Arc Flash Hazards: New Research and Report from 2016 IEEE Electrical Safety Workshop
The Institute for Electronics and Electrical Engineers (IEEE) Industry Applications Society, Electrical Safety Committee held its annual electrical safety workshop (ESW) March 6-11, 2016. "Since its founding in 1991, the IEEE IAS Electrical Safety Workshop has served to accelerate the dispersion of information and knowledge impacting electrical safety."1 The ESW brought together mainly electrical engineers, with some safety professionals to discuss the latest research and other discussions related to electrical safety.
An arc flash workplace incident is the result of an electric arc of sufficient uncontrolled energy to cause a fire, explosion or blast and damage to equipment or installations and skin burns and serious personal injury for persons in the arc path. A tutorial before the beginning of the platform presentations, "What you need to know about electrical arc protection that is not in the safety standards yet," contained technical information and data on how to best characterize the behavior of different types of electric arcs and protection from an arc flash. Arc rating is determined by using ASTM F1959, "Standard Test Method for Determining the Arc Rating of Materials for Clothing"2 or IEC 61482-1-1, "Protective Clothing against the Thermal Hazards of an Electric Arc, determination of the arc rating test methods." Heat energy penetrating through a test fabric is determined as an average reading from calorimeter sensors mounted on the test panel behind the fabric. Heat energy penetrating through the test fabric is compared to the thermal criteria for a second degree (survivable) skin burn called the Stoll Criteria.
The tutorial provided evidence and thought-provoking discussion among attendees that calculation methods for arc flash incident (heat) energy are not conclusive for arc flash risk assessment. Incident energy and arc thermal performance value (ATPV) is attributed to materials that describe their performance to exposure from an electrical arc. But this tutorial discussed that the main value of the ATPV is comparison between different fabrics. The tutorial provided information and differences among the five different types of electric arcs -- open air arc, arc in a box, moving or running arc, ejected arc, and tracking arc. The majority of low-voltage electrical equipment installations are enclosed switchgear, control and power enclosed panels, cabinets and motor control centers. These types of installations from an electric arc perspective all are represented by the arc in the box type of arc flash event. Medium- and high-voltage power lines and substation bus work are represented with a moving or ejected arc.
In the same vein as the tutorial, results of research during a presentation comparing test parameters with respect to arc currents, time for ignition from different arc types and different electrical system geometric configurations showed non-FR clothing ignition variability.3 This research demonstrated that the ATPV is not universal for any electric arc discharge or for all types of arc flash events. For example, the ejected arc resulted in non-FR clothing ignition more quickly than any of the other types of arcs. "ASTM F1959 or IEC 61482-1-1 test method is not a constant and is dependent on many factors. It cannot be used as an absolute and uniform measure of fabric thermal protective properties."4
An argument was made from the tutorial and complementary research testing arc-rated flame-resistant (FR) fabric that time current curves that describe the protective clothing's Stoll Criteria performance can be used to best identify safe and unsafe conditions. "Safe or unsafe conditions can be established on site knowing clothing protective time current curves, fault current and duration even if incident energy calculations have not been done (or are incorrect)."
Concerns with incident energy calculations are many. Models contain several assumptions that may or may not be true. All calculation methods currently used are based on a single arc type. No model or correction factor for incident energy calculations currently exist for an ejected arc, and calculated arc energy is often inaccurate because of the necessary model assumptions.
Arc flash and arc blasts are difficult concepts, and the engineering descriptions of these events currently best describing the hazards are not agreed upon by all. For example, the hazards are dependent on the type of arc involved in an arc flash incident. The calculation of arc energy is dependent on the type of arc that occurs. Electric arc protection is based on a match of PPE arc rating to calculate incident energy based on several factors, such as the available fault current and overcurrent protective device clearing time. This concept forces one to match the PPE ATPV to an "often inaccurate estimation of a potential hazard resulting in inadequate protection." In addition, electric arc testing arrangements for ATPV experimental determination on one hand and the calculation model for incident energy values on the other hand are based on completely different types of arc, so that the experimental ATPV number and the calculated potential hazard are not directly compatible.
All current risk assessments are done based on direct exposure to incident energy, which is an extremely complex quantity that has a large number and variety of influences. The main take-away is that arc flash calculations are an estimate of the predicted heat flux used for helping to determine the type of protective clothing needed. In all circumstances, arc-rated clothing helps to reduce the severity of injuries due to an arc flash incident. The incident energy involved is secondary to providing a minimum amount of protective equipment, which will help mitigate the effects of a discharge of electricity through air with accompanying ionized gas with extreme temperatures, electromagnetic forces, and molten components of electrodes and other flying debris. The methodology for arc risk assessment is currently being discussed among IEEE 1584, IEEE Guide for Performing Arc Flash Hazard Calculations, committee members.
Past practices must be integrated with future research that provides safety professionals with practical information for protection against arc flash and electrocution best done through design interventions. Nonetheless, research presented at past IEEE workshops and confirmed from a 2014 study of OSHA electric fatality data showed electrocution among non-electrical workers is the most common cause for an occupational electrical fatality.5
Gavin Burdge ([email protected]) is a freelance occupational safety and industrial hygiene writer in Lemoyne, Pa. Mikhail Golovkov is the president and owner of ArcFlash-CRT (Calculations, Research and Testing) in Chalfont, Pa.
References
1. IEEE Electrical Safety Workshop, "Changing the electrical safety culture," retrieved from: http://www.ewh.ieee.org/cmte/ias-esw/esw-history.php.
2. Available at: http://www.astm.org/Standards/F1959.htm
3. Golovkov, M. and Schau, H, "Effect of Arc Electrode Geometry and Distance on FR Fabric Protection Properties Against Second Degree Skin Burn," IEEE 2016 Electrical Safety Workshop, IEEE copyrighted paper No 15.
4. Ibid
5. Burdge, G.F. and H.L. Floyd, "Electrical Fatalities Reported by Federal OSHA for Calendar Year 2014 with a Consideration of Design Interventions," IEEE 2016 Electrical Safety Workshop, IEEE copyrighted paper ESW2016-37.
Posted by Gavin Burdge, Mikhail Golovkov on Apr 21, 2016