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.
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