How to Read an Arc Rating Report
When you face arc flash hazards on the job, it’s essential to choose arc rated, flame resistant garments that offer you sufficient protection against the risk. In most cases, you use an Arc Rating or CAT level to determine which garments will protect you from the level of hazard present in your work environment. But where do those number come from?
Arc Ratings are determined using ASTM F1959, the Standard Test Method for Determining the Arc Rating of Materials for Clothing. The ASTM F1959 test exposes 3 swatches of fabric per arc, each a foot way, then conducts 7 arcs. This generates 21 unique data points for analysis.
AR/FR clothing has two main goals:
- Do not ignite and continue to burn (thus saving the wearer’s life); and
- Minimize or eliminate 2nd degree burn or worse (thus reducing or eliminating injury).
A fabric must be FR (ASTM D6413) to qualify for arc rating testing; the arc rating determines how much energy the fabric can block before you’d get a second degree burn through it. You can think of the arc rating test as binary. Red dots on the upper line “failed” the test due to burn or breakopen, and red dots on the bottom line “passed” the test because they neither allowed a 2nd degree burn nor broke open.
That data and analysis is included in an Arc Rating Test Report. Seen here is an excerpt from the results page of one such report calculating Ebt (as we’ve noted in other blog posts, ATPV and Ebt are equally effective measures of protection) :
In the following video, Tyndale VP of Technical, Scott Margolin explains how to read these results to determine the level of protection the tested fabric will provide.
As Scott explains in the video, to determine the fabric’s arc rating:
- Find the point where the black line (which is the computer-generated average of the red dots) tracking the data points intersects with the 50% reading on the vertical axis.
- Then draw a straight line down from that point of intersection to the horizontal access to find the arc rating.
In the case of the test data shown above, the arc rating for the fabric being tested is between 23 and 24, placing it in CAT 2. That rating can now be applied to the fabric that was tested, as well as any garment made with that fabric.
But why test against a 50% probability? Doesn’t that mean there’s as much of a chance you will be burned as there is that the garment will protect you? Not really. As Scott explains in the video, to achieve 100% certainty of protection would require an astronomical number of data points – more in fact that could reasonably be collected. Further, when you match probability to Incident Energy in the chart to the right of the report shown above, you’ll see that at 50% probability the incident energy tracks very closely to the determined arc rating revealed by the test.
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.
