NFPA 70E, the standard for electrical safety in the industrial workplace, is used to provide guidance when selecting thermal personal protective equipment for employees who may encounter electric arc hazards from electrical equipment. The standard includes testing methods for apparel (ASTM F1959), face protective equipment (ASTM F1959) and rainwear (F1891). In each of these methods, the resulting arc rating is expressed as the lower of ATPV (arc thermal performance value) or EBT (energy of break open threshold).
NFPA 70E establishes a hazard assessment requirement in which employers must identify exposure potentials or use the predefined five arc exposure hazard/risk categories ranging from 0 to 4. The results are used to identify the minimum arc rating (cal/cm2) and PPE requirements required for worker safety when performing specific tasks within an arc flash boundary. Selecting materials with arc ratings higher than the estimated exposure hazard generally leads to more protection for the wearer.
ATPV is the incident energy during arc testing on a material, or a multilayer system of materials, which exceeds thermal protective capabilities and results in a model-predicted second-degree skin burn injury.
EBT is the incident energy on a material, or material system, that creates a physical failure based on a specific tear length or hole formation (expressed as “break open”). When material break open occurs, there is potential for heat to pass through the material unimpeded and onto the wearer.
Materials that report an EBT value as their arc rating exhibit break open before the onset of a model-predicted second-degree burn injury.
Is one measure better than another?
ATPV and EBT values are expressions of different attributes of a FR material system and can result in significantly different levels of protection.
For single-layer systems, materials with arc ratings derived from the ATPV value are the typical systems available in the marketplace. In this case, the materials of construction are expected to remain intact during an arc exposure to provide thermal protection to the wearer. If an EBT arc rated material is exposed to energies somewhat higher than its break open value, the potential for direct skin exposure – or worse, non-FR underlayer ignition – may result in additional injuries.
For systems in which layering is employed, it is possible to obtain higher levels of thermal arc protection with materials that have EBT arc ratings. An FR outer layer of material that breaks open can remove a significant amount of thermal energy (a 50+ percent increase in arc thermal protection has been observed). The risk with these types of systems is a chance of directly exposed skin and, at some point, non-FR underlayer ignition.
What can be done to improve ATPV or EBT?
Important factors that can influence a protective systems arc rating (ATPV or EBT) are the weight, fabric construction and fiber components.
A known factor for improving EBT is the addition of fibers or layers that combines high strength retention after thermal exposure and low flame shrinkage (e.g., para-aramids). For ATPV, increasing a system’s insulative property with thickness and numbers of layers are typical means used to improve thermal performance.
When evaluating a garment’s arc protection, a wearer should recognize that standard compliance does not always mean equal protection and FR PPE is meant as a last line of defense.
Editor's note: This article represents the independent views of the author and should not be construed as a National Safety Council endorsement.