Q. Can we suffer a burn from an arc flash if the incident energy is less than 1.2 calories per centimeter squared?
A. Yes, hazards can still exist at this level although, it’s important to understand how to interact with electrical equipment to stay safe.
Estimating the injury due to arc flash
NFPA 70E states-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.
NFPA 70E 130.7(C)(6), Body Protection, states, “Employees shall wear arc-rated clothing wherever there is a possible exposure to an electric arc flash above the threshold incident energy level for a second-degree burn [1.2 cal/cm2].” Essentially what this means is that employees must wear the proper PPE to protect their bodies from a potential arc flash if the incident energy is calculated to be 1.2 calories per centimeter squared or greater. If the incident energy is calculated to be less than 1.2 calories per centimeter squared arc rated PPE would not be required because not enough heat energy would be released to produce a second-degree burn. What is important to understand is that incident energy is calculated at a distance.
Hazards can exist when working below 1.2 cal/cm^2
There is a direct relationship between distance and heat. The closer we are to a source of heat the hotter it is. When an equipment label 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 from the point of the arc), then your face and body should not receive a blister or worse in the event of an arc flash. However, your hands and arms will almost always be closer than this distance consequently they can be exposed to a level of heat energy that is greater than 1.2 calories per centimeter squared.
How do we stay safe?
Because the hands and arms will most likely be inside of the working distance Rozel recommends that employees wear a long sleeve non-melting shirt (i.e., cotton) at a minimum. Preferably, we suggest an arc rated daily wear shirt which generally has an arc thermal performance value (ATPV) of 8 calories per centimeter squared.
Hands should be protected by gloves
OSHA as well as NFPA 70E requires that if your hands are closer than the restricted approach boundary (example: 12″ to 480V) then your hands must be insulated by wearing voltage rated gloves with leather protectors. Remember that a low arc flash hazard doesn’t mean a low shock hazard. Not only will these gloves protect your hands from the shock hazard they will also keep you from receiving a burn.
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 proper PPE on your hands and arms if you place them closer than the working distance.