Do arc flash hazards exist below a certain level, i.e. less than 1.2 cal/cm^2?
answer: YES, hazards still exist. At this level though, it’s important to understand how to interact with electrical equipment to stay safe.
Category 0 disappeared from NFPA 70E years ago so if the rating doesn’t exist, then why do workers have to worry? Arc flash hazards are calculated at a working distance, the further you are from a piece of equipment, the safer you are from an arc flash, regardless of arc flash size. Most likely your labels indicate an arc flash hazard when standing 18″ from that equipment (this is the most common working distance), if instead the labels were calculated based on a 36″ working distance, the calculated arc flash value would be less. Similarly, if the same label were calculated based on a 6″ working distance, the arc flash value would indicate a greater hazard.
Per NFPA 70E 2021, the definition of the arc flash boundary is: When an arc flash hazard exists, an approach limit from an arc source at which incident energy equals 1.2 cal/cm^2 (5 J/cm^2).
Informational Note: According to the Stoll skin burn injury model, the onset of a second degree burn on unprotected skin is likely to occur at an exposure of 1.2 cal/cm^2 (5 J/cm^2) for one second.
When equipment indicates a hazard less than 1.2 cal/cm^2 at an 18″ working distance it simply means, if you’re face and body are where they should be (18″ away), then your face and body should not receive a blister or worse in the event of an arc flash. Your hands and arms however, will almost always be closer than this distance. This reason is why we recommend at a minimum of a long sleeve non-melting (i.e. cotton) daily work shirt, more preferably we suggest an 8 cal/cm^2 ATPV daily wear shirt. Additionally, per OHSA and NFPA 70E if your hands are closer than the restricted approach boundary (12″ for 480V) then you must have on voltage rated gloves with leather protectors. Remember that a low arc flash hazard doesn’t mean a low shock hazard.
All equipment with a rating of less than 1.2 cal/cm^2 should be treated very similarly. Even if your label indicates a hazard of less than 1.2 cal/cm^2 at 18″ (or if an older label says Category 0), you should still have to have proper PPE on your hands and arms to put them closer than 18″.