Serious injuries and fatalities from arc flash incidents continue to plague companies and industries across the globe. Unfortunately, many electrical workers, their supervisors and safety professionals only have a basic concept of this dangerous condition, which hinders an effective protection strategy.
While the majority understand that an arc flash can seriously injure or kill, many believe wearing Arc Rated (AR) Personal Protective Equipment (PPE) is their only option of protection. Their knowledge often doesn’t extend much further than a basic understanding of the enigma known as an arc flash hazard, and they are further perplexed by how Incident Energy (IE) plays into the mix. This can lead to a misconception that PPE directly prevents arc flash hazards. Therefore, the goal of this article is to help the reader gain a better understanding of the dangerous phenomenon known as electric arc flash, the role of PPE and how to consider alternative methods of protection.
The lack of important safety knowledge isn’t the fault of the worker, but from non-existent or inadequate electrical safety training. Ensuring electrical workers receive adequate safety training so they’ll be able to not only identify hazards, but more importantly, mitigate or eliminate them, is part of being a “qualified [electrical] worker”. And the employer is responsible for providing their employees with the necessary electrical safety training.
Let’s first define what an arc flash hazard is, then move into the realm of incident energy, prevention strategies, contributing factors and the governances and standards addressing this danger.
“convective heat energy may ignite clothing and burn skin a significant distance from the fault”
From a US regulatory position, the Occupational Safety and Health Administration (OSHA), requires employers to protect their employees from arc flash but doesn’t provide any substantial guidance for the “how-tos”. OSHA’s directives are also broad by combining “flames and electric arcs”, and as those familiar with electrical safety know all too well, there’s a world of difference between flames and electric arcs.
While OSHA only applies to the US, it’s presented to show governmental regulations are often vague at best by focusing on the “what to do”. But consulting industry consensus standards is the “how to” part that helps employers develop an effective electrical safety programme that protect workers and fulfill regulatory obligations. And the National Fire Protection Association (NFPA) 70E – Standard for Electrical Safety in the Workplace® is the premier standard for electrical safety on an international level. But it too must be augmented by other standards to give us a comprehensive and holistic approach to arc flash safety.
What is an Arc Flash Hazard?
The 2021 edition of NFPA 70E, article 100 defines an Arc Flash Hazard as “A source of possible injury or damage to health associated with the release of energy caused by an electric arc.”
The definition while accurate, tends to be a little indefinite, but if we transition to information derived from the 2018 IEEE 1584 – IEEE Guide for Performing Arc-Flash Hazard Calculations®, we are given a better explanation.
IEEE 1584, section 3.1 provides three pertinent definitions along with Informational Annex E. This quadruple approach should aid our understanding.
Arc Flash – An electric arc event with thermal energy dissipated as radiant, convective and conductive heat.
Arc-Flash Hazard – A Dangerous condition associated with an electric arc likely to cause possible injury.
Arcing Fault Current – A fault current flowing through an electric arc plasma. Syn: arc current.
‘Informational Annex E – What is an arc flash?’ provides the following statement from section E1:
“When an arc flash is initiated, the usually large current generates a strong magnetic field that propels the loose part or tool away. This breaks its contact with the energised parts. As the part moves, the current continues and forms hot arcs that consume conductors, ionises gases and generates a plasma cloud. There is a very bright light and the sound of an explosion. The rapidly expanding gases may blow open doors and propel parts, liquid metal droplets and metal oxide dust. Radiant and convective heat energy may ignite clothing and burn skin on a person a significant distance from the fault. Damage to skin, eyes, ears and lungs can occur, and may be temporary or permanent and in some cases death may result. The injured person may never return to their job. “
When combined, we learn that an arc flash occurs when electric current bridges air [arcing fault current] which is normally an insulator. This causes metals to vaporise [arc plasma] resulting in hot expanding gases [pressure energy] at extremely high temperatures [radiant and thermal energy] to be expelled, followed by a blinding flash [light energy] and deafening noise [sound energy] causing serious injury or death. Note the emphasis on thermal energy released through radiant, conductive and convective forms.
Therefore, an arc flash hazard is essentially a massive amount of heat released nearly instantaneously, resulting in ignition of flammable materials and serious burns to human tissue. While other forms of hazardous energies are accompanied, heat is the main hazard.
How incident energy ties into this leads us to the first half of the original question in the title…
What is Incident Energy?
OSHA regulations use the terms “Incident Energy” or “Incident Heat Energy” several times within their respective standards but fail to provide a definition.
Once again, we defer to consensus standards to help guide our journey to answer this crucial question.
IEEE 1584 defines Incident Energy:
“The amount of thermal energy impressed on a surface, a certain distance from the source, generated during and electric arc event.”
But NFPA 70E article 100, paints a better picture of Incident Energy:
“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).”
