Arc ratings matter for safety

The revised NESC contains significant changes to requirements regarding arc flash protection and the use of flame-resistant protective apparel. FR clothing must be worn; must be the outermost layer; and must cover the body, including torso, arms and legs. Depending on the level of an arc flash hazard, the clothing must have an arc rating of at least 4, 8 or 12 calories.

The standard that determines arc rating is called ASTM F-1959. There is much to like about this standard: all testing is done at one independent lab (Kinectrics, Toronto), the standard is tightly written and thus allows essentially no room to bias results, and ratings are expressed as one number (the higher the better). These things combine to make the ratings accurate, precise, and easy to understand and implement.

Arc rating is defined as the amount of energy a given fabric can withstand before a 50 percent likelihood of the onset of second-degree burn through the fabric. In other words, what we are really measuring is insulation. The arc rating can be expressed either as an Arc Thermal Performance Value or Energy to Breakopen Threshold; fabrics that consistently break open before the onset of second-degree burn are given arc ratings expressed as EBT, rather than ATPV.

The two areas of the standard most often viewed as more problematic are EBT and laundering. When choosing FR fabrics, it is important to understand that all arc ratings are not the same. Arc ratings reported as EBT will break open at their arc rating, potentially exposing flammable undergarments or skin to arc effects at only a calorie or two above the rating. Conversely, most fabrics with arc ratings expressed as an ATPV usually do not break open until about twice the arc rating, allowing a much larger margin of safety. This leads many people to specify only fabrics with ATPVs.

Laundering also is an area worthy of consideration. The ASTM F-1959 standard requires tested fabrics to be washed three times and dried once prior to testing. The FR garments made from these rated fabrics are designed to last anywhere from several months to five years, during which they will obviously be laundered many more than three times. ASTM F-1506, which many people elect to specify as well, requires 25 launderings, but this is still only about six months of normal use. So, while the standards available today do an excellent job of rating protective performance of new FR fabrics, they do not tell us what will still be flame-resistant two, three or four years later.

The standard is a precise and highly valuable tool, but it is not a substitute for careful evaluation. Given the issues of EBT and laundering, it is clear that simply accepting any product that passes the standard is not enough. With so many inferior generics and foreign imitations flooding the market, resulting in several recent public failures and recalls, it is more important than ever to look for and specify market-proven products.