the illumination levels. It is worth mentioning that a bright
summer day will have a midday ground-level illumination in the order of 100,000
lux. Because some light could have been blocked by the calorimeters, more accurate
measurements were obtained when the calorimeters were removed to perform special
arc blast pressure measurements.25
IMPACT
The arc flash research project defined a scientific relationship between the electrical
and hazard characteristics of an arcing fault or arc flash incident. In 2012, the project
team provided a review and evaluation of the first data along with a draft model
that better defines the mechanisms of thermal energy transfer from the arc to the
surrounding area as well as the potential for injury to people. Today, the research is
culminating in the development of a new comprehensive model for arc-flash incident
energy calculations. The model will benefit both the IEEE 1584 Guide for Performing
Arc-Flash Hazard Calculations, and the NFPA 70E Standard for Electrical Safety in
the Workplace.26 The NFPA 70E technical committee will be considering the need for
certified personal protective equipment (PPE) in the next edition of the standard.27
Ultimately, the results of the arc flash incidents research collaboration will provide
information to improve electrical safety standards, predict the hazards associated with
arc flash and provide practical safeguards for employees in the workplace.28
NEW SCIENCE FIRE SAFETY / MITIGATING WORKPLACE ARC FLASH RISKS
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SOURCES
1
“Understanding ‘Arc Flash,’” U.S. Occupational Safety and Health
Administration, 2014. Web: 2 Oct. 2014. https://www.osha.gov/dte/grant_
materials/fy07/sh-16615-07/arc_flash_handout.pdf.
2
“IEEE/NFPA Arc Flash Phenomena Collaborative Research Project,” Institute of
Electrical and Electronics Engineers (IEEE), 27 Jan. 2011. Presentation: 24 Sept.
2014.
3
“Common Electrical Hazards in the Workplace Including Arc Flash,” U.S.
Occupational Safety and Health Administration, 2014. Web: 2 Oct. 2014.
https://www.osha.gov/dte/grant_materials/fy07/sh-16615-07/electrical_
hazards2.ppt.
4
“Understanding ‘Arc Flash,’” U.S. Occupational Safety and Health
Administration, 2014. Web: 2 Oct. 2014. https://www.osha.gov/dte/grant_
materials/fy07/sh-16615-07/arc_flash_handout.pdf.
5
Ibid.
6
“Understanding ‘Arc Flash,’” U.S. Occupational Safety and Health
Administration, 2014. Web: 2 Oct. 2014. https://www.osha.gov/dte/grant_
materials/fy07/sh-16615-07/arc_flash_handout.pdf.
7
Ibid.
8
Dini, D., personal interview, UL, 15 Oct. 2014.
9
“Workplace Electrical Injury and Fatality Statistics, 2003-2010,” Electrical Safety
Foundation International, 2011. Web: 16 Oct. 2014. http://www.esfi.org/index.
cfm/page/Workplace-Electrical-Injury-and-Fatality-Statistics,-2003-2010/
cdid/12396/pid/3003.
10
“Understanding ‘Arc Flash,’” U.S. Occupational Safety and Health
Administration, 2014. Web: 2 Oct. 2014. https://www.osha.gov/dte/grant_
materials/fy07/sh-16615-07/arc_flash_handout.pdf.
11
Dini, D., “IEEE/NFPA Arc Flash Phenomena Collaborative Research Project,”
UL, 10 Apr. 2013. White paper: 16 Sept. 2014.
12
Ibid.
13
Lee, W. J., et al., “IEEE / NFPA Collaboration on Arc Flash Phenomena Research
Project,” The Institute of Electrical and Electronics Engineers (IEEE), 28 Nov.
2011. White paper: 24 Sept. 2014.
NEW SCIENCE FIRE SAFETY / MITIGATING WORKPLACE ARC FLASH RISKS
14
Dini, D., “IEEE/NFPA Arc Flash Phenomena Collaborative Research Project,”
UL, 10 Apr. 2013. White paper: 16 Sept. 2014.
15
Ibid.
16
“IEEE/NFPA Arc Flash Phenomena Collaborative Research Project,”
The Institute of Electrical and Electronics Engineers (IEEE), 27 Jan. 2011.
Presentation: 24 Sept. 2014.
17
Dini, D., “IEEE/NFPA Arc Flash Phenomena Collaborative Research Project,”
UL, 10 Apr. 2013. White paper: 16 Sept. 2014.
18
Ibid.
19
Lee, W. J., personal interview, 16 Oct. 2014.
20
Lee, W. J., et al., “IEEE / NFPA Collaboration on Arc Flash Phenomena Research
Project,” The Institute of Electrical and Electronics Engineers (IEEE), 28 Nov.
2011. White paper: 24 Sept. 2014.
21
Ibid.
22
Ibid.
23
Ibid.
24
Ibid.
25
Ibid.
26
Dini, D., “IEEE/NFPA Arc Flash Phenomena Collaborative Research Project,”
UL, 10 Apr. 2013. White paper: 16 Sept. 2014.
27
Dini, D., UL, personal interview, 10 Nov. 2014.
28
“IEEE/NFPA Arc Flash Phenomena Collaborative Research Project,” Institute of
Electrical and Electronics Engineers (IEEE), 27 Jan. 2011. Presentation: 24 Sept.
2014.
9
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