To better understand arc ratings – what they are and the difference between the two ways they are measured – we define them below and link back to Tyndale’s Arc Week Season 3, which covers the science behind all things arc flash.
Simply put, an arc rating directly measures a fabric’s ability to insulate the wearer from second-degree burn through the fabric. The higher the arc rating, the more protective the garment. The magnitude of an arc rating is measured by its arc incident energy level. Tune in to Arc Week Season 3, episodes 1 and 2, for an explanation of how arc incident energy levels are evaluated to produce an arc rating. There are two types of arc ratings and although they are equal, they are achieved using different methods.
Tune in as Scott Margolin, Tyndale’s Vice President of Corporate Strategy and Technical explains the difference between ATPV and EBT:
The arc rating is the number of calories that the garment is expected to “absorb” if exposed to an electric arc. Arc rating is, in essence, the level of protection provided to you, the wearer.
A calorie is the unit of measure of the heat energy of an arc flash and the protective level of FR clothing. The bigger the calorie number, the greater the heat energy level of arc flash and the greater the protective level of the clothing. You will be protected from an electric arc if your clothing has a higher calorie arc rating than the calories of heat generated by the arc.
ASTM F1959, Standard Test Method for Determining the Arc Rating of Materials for Clothing, is the official method for determining how much heat from an arc flash a certain fabric (or system of fabrics) will block before the onset of second-degree burns to the wearer. There are two ways to achieve an arc rating: Arc Thermal Performance Value (ATPV) and Energy Breakopen Threshold (EBT). Both are reliable measurements of an arc rating; however, the first one to be reached is reported as the arc rating.
All FR fabrics will break open if exposed to sufficient energy. If an arc-rated fabric is exposed to energies higher than its breakopen value, the potential for direct skin exposure or non-FR inner layer ignition may result in additional injuries. Important factors influencing a protective system’s arc rating (ATPV or EBT) are the fabric’s weight, construction, and fiber make-up.
Arc-rated (AR) clothing must meet the ASTM F1506 standard, Standard Performance Specification for Textile Material for Wearing Apparel for Use by Electrical Workers Exposed to Momentary Electric Arc and Related Thermal Hazards. Under this standard it is required that the fabric pass several tests, including wash testing and the vertical flammability test as per ASTM D6413. It is also required that a garment be properly labeled with its arc rating. As defined by ASTM F1506, only the lowest value is used on the clothing label.
If an EBT value is determined to be equal to or below a determined ATPV value, then the EBT value is reported as the arc rating and is noted as “Arc Rating (EBT)” on the garment label. EBT fabrics are typically more insulative than they are strong, and generally, ATPV fabrics are stronger than they are insulative.
A worker exposed to a potential arc flash hazard must understand the arc incident energy level possible and the protection level of the PPE they are wearing (the arc rating). From there, they must match the two so that the PPE they’re wearing meets or exceeds the incident energy level of the potential hazard, ensuring they are not burned. While it does not matter if the fabric has an EBT or ATPV value, it is important to pay attention to the calorie level the fabric can support (as expressed in cal/cm2):
|Level||Minimum Arc Rating (cal/cm2)|
|CAT 1||4.0 cal/cm2|
|CAT 2||8.0 cal/cm2|
|CAT 3||25 cal/cm2|
|CAT 4||40 cal/cm2|
Please note, the 2015 edition of NFPA 70E eliminated the term hazard risk category (HRC), replacing it with the term PPE Category (CAT).