Arc flash testing
How does arc testing differ from real-life arc flash incidents?
Responding is Hugh Hoagland, senior consultant; Stacy Klausing, PPE project manager; and Jill Kirby, lab manager, ArcWear, Louisville, KY.
Arc flash testing, like any test method, is intended to provide a way to compare materials in a controlled testing environment with a high amount of repeatability. In an ideal situation, the test method also would replicate real-life scenarios. This can be difficult to achieve – especially with a hazard as unpredictable and variable as an arc flash.
The commonly used arc flash test method ASTM F1959 is an open-air arc test, and the results are intended to be conservative. The arc rating in testing is determined using a statistical analysis to calculate the incident energy level that results in a 50 percent probability of crossing the Stoll Curve (a theoretical skin burn injury model), which is used to predict the “onset” of a second-degree burn. This has given many people the impression that 50 percent of the time the worker will receive second-degree burns; this is not the case (see “Update of field analysis of arc flash incidents, PPE protective performance and related worker injuries” IEEE XPlore, Dan Doan, et.al.). Several factors make this test conservative:
- When an arc flash occurs in real life, it typically is a focused event that creates exposure to less than 25 percent of the body. If the clothing does not melt and drip or ignite, and continues to burn, the areas of the body exposed to the damaging effects of an arc flash are minimized.
- Clothing ignition increases the risk of body burns and greatly reduces survival rates; the ASTM F1959 standard-tested materials are never allowed to continue burning or to melt and drip onto workers’ skin if they receive an arc rating.
- The test is set at one fault current, which is near the worst case, while allowing 100 cal/cm² exposures without destruction of the test apparatus. Higher fault currents produce the energy faster and would produce a higher rating.
- The metal used for electrodes in the test is stainless steel. This delivers more energy to the fabric surface because of the smoke from copper or aluminum from normal equipment.
- The test is performed on a flat fabric panel tight against the sensors so any folds, double layers or curves in the fabric – as in a real garment – will result in less exposure.
The test has been developed and changed since the early 1990s, and substantial research has gone into the test method and hazard assessment methods, such as IEEE 1584. The predictions from IEEE 1584 are conservative, so matching to the hazard with an arc rating gives a very low risk of any burns to a worker.
IEEE, ASTM and IEC standards committees continue to refine the standards to prevent injury while balancing practical personal protective equipment for workers when they must work energized or are exposed to energized parts that could fail and cause worker harm. Although the open-air arc flash test method does not replicate every real-life scenario (and it is not intended to), it is a conservative test that adequately measures a material’s ability to protect and reduce injury when exposed to the hazard. When matched with NFPA 70E or IEEE 1584 engineering calculations, workers typically walk away from an incident unscathed.