Trying to grasp a full understanding of incident energy as electrical workers without considering the other related criteria is like driving from California to Florida without a good navigational system. You’ll probably arrive at your destination, but you’ll most certainly be traveling the long scenic route. When it comes to safety, we should rely on the most direct path to the answer.
Let's Recap
In Part 1, we learned an arc flash hazard is a dangerous event where a large amount of heat is released. And in Part 2, we delved into the question, “What is incident energy?” which is the quantitative amount of heat energy expressed as cal/cm2. Now, in Part 3, we’ll tie everything we’ve learned to the most important question, “Why is it [incident energy] important to my job as an electrical worker?”
Top 5 Factors
Many factors must be considered by the electrical worker as they relate to incident energy, but the following five float to the top of the list.
- Incident Energy Level – The amount of incident energy when the employer is required to provide arc flash protection for employees differs slightly between standards.
For those incorporating NFPA 70E® into your electrical safety program, the incident energy threshold starts at 1.2 cal/cm2, which is the amount of heat energy necessary to cause the onset of a second degree burn of unprotected skin. For electric utilities who limit themselves to OSHA 1910.269 subpart R or 1926 subpart V, the demarcation is anything greater than 2.0 cal/cm2.
- Working Distance – Both NFPA 70E® and IEEE 1584® define working distance essentially the same:
NFPA 70E®: “The distance between a person’s face and chest areas and a prospective arc source.”
IEEE 1584®: “The distance between the potential arc source and the face and chest of the worker performing the task.”
The reason for the emphasis on the worker’s face and chest area is because these two areas of the body constitute the greatest area of skin. The greater percentage of the body burned directly affects the survivability of the burn victim.
The working distance is normally shown in one of three preestablished distances of 18” [457.2 mm], 24” [609.6 mm] or 36” [914.4], 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 identified at a certain working distance, for example, ‘24 cal/cm2 @ 18 inches.’ This means if any body part is closer than 18” to the arc source, then those parts will have an exposure greater than 24 cal/cm2. Thankfully, the opposite is also true; the further you’re away from the arc source, the less incident energy you’re exposed to.
This is known as the “Inverse Square Law,” and NFPA 70E® and IEEE 1584® speak of it as informational notes 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.” **
- Body Positioning and Exposure time – The amount of time the worker is exposed to an arc flash is just as important as the working distance.
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 is expelled.
While electrical workers don’t have much control over the type of OCPD used, they must be aware of two conditions that can directly affect their safety.
- First is the condition of maintenance of the equipment they are about to work on. If the company doesn’t have a good preventative maintenance program, 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 the incident energy.
See NFPA 70E® article 130.5(G) and Table 130.5(C) Informational Note #2 for more information.
2. Second is the body position of the worker when performing a task. Engineers often rely on the “two-second rule” during calculations. This is an engineering assumption that the average worker will instinctively move away from an arc flash, based on the normal human physiological reaction when unexpected threats are experienced. While true in most cases, the worker must also consider his body positioning to perform the task or whether his emergency egress route will be impeded.
For example, a worker lying prone on the floor taking voltage readings will need more time to move away from an arc flash compared to standing up.
- APTV or EBT of PPE – The Arc Performance Thermal Protection (APTV) or Energy Breakopen (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 whenever practical, which will provide an additional safety margin.
- Likelihood of Occurrence – The likelihood of an arc flash occurring is somewhat subjective. However, the specific task performed will mainly affect the likelihood.
A good rule of thumb to determine the likelihood of an arc flash occurring is to review the specific activity or task the employee is about to perform, e.g., “when interacting with energized electrical equipment.” †
Table 130.5(C) Estimate of the Likelihood of Occurrence of an Arc Flash Incident for ac and dc Systems is another great resource.
Work Responsibly
In conclusion, having a strong understanding of what incident energy is and why it plays such an important role in your job as an electrical worker is paramount to your personal safety.
While it is the employer’s responsibility to provide quality electrical safety training to employees, inversely, it is the employees’ responsibility to put that training into practice.
This two-way approach to safety will ultimately sustain a strong electrical safety program. But 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