How is Arc Rating Determined?

Wearing flame resistant (FR) clothing is critical in the event of an arc flash because it will not ignite and continue to burn, which is what causes most of the catastrophic injuries and fatalities. Arc-rated FR clothing also performs a second function – insulating you against the hazard so that you’re not burned through the fabric. An arc rating is a direct measure of the fabric’s ability to insulate the wearer from second-degree burn through the fabric. How is this determined? Through ASTM F1959, the Standard Test Method for Determining the Arc Rating of Materials for Clothing.

Just as the name states, ASTM F1959, is the official arc rating test standard. Arc rating can be measured as Arc Thermal Performance Value (ATPV) or Energy to Break Open Threshold (Ebt). There is no difference in the protection provided; whether the arc rating is ATPV or Ebt, you are protected from second degree burn at or below the arc rating of the fabric.  The only difference is whether the fabric allows a burn through it (ATPV) or a hole in it (Ebt).

Listen to Scott Margolin, Tyndale’s VP of Technical, as he describes how the test works in determining the arc rating of FR fabric:

As Scott states, the ASTM F1959 test exposes 3 swatches of fabric per arc, each a foot away, to 7 arcs. This generates 21 unique data points for analysis. These data points are shown as red dots on the arc rating report. The red dots on the bottom, horizontal axis mean neither a burn through the fabric or hole in the fabric. Red dots at the upper, horizontal axis mean there was either a burn through or hole in the fabric. To analyze the data, the computer draws a back line, which is the average of the red dots. To determine the arc rating, find the point where the black line intersects with the 50% reading on the vertical axis, then draw a straight line down from that point of intersection to the horizontal access.

You may wonder, why test against a 50% probability. But as Scott explains to achieve 100% certainty of protection would require an astronomical number of data points – more that could reasonably be collected.  The test is so good that there is very little difference between low and high probabilities; usually less than a calorie, as you can see in the chart to the right of the graph in an arc rating report.

Once we have an arc rating you can compare it to the incident energy that’s possible in the equipment you’re working on and as long as the arc rating is greater than the incident energy that could be coming out in the arc, you can be confident you won’t be burned.

So, while you may never be called upon to read an arc rating report, you now have a fundamental understanding of the work that goes into confirming that the AR/FR garments you wear on the job provide the level of protection you need to stay safe.

Have other technical questions? Schedule a free 15 minute consultation with Tyndale’s VP of Technical, Scott Margolin.

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