These descriptions can be a little baffling in terms of how they relate to safety protocols needed to protect workers. But if we tackle each element individually using common analogies, this should help with a better concept of the whole.
Dissecting the definition of incident energy into its five subparts:
- An amount of thermal energy – Everyone who has ever stood near a raging bonfire or suffered a minor burn understands what an amount of thermal energy is and its effects.
- That’s impressed on a surface – The term ‘impressed on a surface’ means your body, for example, if you grabbed the hot handle of a cast iron skillet with your bare hand. The heat from the handle is transferred or ‘impressed’ upon your palm and fingers resulting in pain and blisters, followed by a few choice words.
- 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 the raging bonfire may be comfortable on a chilly evening but moving a foot away won’t be.
- Generated from an electric arc – Anyone who’s undertaken or observed metal welding using electricity has witnessed an electric arc and its capabilities, i.e. melting and fusing metal. An electric arc occurs when current passes through air, an insulator, which develops a conductive plasma cloud. But the arc during welding is a controlled process while an arc flash is not.
- Expressed in cal/cm2 – Calories per square centimeter (cal/cm2) is a measurement of heat energy transferred across a certain area and tends to be the most challenging of the five to grasp.
Heat is measured in calories [lower case ‘c’] abbreviated ‘cal’ which is the amount of energy needed to raise one cubic centimeter (cm3) or gram of water one degree Celsius. The ‘cm2’ is a measurement in a two-dimensional plane, a square area measuring one centimeter by one centimeter which is approximately a ½ inch x ½ inch. When expressed as calorie per centimeter squared (cal/cm2), is the amount of heat needed to raise one gram of water one degree Celsius spread over an area of one centimeter.
“an arc flash hazard is a massive amount of heat released instantaneously, resulting in ignition of flammable materials”
From the five parts listed above, at its basic level, incident energy is nothing more than an amount of heat energy quantified in cal/cm2 released during an arc flash. Obviously, the higher the cal/cm2 the greater the heat energy. However, limiting our explanation of what incident energy is, is inadequate in and of itself and won’t aid in the reader with a full understanding of its dangers unless all the other related criteria are also considered.
This leads to the title’s second question “Why is Incident Energy important to my job as an electrical worker?”
Why Incident Energy is Important
Trying to grasp an adequate comprehension of incident energy without considering the other related criteria is like driving from California to Florida (4,418 kilometers) without a good navigational system. You’ll probably arrive at your destination after a long arduous journey because you’ve traveled the long, not-so-scenic route. But when it comes to safety, we should rely on the most direct path to the answer.
Many factors must be considered by the electrical worker as they relate to incident energy but the following six top the list.
1. Incident Energy Level Threshold
The level of IE that triggers when the employer is required to provide arc flash protection of employees differs slightly between standards. For those incorporating NFPA 70E into your electrical safety programmes, the IE threshold starts at 1.2 cal/cm2, which is the amount of heat energy to cause the onset of a second degree burn of unprotected skin. But electric utilities in the US are permitted to increase the level to 2.0 cal/cm2. The lower value of 1.2 cal/cm2 should be strongly considered by electric utilities to provide a greater safety margin for their employees.
2. Working Distance
Both NFPA 70E and IEEE 1584 defines Working Distance nearly identically:
“The distance between a person’s face and chest areas and a prospective arc source” versus, “The distance between the potential arc source and the face and chest of the worker performing the task”, respectively.
The reason for the emphasis on the worker’s face and chest area, is these two areas of the body constitute the greatest surface area of skin. The greater the percentage of the worker’s skin damaged by third degree burns directly affects the victim’s chance of survival.
“the greater the percentage of the worker’s skin damaged by third degree burns directly affects the victim’s chance of survival”
Working distance is shown in one of three preestablished distances of 18” [457.2 mm], 24” [609.6 mm] or 36” [914.4 mm] depending on the nominal operating voltage and the class of electrical equipment. When an arc flash analysis is performed, the calculated incident energy level is based on a certain working distance, for example, “24 cal/cm2 @ 18 inches”. This means, if any body part is closer than 18 inches to the arc source, then those parts will experience an exposure greater than 24 cal/cm2. Thankfully, the opposite is also true, as you move further from the arc source it also lessens incident energy exposure.
This is known as the “Inverse Square Law” and both standards speak of it in their definitions.
* “Incident energy increases as the distance from the arc source decreases.”
** “…Incident energy increases as the distance from the potential arc source decreases and the incident energy
decreases as the distance increases.” and “Parts of the body closer to the potential arc source other than the face and chest receive a greater incident energy.”
