I often get questions on which standard, equation or computer program should be used to calculate the incident energy when the incident energy analysis method is used. Some argue that the information should be in NFPA 70E, Standard for Electrical Safety in the Workplace or they use the Annex D equations without understanding their applicability. NFPA 70E covers electrical safety not calculating incident energy. Science and testing have shown that an energy of 1.2 cal/cm2 is enough to cause a second-degree burn to exposed skin. That is the limit used in NFPA 70E as the basis for arc-flash protection in an electrical safety work program. Employers must protect employees from this thermal hazard. An incorrectly applied equation will not change the fact that the system presents a thermal hazard to the employee.
How to calculate the energy is beyond the scope of NFPA 70E since the 1.2 cal/cm2 threshold is, for the application of NFPA 70E, absolute. There is a lot of science that goes into converting electrical system parameters into incident energy for any given piece of equipment. IEEE 1584, Guide for Performing Arc Flash Hazard Calculations is probably the most used standard for calculating incident energy. That guide went through a revision and NFPA 70E Annex D information has been updated to reflect that. Users of the 2021 NFPA 70E will find that Annex D summarizes IEEE 1584-2018. This was done in order to protect people from themselves. It was never intended that the information in the NFPA 70E annex be applied without proper use of the entire IEEE 1584 standard. However, unqualified persons were solely using the NFPA 70E annex for calculation methods.
NFPA 70E does not require the use of any specific calculation method but does require that the arc-flash boundary be established at 1.2 cal/cm2. One equation is not applicable to all equipment or electrical systems. NFPA 70E Annex D provides information on several incident energy calculation methods. IEEE 1584 points out that there are other methods which could be used. Calculation methods have specific limitations on the type of electrical system and equipment as well as for many of the parameters used in the equation. Using any equation, computer software or contract company does not absolve an employer from making sure that an appropriate equation is used to calculate the incident energy as accurately as current methods permit. An employee’s life depends on it.
For more information on 70E, read my entire 70E blog series on Xchange. Hear personal stories of people impacted by electrical incidents through NFPA’s Faces of FireTM/Electricaleducation campaign that was created in collaboration with the Phoenix Society for Burn Survivors and raises awareness about electrical hazards in the workplace.
Next time: Comparing four decades of electrical fatalities.
Want to keep track of what is happening with the National Electrical Code (NEC)? Subscribe to the NFPA Networkto stay informed of new content. The newsletter also includes NFPA 70E information such as my blogs.