Mitigating Workplace Arc Flash Risks
UL was a principal sponsor and participant in the IEEE/NFPA Arc Flash Collaborative Research Project, conducting experiments to understand and find ways to mitigate the risks of arc flash incidents.
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
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 range 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
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
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 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 1584 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
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
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
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 (IE). 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. The 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:
- Bolted fault current level
- Duration of the arc
- Electrode orientation/presence of an enclosure
- 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.
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 (cochleae). 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
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 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
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