ASTM Standard Definitions:
ATPV (Arc Thermal Performance Value) is the incident energy on a material that results in a 50% probability that sufficient heat transfer through the specimen is predicted to cause the onset of second-degree burn injury based on the Stoll Curve, cal/cm².
Energy Breakopen Threshold (EBT) The incident energy on a material that results in a 50% probability of breakopen. Breakopen is defined as any open area at least 1.6 cm² (0.5 in.²)
Which is Better?
The question often arises as to whether an Arc Thermal Performance Value (ATPV) or an Energy Breakopen Threshold (EBT) is better protection in a garment system. When ASTM F1959 / F1959M (the Standard Test Method for Determining the Arc Rating of Materials for Clothing) was in development, we quickly recognized that some materials reach the skin burn prediction threshold (also called the Stoll Curve) before they “broke open” and others would not. Initially, these differing results became two ratings; an ATPV and an EBT. One was marketed as “better” than the other because some materials tested did not exhibit the “breakopen” phenomena before they reached the skin burn prediction threshold. This belief was erroneous and it was later determined that every fiber type could breakopen before the burn prediction level was reached. This happened most commonly in knits vs. wovens; knit materials typically receive an Arc Rating (EBT) rather than an Arc Rating (ATPV). Both are Arc Ratings and both are applicable for use in hazard assessment; they are functional equivalents.
Each construction and all fibers have advantages and disadvantages. The committee decided to encompass both ATPV and EBT and rename the term “Arc Rating”. After much consideration, the committee decided to leave the terms EBT and ATPV as a subscript or an addendum to the Arc Rating. Both are a 50% probability of the behavior at which the material compared to a burn model can cross the second-degree burn threshold under the tested conditions (this does NOT mean you have a 50% chance of receiving a second-degree burn). The EBT rated material has not exhibited second-degree burn threshold crossing on the sensors because the material has a one inch (2.5 cm) crack or a ½ square inch hole which is not directly over the sensor. Theoretically, if this had happened over the sensor a burn would be predicted, so the rating is cut off at this point. Both EBT and ATPV Arc Ratings are expressed in calorie/cm²; the lowest of the two is always reported. While the material receives either an EBT or an ATPV as the Arc Rating, both can be reported from testing. Only the lowest can be used as the Arc Rating on the clothing label according to the ASTM F1506 specification and as specified by NFPA 70E and CSA Z462. The same is true in the ISO/IEC 61482-1-1 standard.
A Few Things to Keep in Mind
- EBT means the fabric has NOT recorded a burn. The fabric has not superheated to the point of causing a burn reading on the sensor and only has a VERY small hole in it.
- ATPV is a 50% probability of the sensor crossing the Stoll Curve
- Arcs are VERY focused events and the worst energy is usually in the area of about the size two opened human hands. Prevention of ignition of garments and undergarments usually guarantees survival. Limiting burns to a small surface area makes for a better outcome. Prevention of all burns is great, but uncovered skin is likely to be burned.
- The arc rating is a comparative rating and has been proven to be VERY conservative. Most incidents investigated result in little or NO burns when the arc rating has been matched by the PPE. The calculation methods build in factors which have historically overprotected in almost all the cases. Typically, only “ejected arcs” (which are quite rare in working conditions), “tracking arcs” (high voltage arcs, >600V, in which the worker makes contact with the conductor) and oil filled equipment explosions produce energy higher than predicted by software. Over-dressing can lead to sweating, which frequently lowers clothing protection values.
The Big Picture
ATPV and EBT are both evaluated in the same test, but the first point to be reached is reported as the Arc Rating. If the material has more thermal insulative value than its tensile strength to heat, it exhibits breakopen first and receives an EBT rating. If the opposite is true, it will typically predict burns BEFORE it exhibits breakopen. Which is better? Neither! Wearing the PPE and matching to the hazard is the critical point.
If an EBT value is determined and it is found to be equal to or below a determined ATPV, then the EBT value shall be reported as the arc rating value and noted as Arc Rating (EBT).
If an EBT value is determined and it is found to be above a determined ATPV, then the ATPV result shall be reported as the Arc Rating (ATPV) of the tested specimen.
Another way to look at these numbers:
Arc Rating can be of two types:
- ATPV: This is 50% probability of predicted second-degree burn in the 8kA arc test on a flat panel.
- EBT: This is the 50% probability of a one inch crack in the material.
Neither is better. Basically, EBT fabrics are typically more insulative than they are strong and ATPV materials are stronger than they are insulative. Usually, EBT indicates the garment is a knit and is more comfortable but essentially no less protective to the user.
Hugh Hoagland performs arc testing at Kinectrics High Current Lab in Toronto and performs about 40-60 ratings each month on fabrics and PPE, as well as special projects and research and development. His company is ArcWear, LLC, a Textile Test Lab in Louisville, KY which is part of the FlashCert™ Consortium. Contact: firstname.lastname@example.org