I have received conflicting information on arc ratings from two people I respect in the industry. This concerns wearing the minimum required PPE when using the incident energy analysis method 130.5(G) and T130.5G and informational, note 2 & 3 for the definition Arc Rating.

The first piece of information is that if you wear the minimum PPE, you have a 50% chance of getting a second-degree burn, so never dress to the minimum.

The second piece of information is that this is not how the stoll curve is meant to be applied, and wearing the minimum PPE will not get you burned if the incident energy does not exceed the PPE rating.

On a side note, are manufacturers permitted to give the clothing an arc rating higher than that of their lowest-tested specimen? Is this a concern?

Informational Note No. 2: ATPV is defined in ASTM F1959/F1959M, Standard Test Method for Determining the Arc Rating of Materials for Clothing, as the incident energy (cal/cm2) on a material or a multilayer system of materials that results in a 50 percent probability that sufficient heat transfer through the tested specimen is predicted to cause the onset of a second degree skin burn injury based on the Stoll curve.

I don't know this for a fact, but as you may know, some manufacturers will list, for example, a suit with a 40 cal/cm^2 rating. But when inspecting the tag, you'll see an arc rating of 45 cal/cm^2. I always thought this was to cover the 50% break-thru (for 40 cal/cm^2), but I have never asked the question.


Mike

You're right that the ATPV (Arc Thermal Performance Value) and EBT50 are statistical values derived from test data, indicating a 50% probability of exceeding the threshold for second-degree burns. This means that if you purchase PPE rated at an ATPV of 22 cal/cm² and your incident energy calculation for a workplace is 21.5 cal/cm², you might still be within the "probability zone" for injury.

This uncertainty is one reason why PPE with such ratings struggled to gain certification in Europe for some time, leading to the introduction of the ELIM (Energy Limit Value) in IEC 61482-1-1. Unlike ATPV, ELIM represents an energy value that is always below the lowest incident energy that caused a failure during testing.

The attached image illustrates how these values are calculated from test results. As shown, one test point marked as a "fail" is slightly below the calculated ATPV.

To ensure greater safety, you can request the ELIM value from the PPE manufacturer or choose an arc rating slightly higher than your maximum incident energy. Personally, I advocate for choosing PPE that is rated as high as necessary but as low as possible for better ergonomics.

Regarding why some PPE might seem "under-rated" (e.g. marked as 40 cal but tested as 45 cal), this often occurs when PPE is sold as a kit. Individual items in the kit may have higher arc ratings, but to avoid confusion, all items are marked with the rating of the lowest-rated item.