Evaluation of Onset to Second Degree Burn Energy in Arc Flash

Our interest in determining accurate onset to second degree burn energy and its significance in computing the arc flash boundary is focused on the prevention of injury to the skin of a human who might be exposed to an arc-flash. During the last two decades different formulas have been proposed to calculate incident energy at an assumed working distance, and the arc flash boundary in order to determine arc rated personal protective equipment for Qualified Electrical Workers. Among others, the IEEE Standard P1584 Guide for Performing Arc-Flash Hazard Calculations [1584 IEEE Guide for Performing Arc-Flash Hazard Calculations. IEEE Industry Applications Society. September 2002] and formulas provided in Annex D of NFPA 70E [NFPA 70E Standard for Electrical Safety in the Workplace. 2012.] and CSA Z462 [ CSA Z462 Workplace electrical safety Standards. 2012.] Workplace Electrical Safety Standard are the most often utilized in the industry to perform arc flash hazard analysis. The formulas are based on incident energy testing performed and calculations conducted for selected range of prospective fault currents, system voltages, physical configurations etc.

Use of Incident Energy as a Measure of Burn Severity in Arc Flash Boundary Calculations
The IEEE P1584 Standard was developed by having incident energy testing performed based on methodology described in the ASTM F1959-99 standard. The incident energy to which the worker’s face and chest could be exposed at working distance during an electrical arc event was selected as a measure for determining hazard risk category and calculating the arc flash boundary. The incident energy of 1.2 cal/cm2 ( 5.0 J/cm2 ) for bare skinwas selected in solving the equation for the arc flash boundary in IEEE P1584 [1584 IEEE Guide for Performing Arc-Flash Hazard Calculations. IEEE Industry Applications Society. September 2002. page 41]. Also, NFPA 70E [NFPA 70E Standard for Electrical Safety in the Workplace. 2012. page 10] states that “a second degree burn is possible by an exposure of unprotected skin to an electric arc flash above the incident energy level of 1.2 cal/cm2 ( 5.0 J/cm2 )” and assumes 1.2 cal/cm2 as a threshold incident energy level for a second degree burn for systems 50 Volts and greater [NFPA 70E Standard for Electrical Safety in the Workplace. 2012. page 26].The IEEE 1584 Guidestates that “the incident energy that will cause a just curable burn or a second degree burn is 1.2 cal/cm2 (5.0 J/cm2 )” [1584 IEEE Guide for Performing Arc-Flash Hazard Calculations. IEEE Industry Applications Society. September 2002. page 96]. To better understand these units, IEEE P1584 refers to an example of a butane lighter. Quote: “if a butane lighter is held 1 cm away from a person’s finger for one second and the finger is in the blue flame, a square centimeter area of the finger will be exposed to about 5.0 J/cm2 or 1.2 cal/cm2 “. However IEEE P1584 equations (5.8) and (5.9) for determining the arc flash boundary can also be solved with other incident energy levels as well such as the rating of proposed personal protective equipment (PPE). The important point to note here is that threshold incident energy level for a second degree burn or onset to second degree burn energy on a bare skin is considered constant value equal to 1.2 cal/cm2 (5.0 J/cm2) in IEEE P1584 Standard.

Flash Fire Burn Experimentations and Observations

Much of the research which led to equations to predict skin burns was started during or immediately after World War II. In order to protect people from fires, atomic bomb blasts and other thermal threats it was first necessary to understand the effects of thermal trauma on the skin. To name the few, are the works done by Alice M. Stoll, J.B.Perkins, H.E.Pease, H.D.Kingsley and Wordie H. Parr. Tests were performed on a large number of anaesthetized pigs and rats exposed directly to fire. Some tests were also performed on human volunteers on the fronts of the thorax and forearms. A variety of studies on thermal effects have been performed and thermal thresholds were identified for different degree burns. We will focus on second degree burn as this is the kind of burn used to determine the arc flash boundary in engineering arc flash analysis studies.

Alice Stoll pursued the basic concept that burn injury is ultimately related to skin tissue temperature elevation for a sufficient time. Stoll and associates performed experimental research to determine the time it takes for second degree burn damage to occur for a given heat flux exposure. Stoll showed that regardless of the mode of application of heat, the temperature rise and therefore the tolerance time is related to heat absorbed by the skin[Stoll, A.M., Chianta M.A, Heat Transfer Through Fabrics. Naval Air Development Center. Sept. 1970]. Results of this study are represented in Figure 1 line (A) along with other studies discussed below. READ MORE

Time To Second Degree Burn Graph

Arc Flash Boundary – Calculated, Larger, Something Else?

It has been a while since a question of the week was asked about the Arc Flash Boundary. This is the distance from a prospective arc flash where the incident energy is 1.2 cal/cm^2 which is the generally accepted value for the onset of a second degree burn. IEEE 1584 has a method for calculating this distance.

Since electrical safety practices continue to evolve, this week’s question is about the Arc Flash Boundary. Although the AFB is required to be on the warning label and is a calculated value, many are opting to keep unprotected/unqualified workers further away from a possible arc flash during live work (which should be kept to a minimum). This week’s question:

For your (client’s) electrical safety practices, do you use:
Select up to 2 answers

Calculated AFB
Something larger (please explain)
Keep unprotected people out of the electrical room
It depends

Please feel free to elaborate READ MORE

2015 NFPA 70E – 10 Item Check Up

With the 2015 Edition of NFPA 70E being published and all of the changes that it brings, it is time to review your arc flash study, labels and overall practices.  There are many key areas that should be evaluated.  Here ten of the more important areas to look at to give your site a check up. Continue reading

Arc Flash Hazard Calculation Studies

In the earlier years of NFPA 70E and the emergence of arc flash protection requirements, many people would use the NFPA 70E Hazard/Risk Tables to determine what arc rated PPE to wear. This approach continues to shift towards the use of arc flash studies involving incident energy and arc flash boundary calculations based on IEEE 1584. Continue reading

IEEE 1584 – 125 kVA Transformer / Less than 240 Volts Exception

Download FREE Arc Flash Calculations

One sentence in the IEEE 1584 Standard, IEEE Guide for Performing Arc-Flash Hazard Calculations, frequently has people scratching their heads: Equipment below 240V need not be considered unless it involves at least one 125 kVA or larger low-impedance transformer in its immediate power supply. What does this sentence mean? What is so significant about 240 volts and 125 kilovolt-amperes?

Download:  125 kVA / 240 Volt Exception

 

 

 

About Jim Phillips: Electrical Power and Arc Flash Training Programs – For over 30 years, Jim Phillips has been helping tens of thousands of people around the world, understand electrical power system design, analysis, arc flash and electrical safety.

NFPA70E 2018 Update video by Jim PhillipsJim is Vice Chair of IEEE 1584, International Chairman of IEC TC78 Live Working and Steering Committee Member – IEEE/NFPA Arc Flash Collaborative Research Project. He has developed a reputation for being one of the best trainers in the electric power industry.  Learn More


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How to Perform an Arc Flash Calculation Study

This article by Jim Phillips provides an overview of how to perform an arc flash study.  It was originally presented at the 2010 NETA Conference.  InterNational Electrical Testing Association.

Arc Flash Hazard Calculations Studies guide

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Arc Flash Calculation Study
Many separate codes, standards and related documents are available regarding electrical safety and arc flash. However, a standardized recommended practice or guide that integrates all of the components into an Arc Flash Calculation Study does not presently exist. Continue reading