3. Arc Flash Boundary
The arc flash boundary, abbreviated as “AFB”, is a different but related element. It’s a physical approach distance from the arc source to a point where the incident energy level is 1.2 cal/cm2 or greater. And unlike the Limited Approach and Restricted Approach Boundaries for electric shock protection, which is based on system voltage alone, the AFB, can only be determined from an arc flash study. For this reason, two identical pieces of electrical equipment operating at the same nominal voltage can have two different IE’s and AFB’s.
4. Exposure Time and Body Positioning
The amount of time the worker is exposed to an arc flash is just as important as the working distance and arc flash boundary. When engineers calculate incident energy, they use the clearing times of Overcurrent Protection Devices (OCPD), such as protective relaying settings, breaker trip curves and fuse types during the analysis. The faster the device opens, the less incident energy released. While electrical workers don’t have much control over the type of OCPD used, they must be aware of two other associated conditions that can directly affect their personal safety.
• First is the condition of maintenance of the electrical equipment. If your company doesn’t have a routine preventative maintenance programme, then there’s a good chance the OCPD may not operate within it’s designed parameters. Any delay in clearing a fault will contribute to an increased incident energy. See NFPA 70E article 130.5(G) and Table 130.5(C) Informational Note #2 for more information.
• Second is body positioning when performing a task. Engineers often rely on the “two second rule” during calculations which is an engineering assumption the average worker will instinctively move away from an arc flash due to the normal human physiological reaction when unexpected threats are experienced. While true in most cases, the worker must also consider if his emergency egress route is impeded by body position when performing a task. For example, a worker lying prone on the floor taking voltage measurements will need more time to move away from an arc flash compared to standing up.
“the arc rating of PPE should exceed the incident energy which will provide additional safety margin”
5. ATPV or EBT of PPE
The Arc Thermal Performance Value (ATPV) or Energy Breakopen Threshold (EBT) of arc rated PPE clothing and equipment must meet or exceed the incident energy that can be encountered. At e-Hazard, we strongly recommend the arc rating of PPE should exceed the incident energy which will provide additional safety margin. However, regardless of its rating, PPE does not prevent or reduce the risks of an arc flash occurring but rather, it reduces the risks of injury if the unexpected were to occur. This is one reason why NFPA 70E ‘Hierarchy of Risk Controls’ per article 110.5(H)(3), lists PPE as the least effective control method.
6. Likelihood of Occurrence
The likelihood of an arc flash occurring is somewhat subjective, because the task being performed will affect the probability. A good rule of thumb to determine the likelihood of an arc flash is when +“interacting with electric equipment” and NFPA 70E Table 130.5(C) Estimate of the Likelihood of Occurrence of an Arc Flash Incident for ac and dc Systems is a great resource. Another factor impacting the likelihood is the condition of maintenance or lack of maintenance. As previously related to OCPD exposure time, routine maintenance also affects the probability of an occurrence in a positive direction by reducing the chances.
How does this Information Help Prevent Arc Flashes?
In truth, arc flashes are never 100% preventable because it’s an unexpected condition influenced by unforeseen equipment failure, poor maintenance practices, human errors, and a host of other contributors. But its impact on you can be minimised or eliminated if a thorough arc flash risk analysis is performed prior to performing every job. NFPA 70E article 130.5 “Arc Flash Risk Assessment” actually mandates its performance and contains very concise step by step instructions along with Informative Annex F – Risk Assessment and Risk Control.
“a strong foundation of what incident energy is and why it plays an important role in your job is paramount to your personal safety”
At the same time, if the arc flash study reveals the electrical distribution system has areas with very high incident energy levels, then this information can be used to justify the need to procure mitigation or preferably elimination technologies. For example, there are various types of arc flash reduction methods that can be installed to existing equipment. During new construction, arc resistant electric equipment which is designed to direct incident energy away from the worker can be considered during the planning phase. And while they won’t eliminate the risk of an arc flash, the incident energy level exposure is significantly reduced thus requiring less PPE. But elimination of exposure from an arc flash is the preferred approach, and can be accomplished by the use of such things as remote racking and/or remote switching devices. Since a good percentage of arc flashes happen during breaker manipulation, remote racking and switching devices position the worker physically outside of the arc flash boundary thus eradicating worker exposure and the need for PPE. With elimination, the hazard still exists but the risk of damage to the health and safety of the worker(s) does not.
A strong foundation of what incident energy is and why it plays an important role in your job is paramount to your personal safety. While the employer is responsible to provide quality electrical safety training to employees, conversely, it’s the employee’s responsibility to put that training into practice by following all safety rules. This two-way approach to safety will sustain a strong electrical safety programme. But the bottom line is, you as the qualified electrical worker must never forget “You Are Responsible for Your Own Safety!”.
* NFPA 70E informational note for the definition of working distance, article 100
** IEEE 1584 note for the definitions of incident energy and working distance, section 3 + NFPA 70E informational note for the definition of arc flash hazard, article 100