many years ago the U.S. Electric utilities looked at the best harnesses on the market and determined that Nylon and Kevlar harnesses with the proper keepers, arc rated coverings for labels and a few flame-resistant modifications could meet 40 cal/cm². This was considered the “worst case scenario” at the time.
Since that time, further research has indicated that with weathering, Nylon harnesses will fail at 1/2 that level over time, so the minimum had remained. The committee has been considering this issue for years but with no recorded incidents of failure in the field, the old level has held. Over time there could be a move to weather by UV exposure and re-test to the arc, or a move to require a non-melting AR harness material, but without financial or accident support, this is unlikely. Our opinion at ArcWear is that at lower levels in arc testing, Nylon harnesses would be more dangerous. The 40 cal/cm² was picked long before the NFPA 70E arc PPE (ARC) was set at 40 cal/cm², so these are unrelated.