An arc rating is derived by exposing fabric samples to a controlled electric arc, measuring how much heat energy passes through the material using copper sensors, and then using statistical analysis to find the energy level at which the fabric has a 50% chance of allowing a second-degree burn. The final number, expressed in calories per square centimeter (cal/cm²), represents the maximum incident energy a garment can handle before it stops providing adequate protection.
The process combines physical testing, burn injury science, and regression statistics into a single value that workers and safety managers can use to match protective clothing to electrical hazards.
The Test Setup
Arc rating tests follow standardized methods, primarily ASTM F1959 in North America and IEC 61482-1-1 internationally. Both use an open electric arc under controlled laboratory conditions. Flat fabric specimens are mounted in a static, vertical position and exposed to the arc. The fabric doesn’t move during the test except from the force of the exposure itself.
Behind each specimen sits a copper disc calorimeter, a small sensor that absorbs the heat passing through the fabric and converts the resulting temperature rise into a heat energy measurement. Additional copper sensors face the arc directly, without fabric in front of them, to measure the total incident energy the arc delivers. At least 20 fabric samples are tested in a single evaluation. Because arc exposures are inherently variable, different sensors can register different heat transfer values even under the same conditions, which is why so many samples are needed.
How Copper Sensors Measure Heat Energy
The copper disc sensors are the backbone of the measurement. Each sensor has a known mass and exposed surface area, and the test records its temperature before and after the arc exposure. The temperature rise is then converted to heat energy using a detailed equation that accounts for how copper’s ability to absorb heat changes at different temperatures. This isn’t a simple multiplication. The calculation uses polynomial terms that adjust for copper’s heat capacity across a wide temperature range, producing a result in kilojoules per square meter.
Two sets of readings come out of every test. The “incident energy” sensors (unshielded) tell you how much total energy the arc delivered. The “transmitted energy” sensors (behind the fabric) tell you how much heat got through. The transmitted energy is what determines whether the fabric would have caused a burn.
The Stoll Curve and Burn Prediction
Whether a given amount of transmitted heat would cause a burn isn’t a simple threshold. It depends on both the intensity of the heat flux and how long the skin is exposed. In the 1960s, researcher Alice Stoll developed a curve mapping the relationship between heat flux and the time it takes to produce a second-degree burn on bare skin. Her experimental data covered heat fluxes from about 4.2 to 16.8 kW/m², later extended theoretically up to 42.0 kW/m².
In arc rating tests, the transmitted energy curve from each sensor behind a specimen is plotted against the Stoll curve. If the transmitted energy exceeds the Stoll curve at any point during the exposure, that sample is scored as a predicted burn. If it stays below, no burn. Each of the 20-plus specimens gets a simple binary result: burn or no burn.
Logistic Regression Produces the Final Number
With all samples tested, the data set contains pairs of values: the incident energy each specimen received and whether that specimen resulted in a predicted burn. A logistic regression analysis fits a probability curve to this data, modeling the relationship between incident energy and the likelihood of a burn.
The arc thermal performance value (ATPV) is the point on that curve where the probability of a burn reaches exactly 50%. This is the number reported as the arc rating. The regression approach accounts for the natural variability in arc exposures, since not every specimen receives the same incident energy, and turns scattered pass/fail data points into a single, statistically meaningful value.
One important source of uncertainty in this process comes from errors in the incident energy measurements themselves. The logistic regression assumes the incident energy values are precise, but since they’re calculated from high-temperature calorimeter readings, they carry their own measurement uncertainty that can influence the final ATPV.
ATPV vs. EBT: Two Possible Ratings
The same test evaluates two failure modes simultaneously. ATPV measures heat transfer through intact fabric. But some materials physically break open under arc exposure before they transfer enough heat to cause a burn. Breakopen is defined as any opening in the fabric of at least 1.6 cm² (about half a square inch).
The energy breakopen threshold (EBT) is the incident energy at which there’s a 50% probability the fabric will develop such an opening. If a material’s EBT is lower than its ATPV, the fabric tears apart before it would transfer a dangerous amount of heat, and the EBT becomes the reported arc rating. If the ATPV is lower, meaning the fabric would allow burns before it breaks open, the ATPV is reported instead. Either way, the lower of the two values is what appears on the garment label, because it represents the point where protection fails first.
How Layered Systems Are Rated
When workers wear multiple layers of arc-rated clothing, the total protection isn’t calculated by adding individual arc ratings together. Layered systems must be tested as they would actually be worn, because the combined rating is often significantly higher than the sum of the parts.
A practical example illustrates why: a long-sleeve base layer rated at 6.4 cal/cm² worn under a coverall rated at 9.0 cal/cm² doesn’t produce a system rating of 15.4 cal/cm². When tested together, the combination rates at 21 cal/cm². The air gap trapped between layers acts as additional insulation, boosting protection beyond what either garment provides alone. How much extra protection you get depends on the specific fabrics, their weight, and how they fit together.
Open Arc vs. Box Test Methods
The open arc method described above, used in both ASTM F1959 and IEC 61482-1-1, produces a specific cal/cm² value. Europe and other international markets also use a second approach defined in IEC 61482-1-2, commonly called the “box test,” which uses a constrained arc inside an enclosure rather than an open arc.
The box test doesn’t produce a cal/cm² rating. Instead, it classifies fabrics into arc protection classes: APC 1 and APC 2, corresponding to two fixed test energy levels. The open arc method gives more granular information for hazard matching, while the box test offers a simpler pass/fail classification. Many manufacturers test to both standards to sell protective clothing globally.