I still hear this more than I should. Someone wants to list “no arc flash hazard exists” on the arc flash warning label. When I ask why, the response is usually along the lines of, “Because the incident energy is less than 1.2 cal/cm2 (calories per centimeter squared).” Really?
It’s no secret that 1.2 cal/cm2 is the generally accepted level of incident-energy exposure where the onset of a second-degree burn may occur. This value also triggers the requirement for arc-rated clothing and personal protective equipment, according to NFPA 70E, Standard for Electrical Safety in the Workplace.
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].”
However, some people also incorrectly interpret 1.2 cal/cm2 as defining when the arc flash hazard begins. An incident energy below this value does not mean there isn’t an arc flash hazard or possibility of injury. It just means arc-rated clothing and PPE are not required.
Although incident energy below 1.2 cal/cm2 may not translate into a dramatic explosion like you see in many arc flash videos, hazards still may exist.
Incident energy and working distance
Let’s look at what the incident energy on the label really means. The incident energy is determined by performing complex calculations using equations from IEEE 1584—IEEE Guide for Performing Arc-Flash Hazard Calculations. NFPA 70E 130.5(H)(3) requires that when the incident energy is listed on the label, the corresponding working distance is listed as well.
The working distance is defined by NFPA 70E as the “distance between a person’s face and chest area and a prospective arc source.” If any part of the body is closer than the working distance during an arc flash, the incident energy increases dramatically. So, even if it is less than 1.2 cal/cm2 at the working distance, that does not necessarily mean there is no risk of injury, and moving closer results in a greater incident-energy exposure. That is why understanding and maintaining the working distance when an arc flash hazard exists should be an integral part of any electrical safety training program.
Also, what about first-degree burns? What about hands or other body parts that may be closer than the working distance? What about … ? You get the idea. There is still a hazard, it is just not commonly considered a major hazard.
The table above illustrates how incident energy can increase as the working distance decreases. This example was based on 480V equipment with a bolted short-circuit current of 18 kA, VCB electrode configuration and a two-cycle arc duration. At a typical working distance of 18 inches, the incident energy is 1.1 cal/cm2, slightly below the 1.2 cal/cm2 threshold. But with only a 3-inch decrease in the working distance, the incident energy increases to 1.4 cal/cm2. At 12 inches—which is also the restricted approach boundary where shock protection is required—it nearly doubles to 2.1 cal/cm2.
An arc flash produces more than thermal hazards. From my own observations during arc flash tests, a “low” level of incident energy doesn’t mean there aren’t other hazards. On several occasions where a test resulted in a very minimal amount of incident energy recorded, there was still a spray of sparks and molten metal as well as ejected debris. The sound pressure and ultraviolet light can also be significant.
Label colors—further confusion
Label colors can create even more confusion. On a recent trip to our local big box store, I saw an electrical panel with an, ahem, interesting arc flash label. It used the typical signal word “warning.” However, it was on a green background, rather than orange. Looking closer, I noticed the incident energy was listed as “<1.2 cal/cm2.” Then the store manager tapped me on the shoulder.
ANSI Z535.4, Product Safety Signs and Labels, states that the signal word “warning” is used with an orange background, not green. Perhaps green was also meant to indicate “no arc flash hazard exists.” This label was only 3 years old.
So, the next time someone wants to list “no arc flash hazard exists,” imagine how it could play out in a legal setting.
Attorney: “I am sorry to hear that you received a burn injury. Why weren’t you wearing arc flash protection?”
Response: “Because the label stated, ‘No arc flash hazard exists.’”