How to Calculate Arc Flash Incident Energy

How to Calculate Arc Flash Incident Energy - there are two basic standards which establish requirements for arc flash hazards. The first is NFPA 70E, Standard for Electrical Safety in the Workplace, which defines the basic practices to be followed for electrical safety, including personal protective equipment ppe levels which must be worn for given levels of arc flash incident energy and what steps must be taken prior to live work on electrical equipment. The second is the IEEE Guide for Performing Arc-Flash Hazard Calculations, IEEE 1584-2018 which gives the engineer the methods for calculating the severity of arc flash incident energy levels.

Incident Energy is defined as the amount of energy impressed on a surface, a certain distance from the source, generated during an electrical arc event. One of the units used to measure incident is calories per square centimeter (cal/cm2).

When determining how to calculate the incident energy for voltages over 600V, the method for calculating the arc flash incident energy for a given working distance from live parts is not specified in NFPA 70E code text itself but several methods are given in Annex D of NFPA 70E. The preferred methods for determining how to calculate arc flash incident energy are given in IEEE 1584. The option is also given to use pre-prepared tables given in NFPA 70E based upon given levels of bolted fault and fault current and open air circuit breaker arcing time to select personal protective equipment in lieu of a formal arc flash study.

Over the last few decades, arc flash hazards have been a significant concern for many electrical workers and employers. Many calculation methods have been developed through the years to assess incident energy. The most common is that of the IEEE 1584, Guide for Performing Arc-Flash Hazard Calculations. These equations have remained the same since the standard was first published in 2002. Based on the results of more than 1,800 tests, the 2018 edition of the guide provides new formulas that are both more accurate and more complex. These formulas also take more parameters into account now, including the conductor orientation at the arc’s location and the compartment enclosure dimensions where the arc occurs.

 
IEEE 1584 2018 EDITION: WHAT PARAMETERS TO USE
IEEE 1584 is the guide for determining arc flash incident energy levels and protection boundaries. It contains an empirical calculation method based upon extensive test results using a Design-of-Experiments (DOE) method, resulting in a 95% confidence level that the arcing fault current will be higher than calculated. In situations where the empirical method does not apply, the “Lee” method from is recommended, and is described in IEEE 1584.
IEEE 1584 only takes into account the heat of an arc, and not the secondary effects such as molten metal spatter and pressure-wave effects.

Given its complexity and need for more parameters, before using this new method for first time, it is important to understand the differences in results compared to the 2002 edition. First, should users of the 2018 edition expect higher or lower values for the same piece of equipment? Second, how will a parameter error affect the results? Some parameters required by the IEEE 1584 2018 edition, i.e., the gap, the conductor orientation or the enclosure dimension, can vary a lot for the same piece of equipment or even in the same compartment. It might not be worth spending a lot of time and resources to get precise parameters; a standard value may be enough. On the other hand, standard values may not be correct and may lead to a significant incident energy error. It is important to determine which parameters need to be accurate.

 

UNDERSTANDING PARAMETER IMPACTS

The complexity of the model must reflect industrial realities: the expert must select, among the proposed parameters, those that are most representative of, or detrimental to, the actual installations and the work to be carried out on these installations. There are multiple software programs available on the market to perform incident energy analyses. Default values are not necessarily the most appropriate; it would depend on the application. It is still up to the engineer to use the proper parameters and understand their worth. A failure to understand the impact of a single parameter may lead to workers being exposed to higher than expected incident energy.

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