Arc Flash and Arc Flash Hazard
In Part I, we learned what an electric arc flash and arc flash hazard are. If you recall, we summarized an arc flash hazard is essentially a massive amount of extremely high temperature heat energy released nearly instantaneously from an electric arc resulting in ignition of flammable materials and serious burns to human tissue. The key words are “rapid release of high temperature heat energy.”
This article will answer the first half of the original question, “What is incident energy?” We will reserve the second half of the question, “Why is it important to my job as an electrical worker?” for Part 3.
What Does OSHA Say Incident Energy Is?
Both OSHA regulations 29Code of Federal Regulations (CFR) 1910 and 1926 incorporate the terms “Incident Energy” or “Incident Heat Energy” several times within their respective standards. But OSHA fails to provide us with a definition of incident energy, similar to what we discovered in Part 1 for the term arc flash hazard.
So, once again, we must lean on definitions derived from industry consensus standards to help guide our journey to answer the question, “What is incident energy?”
IEEE 1584 and NFPA 70E® Definitions of Incident Energy
The 2018 IEEE 1584 defines incident energy as the following:
“The amount of thermal energy impressed on a surface, a certain distance from the source, generated during an electric arc event.”
The definition from the 2021 NFPA 70E® article 100 provides a little more information.
“The amount of thermal energy impressed on a surface, a certain distance from the source, generated during an electrical arc event. Incident energy is typically expressed in calories per square centimeter (cal/cm2).”
Breaking It Down
While accurate, these definitions can be a little baffling to those not familiar with arc flash hazards and how they relate to the safety protocols we employ to protect our workers. But if we tackle each element of the 70E definition individually using common analogies, that should help with a better understanding of the whole.
Dissecting the definition of incident energy into its five subparts, we see these stated facts:
- An amount of thermal energy – Everyone who has ever stood by a bon fire or suffered a minor burn understands what an amount of thermal energy is and its effects.
2. That’s impressed on a surface – The term ‘surface’ can mean your body; for example, when you grabbed the hot handle of a cast iron skillet with your bare hand. The heat from the handle was transferred, or ‘impressed,’ upon your palm and fingers, resulting in pain and blisters, and usually accompanied by a few choice words.
3. A certain distance from the source – The distance from the heat source plays a significant impact on the amount of heat you’re exposed to. Standing five feet from a raging bonfire may be comfortable on a chilly night; moving a foot away won’t be.
4. Generated from an electric arc – Anyone who’s undertaken or observed welding of metal using electricity has witnessed an electric arc and what it’s capable of [melting and fusing metal]. An electric arc occurs when current passes through air (that is normally an insulator), which develops a conductive plasma cloud. But arc welding is a controlled process while an arc flash is not.
5. Expressed in cal/cm2 – The calories per square centimeter (cal/cm2) is a measurement of heat energy transferred across a certain area. This tends to be the most challenging of the five to grasp.
Unit of Measurement: Cal/cm2
Heat is measured in calories [lower case ‘c’], abbreviated ‘cal,’ which is the amount of energy needed to raise the temperature of 1 cubic centimeter (cm3) or gram of water 1 °Celsius.
Interesting side note: related to food and nutrition, a Calorie is comprised of 1000 calories, [1 kilocalorie or 1kcal], hence the designation with an upper case ‘C’. Now let that sink in the next time you wolf down that King Size Snickers bar!
The ‘cm2’ is a measurement in a two-dimensional plane or area, a square measuring 1 cm x 1 cm, which is approximately ½” x ½”.
This number, expressed as calorie/centimeter squared (cal/cm2), is the amount of heat needed to raise 1 gram of water 1 °C, spread over an area of roughly ½” x ½” .
At its basic level, incident energy is nothing more than a certain amount of heat energy expressed in cal/cm2 expelled during an arc flash. The higher the cal/cm2, the greater the heat energy. However, keep in mind that our explanation of what incident energy is, is inadequate in and of itself and won’t aid the reader with a full understanding of its dangers unless all the other related criteria are also considered.
In Part 3, I’ll expound on the other factors that will ultimately answer the second and most important question, “Why is incident energy important to my job as an electrical worker?”
Aloha George, Welcome to E-Hazard, and hoping to meet you one day.
Aloha back to you SeaRay. Will you attending the IEEE Electrical Safety Workshop in Jacksonville, FL in a few weeks? If so, I’ll be there and it will great to meet you face to face as well my friend.
Take care and if not at the ESW then hopefully sometime in the future, maybe in Hawaii, if you need to come out and teach a couple of electrical safety classes on the beach. 🙂