Ground Rules by John Cadick: Electrical Safety Workshop 2000 - Part 2, Predicting Incident Arc Energy
This week I continue with my coverage of the Electrical Safety Workshop held in San Antonio February 16-18. A few weeks ago in my column I covered the electric arc injury. During the Electrical Safety Workshop Mr. Dick Doughty presented an excellent paper on some very recent research into electrical arc incident energy. The paper is co-authored by Thomas E. Neal and H. Landis Floyd. It was published in the January/February 2000 issue of the IEEE Transactions on Industry Applications. If you are interested you can contact the IEEE for a copy.
The state-of-understanding of energy received from electrical arcs is relatively immature. This is in spite of the fact that the original work was done by Ralph Lee over twenty years ago. In this paper the authors describe a test program that measured the incident energy from a 6 cycle (60 Hz base) arc on a 600-Volt system.
The test system creates a three phase electrical arc and then measures incident thermal energy at various distances from the arc. The paper reports measured energies for a variety of different arc currents and distances; moreover, the authors performed tests for open-air arcs (energy is free to radiate 360 degrees) and for what they refer to as "arc in a box." The arc in a box test confines the arc into a small (20 cubic inches) compartment. The effect of this is to "focus" the arc energy out of the open side of the box.
The paper reports some rather astounding results. Most significant is that the incident energy received when in front of the arc-in-a-box are from 1½ to 3 times greater than that received from an open air arc. This is especially critical in light of the fact that almost all "real world" arc exposure is in the form of an open front box (cubicle).
Received energy for the arc-in-a-box is reported in tabular form. They report values from a low of 0.7 cal/cm2 to (16 kA at 60 inches) to a high of 17.5 cal/cm2 (50 kA at 18 inches). All but the lowest values are capable of producing severe burns of human skin. Based upon their experimental results, they developed equations for determining the amount of energy received for open air (equation 1.1) and for arc-in-a-box (equation 1.2).
EMB = 1038.7D–1.4738tA[0.0093F2–0.3453F + 5.9675] (equation 1.2)
Where:
Of course, while such research is important unto itself, of greater importance is how the information is used in the provision of additional safety procedures. The authors use their research date to present a seven step process for the selection of protective clothing. This procedure is paraphrased below:
- Determine the maximum available three-phase fault current at the location of the exposure to the arc. (Short circuit analysis)
- Determine the arc fault clearing time based on the electrical protection system. (Protective device coordination)
- Determine if the exposure is of the "arc-in-the-box" or the open air type.
- Determine how close the worker must approach the arc location. (Minimum approach distance)
- Use equations 1.1 or 1.2 to determine the maximum incident energy at the minimum approach distance.
- If the calculated energy is less than 1.2 cal/cm2 clothing may not be required for arc incident energy. (Remember that other burns are possible from the plasma cloud and/or flying debris)
- If the arc incident energy is greater than 1.2 cal/cm2 the authors provide a table recommending the appropriate level of protective clothing. (Note that a similar table will be published in NFPA 70E 2000 edition due to be released within the next couple of months)
All put together this paper is a very well written and technically excellent effort. I strongly recommend it as a reference for anyone concerned with or establishing an electrical arc safety program.
The URL for the IAS Electrical Safety Workshop website is http://www.ewh.ieee.org/soc/ias/pcic/safety/esw.htm. There you can find the Power Point presentations for many of the presentations made in 1999. The year 2000 presentations will be posted in a few weeks.
A registered professional engineer, John Cadick has specialized for three decades in electrical engineering, training, and management. In 1986 he created Cadick Professional Services (forerunner to the present-day Cadick Corporation), a consulting firm in Garland, Texas. His firm specializes in electrical engineering and training, working extensively in the areas of power system design and engineering studies, condition based maintenance programs, and electrical safety. Prior to the creation of Cadick Corporation, John held a number of technical and managerial positions with electric utilities, electrical testing firms, and consulting firms. Mr. Cadick is a widely published author of numerous articles and technical papers. He is the author of the Electrical Safety Handbook as well as Cables and Wiring. His expertise in electrical engineering as well as electrical maintenance and testing coupled with his extensive experience in the electrical power industry makes Mr. Cadick a highly respected and sought after consultant in the industry.